How to Make FM Transmitter?
This tutorial is for making simplest FM transmitter using only one transistor. VC1 is a small, screw-adjustable, trimmer capacitor and its rating should be around 10-100pF. Set your FM receiver for a clear, blank station.
Then, with a non-conductive tool, adjust the capacitor for the clearest reception, rotate it till the receiver receives a sound from the microphone of transmitter. Use the following formula for determining the frequency.
The schematic of FM transmitter:
The following shows the components used to make FM transmitter:
1. Transistor, 2N3904:
2. Capacitors, 4.7pF, 20pF, 0.001uF, 22nF. (0.001uF has code 102 and 22nF has code 223)
3. Variable Capacitor, For VC1 you can use a trimmer capacitor that looks like this:
4. Resistors, 4.7K, 470R
5. Mic, Condenser Electret Microphone:
6. Inductor, 0.1uF 6-7 turns using 26SWG wire:
Learning how to make an inductor for FM transmitter:
You need to scrap the ends of inductor, otherwise, the inductor does not work, you can use commercial 0.1uH inductor.
Insert transistor, resistors and capacitors on breadboard. You can see the values of the components on the previous picture. Then insert electret microphone. Use 15cm long antenna. You can use a normal wire for antenna.
1.5 Volt FM Broadcast Transmitter with TR 2N4401
The objective of this 1.5V FM Broadcast Transmitter design is to provide a simple low-power transmitter solution for broadcasting audio from various audio sources. This transmitter accepts stereo input via two 470K resistors. Since there is no audio level control on the input, the audio level out from the source needs to be adjusted. Or, you can just add a 10k as an input level control. Transmitter’s frequency, as built is tunable via spreading or compressing the coil to the desired frequency, and the coil can be glued down. If you want to make one that’s tunable, it might be easiest to reduce the 18 pf capacitor and put a small trimmer capacitor in parallel with the inductor (across the reduced value capacitor). Voltage variable capacitors would be an nice alternative to a mechanical variable capacitor but they don’t offer much tuning range with only a 1.5V power supply.
Notice: Before operating a radio transmitter, find out what kind of transmitter operation, if any, is permitted in your locality. Radio transmitter operation is a serious legal matter. In the United States, operation of unlicensed intentional radiators is covered by Part 15 of Title 47 of the Code of Federal Regulations. This design can be readily adapted to different frequencies and different power levels. If you choose to build and operate the transmitter described here, you do so at your own risk. I’m only publishing this as an example of what can be done.
This implementation is adapted to rebroadcast the output of a CD player, television receiver, or radio receiver. I use it so that I can move about the house and listen to my favorite programs without disturbing others. Within and the house, I find that I can get 10 to 20 meters away from the transmitter with the small pocket FM receiver I carry in my shirt pocket. Your mileage may vary. The transmitter as built and pictured below (the transmitter is in the blob of hot melt glue on the end of the battery holder) does not have an on-off switch. I put a 1.5 AA cell that was run down too far to run my CD player in this transmitter and it ran for over a month before I replaced it. The one in the transmitter at this moment has been running it continuously for over three months. Current draw is only about a milliamp with a new battery (assuming you don’t have a super-high beta transistor in which case the theoretical limit is about 2.5 ma). An on-off swich is not necessary, though it may satisfy an emotional need.
Tips to get it working: Wind the coil on a 4 or 5 mm diameter Philips blade screwdriver or similar form then slip it off. I used some vinyl insulated #24 hookup wire as well as #30 enameled wire. In both cases, I played with the length of the coil to tune the transmitter to a dead spot on the FM band. The coil is held in place with hot melt glue. If you don’t have a spectrum analyzer or frequency meter, use a good-quality FM receiver to make sure its tuned where you think it is. While adjusting the coil, keep in mind that all superheterodyne receivers have images. If you find that two or more adjustments make the transmitter show up on the same spot on the receiver, it might be necessary to take a short walk and find out which adjustment drops out first -this would be the image, because the receiver’s front end (if it has a tuned front end) will reduce its sensitivity to the image.
Many kinds of transistors will work fine in this application. After all, its only an oscillator (frequency modulation is obtained my modulating the base-collector voltage, thereby modulating the depth of the depletion layer of the reverse-biased base-collector junction, which results in a change in capacitance at the collector, which results in a change the resonant frequency of the collector circuit.). I used an 2N4401 because I have a lot of them. I like 2N3904 and MPSH34 for this too.
1.1 – 3Volts, 30-50m Simple FM Transmitter TR 2N2222
Mini FM transmitters take place as one of the standard circuit types in an amateur electronics fan’s beginning steps. When done right, they provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance. I’ve seen lots of designs through the years, some of them were so simple, some of them were powerful, some of them were hard to build etc. Here is the last step of this evolution, the most stable, smallest, problem-less, and energy saving champion of this race. Circuit given below will serve as a durable and versatile FM transmitter till you break or crush it’s PCB. Frequency is determined by a parallel L-C resonance circuit and shifts very slow as battery drains out.
Supply voltage : 1.1 – 3 Volts
Power consumption : 1.8 mA at 1.5 Volts
Range : 30 meters max. at 1.5 Volts
Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations Smile
Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.
Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.
The critical part is the inductance L which should be handmade. Get an enameled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance it’s value. When you find the right point, adjust inductance’s looseness and, if required, number of turns.
Once it’s OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio lost in a few meters range, than it’s probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the center frequency. Place radio far away from the circuit and re-adjust. An oscilloscope would make it easier, if you know how to use it in this case. Unfortunately I don’t have any Sad
Every part should fit on the following PCB easily. Pay attention to the transistor’s leads which should be connected right. Also try to connect trimmer capacitor’s moving part to the + side, which may help unwanted frequency shift while adjusting. PCB drawing should be printed at 300DPI, here is a TIFF file already set.
PCB design for the FM Transmitter
The one below is a past PCB work of mine, which was prepared to fit into a pocket flashlight. Since it was so crowded, use the new computerized PCB artwork instead, yet very small. Take a look at my PCB design page to get information on my work style.
Transmitter PCB compared in size with an AA battery
Here is a completed and perfectly running circuit, mounted in a pocket light, taking the advantage of the 1.5V AA cell slot near it. Microphone is fixed into the bulb’s place and antenna is made out of a 30cm soft cable. When cover is placed, it becomes very handy!
Transmitter placed in a pocket flashlight
Do not forget, restrictions on radio frequency transmitting devices may differ in your local area. This circuit has a power output that should be less than 1mW so have to be safe under many kinds of legal conditions but particular attempts such as listening to other people’s private life will always be disapproved everywhere.
1,25-3 Volts, Stereo FM Transmitter Using BA1404
A high quality stereo FM transmitter circuit is shown here. The circuit is based on the IC BA1404 from ROHM Semiconductors. BA1404 is a monolithic FM stereo modulator that has built in stereo modulator, FM modulator, RF amplifier circuitry. BA1404 FM transmitter can be operated from 76 to 108MHz and power supply for the circuit can be anything between 1.25 to 3 volts.
In the circuit R7, C16, C14 and R6, C15, C13 forms the pre-emphasis network for the right and left channels respectively. This is done for matching the frequency response of the FM transmitter with the FM receiver. Inductor L1 and capacitor C5 is used to set the oscillator frequency. Network C9,C10, R4,R5 improves the channel separation. 38kHz crystal X1 is connected between pins 5 and 6 of the IC. Composite stereo signal is created by the stereo modulator circuit using the 38kHz quartz controlled frequency.
1,5-3 Volts, Simple MP3 FM Transmitter TR3904
A simple MP3 FM transmitter circuit shown here can be built easily in few minutes if all parts are available to you. All the components used in this transmitter circuit are general purpose and low cost. The circuit will work as a best FM transmitter for simply broadcasting your music around your house and yard, and can be used to broadcast the output of any equipment like mp3 player, ipod, satellite, etc.
The coil L1 is equal to 8 to 10 turns of 22 guage wire wound on 6mm form, remove the form after winding the coil. At the adjacent to coil there is a 1-30 pF trimmer capacitor, which is used to adjust the frequency of this mp3 FM transmitter.
After building the circuit apply power and connect it to the audio output of your device, then set your FM radio frequency on a blank spot and slightly adjust the transmitter’s trimmer with a non conductive tool and stop where you found the sound of your music. Minimum antenna length should be 12 inches but if you want to increase the range then increase the antenna length to 30 inches. You can power this FM transmitter circuit with any size/type of 1.5V or 3V battery.
3 Volts, FM Micro Transmitter Bug TR BF324
Presented FM transmitter bug is built using BF414 / BF324 / BF606 transistor. The 30cm antenna has a range of about 30m in the building, more in the open field. Power supply 2x AAA batteries have been used with voltage of 2.75 V. I added resistor 10K in parallel with 1.5pF capacitor so that the system works well when connected to an external source (mp3 player / computer). On the computer I had to reduce sound to about 35% of capacity, so that I do not have clipping. I managed to improve transmitter stability with simple shielding. The coil is 5 turns of enameled copper wire wound on 1 mm ø = 5 mm.
Included on pages dedicated electronics simple micro transmitter bug. It’s more than a wireless transmitter microphone bug, would be good to amplify the signal from the microphone to be heard and distant call. The transmitter operates in the CCIR FM (87-108 MHz), to re-tune to another frequency it would probably not be a problem.
First FM wireless transmitter microphone
Tab. First Tried types of transistors Table 1 Tested transistors
Type C2 C3
C e b BF606 excellent / best 33 pF 47 pF
C b e BF324 excellent / best 33 pF 47 pF
e b C 2N3906 good / good 27 pF 33 pF
e C b 2SA854 good / good 22 pF 27 pF
C b e BC307, BC558 bad / poor
low fT / low fT
R1: 4.7 kOhm
R2: 330 Ohm
C1: 1.5 nF ceramic.
C2: 33 pF, ceramic.
C3: 47 pF, ceramic.
Mic: Electret microphone
L: 6 z Cul diameter 0.8 mm self-supporting wound on a mandrel 5 mm 6 turns on 5 mm diameter. 11.5 cm length wire 0.8 mm diam.
Description of involvement
Involvement of wireless transmitter microphone is on Figure 1 . Signal from the electret microphone is modulated oscillator with T1. Microphone at the same time provides the oscillator DC bias. Voltage characteristics microphone MCE100 (GM electronic) on Figure 2 . It shows that from a voltage of about 1 V, the current flowing microphone little changed. This is used to stabilize the operating point of the oscillator. The oscillator I have tried several different transistors. For reliable operation it is necessary that the cutoff frequency of the transistor is at least 200 MHz, preferably 300 to 500 MHz.
Fig. Second VA characteristic electret microphone MCE100 Fig Second MCE100 electret microphone VA characteristic
The preamplifier is built on the board according to Figure 3 , the layout of components on Fig 4. Coil the prototype had 6 turns enameled wire with a diameter of 0.8 mm, self-supporting wound on a mandrel (I used a drill bit shank) with a diameter of 5 mm. Need wire 11.5 cm. The listed components is an oscillator tuned to the lower end of the range, about 90 MHz. To the upper end of band reels only 5 loops. Oscillator tune stretching and compressing coil turns.
Another type of microphone can browse through more or less current. Measure the voltage because the R1. Resistor R1 decrease, if voltage on it exceeds 1.5 V. If the measured voltage is less than 1, it is necessary to increase the resistance of the resistor R1.
The transmitter is designed for power supply voltage of 3 V. The current consumption is about 3 mA. You can also use 2 or 3 NiCd – 2.4 or 3.6 V. When powered from the AC power in the broadcast signal appeared hum. Sufficed for its suppression capacitor 10 uF connected to the power supply directly to the board. When battery power was a signal without hum.
Serves as an antenna 20 cm long piece of wire. If longer necessary to connect the antenna, it will be necessary to separate the capacitor with the capacity of a few pF.
The involvement of bedbugs, I would recommend to add in parallel to the source of about 100 pF capacitor to ensure small internal source resistance at working frequency oscillator. This will improve performance and stability especially oscillator. I know from experience that some oscillators without this capacitor not run at all.
3 Volts, Simple Coilless FM Transmitter
For months I’ve been looking for a simple FM BUG project, the ones online require inductors which you either have to acquire or build, if you don’t have a LCR meter it becomes rather hard to get the circuit working, specially if you’re a beginner without an oscilloscope! – Sometimes they don’t even tell you which inductance is required and you have to calculate an estimate, which is the main reason why many high frequency RF projects fail in the first place. This circuit on the other hand performs pretty well, even if you’re manipulating the board or touching the coax it will stay within the tuned frequency (unless you touch the transistor or timing capacitor!).
From all the projects out there I’ve only seen one which didn’t require an external inductor since it simply used a pcb / trace inductor, however the board was big and the circuit itself had lots of stability issues, etc. I wasn’t going to waste my time with it. My first FM BUG was based on one of the many schematics out there, it seldom worked. It was microphonic (due to the air core inductor) but the electret capsule itself did not modulate the output at all, needless to say it was very unstable and it never worked properly.
No surprises there, it’s very similar to other schematics so therefore I won’t go into the working details. Actually I might argue this one is a bit on the low efficiency side, but at the moment I don’t have the equipment to further improve it’s design. But let’s not focus on that and instead let’s talk about the inductor / antenna, it’s based on a 12cm long 50-75 Ohms coaxial cable with one of it’s ends soldered (mesh and core are joined together) this is a big plus, we don’t have to use a flimsy air core inductor and we don’t need a lengthy wire antenna either – Great!
This tiny ghetto transmitter is guaranteed to work first time, as long as you double check all connections and you make sure your transistor is properly placed and in working order. I used the BF199 because it’s got a low capacitance and it’s ideal for this type of application, but you may use a 2n2222 or similar general purpose NPN BJT. I know, the schematic calls for a BF259 which is what I used in my simulation, but believe me it works beautifully with the BF199.
I didn’t have a 8.2pF capacitor so I used a smaller one, which naturally led the transmitter to work on the upper FM broadcast band, but hey — better than nothing! The sound quality is not the best, this FM Bug could clearly benefit from an audio amplification stage. Another idea would be to scrap the electret and use an MP3 player instead. I reckon R4 could be ignored in that case but you might want to use a resistor in series with the input capacitor.
Another important component is the variable capacitor which forms the tuning circuit, I used a 4.5-20pF trimmer type capacitor, it’s the first one I found in the junk box, it was a bit flimsy and it required lots of patience to tune but I eventually got it to the point I wanted. I recommend you get a ceramic screwdriver for the tuning or you can improvise one with a piece of plastic, it’s important because otherwise your body capacitance will affect the transmitter!
For those interested I also included the PCB layout, it’s very easy to etch your own PCBs so you should definitely give it a try! – Remember to keep the component leads to a minimum, we’re working with high frequencies, any parasitic capacitance, etc. will modify the behaviour of the circuit one way or another!
One improvement over the current layout would be to get rid of the header for the power connection and use a button cell with a holder instead, another one would be to use a 3.5mm connector instead of the electret being soldered directly to the board.
And in case you’re wondering, here’s the BOM:
1x BF199 or BF299 or 2N2222 or similar RF NPN BJT.
1x 8.2pF Capacitor.
1x 2.2nF Capacitor.
1x 0.1uF Capacitor.
1x 50nF Capacitor.
1x 100 ohm resistor.
2x 4.7k resistor.
1x 5.6k resistor.
1x 3-30pF variable capacitor.
1x 3V source (I used 2x 1.5V AAs with a battery holder).
1x 12cm long piece of 50-75 Ohms coaxial cable to serve as the inductor/antenna.
1x electret microphone (double check the polarity!)
Standby power consumption is ~5mA. During normal operation you should expect peaks of 10mA but on average 6mA is about right. L1 would be lucky to see peaks higher than 15mW! If you want to increase the transmission distances you may use a metallic enclosure and solder the negative of the battery to the enclosure itself.
Also keep in mind my design was not meant to be used as a concealed spy bug, if you really wanted to build a proper spy bug you’d have to use SMT parts, these are generally not available to the beginner and therefore I went with through-hole components instead. Plus I’m against spying people without their consent… That’s all for now, hopefully you’ll build and enjoy this tiny FM BUG.
3 Volts, 50-100m FM Transmitter TR C2058
The objective of this 3V FM Transmitter design is to provide a simple low-power transmitter solution for broadcasting audio from various audio sources. This transmitter transmits audio using small sensitive microphone. Transmitter’s frequency, as built is tunable via 15pF trimmer to the desired frequency, and the coil is embedded on the circuit board. This implementation is adapted to rebroadcast the output of a CD player, television receiver, or radio receiver. I use this transmitter so that I can move about the house and listen to my favorite programs without disturbing others. Within and the house, I find that I can get 50 to 100 meters away from the transmitter with the small pocket FM receiver I carry in my shirt pocket.
The transmitter as built and pictured below (the transmitter is in the blob of hot melt glue on the end of the battery holder) does not have an on-off switch. I put a two 1.5V AA cells that was run down too far to run my CD player in this transmitter and it ran for over a month before I replaced it. The one in the transmitter at this moment has been running it continuously for over three months. Current draw is only about a milliamp with a new battery (assuming you don’t have a super-high beta transistor in which case the theoretical limit is about 2.5 ma). An on-off switch is not necessary, though it may satisfy an emotional need.
Many kinds of transistors will work fine in this application. After all, its only an oscillator (frequency modulation is obtained my modulating the base-collector voltage, thereby modulating the depth of the depletion layer of the reverse-biased base-collector junction, which results in a change in capacitance at the collector, which results in a change the resonant frequency of the collector circuit.). I used an BC549 because I have a lot of them. I like BF199 and MPSH34 for this too.
3 Volts, 100m Simple FM Transmitter TR BF194B
Here is a very interesting and simple FM transmitter used to transmit audio in the wide range up to 100M using only one transistor. The entire circuit of FM transmitter is divided into three major stages oscillator, modulator and amplifier. The transmitting frequency of 88-108 MHz is generated by adjusting VC1. The input audio generated by microphone is changed into electric signal and is given to base of transistor T1. Transistor T1 is used as oscillator which oscillates the frequency of 88-108 MHz. The oscillated frequency depends upon the value R2, C2, L2 and L3. Transmitted audio from FM transmitter circuit can be received by standard FM receiver.
Resistors (all ¼-watt, ± 5% Carbon):
R1 = 180 KΩ
R2 = 10 KΩ
R3 = 15 KΩ
R4 = 4.7 KΩ
C1 = 10 KPF
C2 = 10 PF
C3 = 20 KPF
C4 = 0.001 µF
C5 = 1 µF/10V
C6 = 4.7 PF
C7 = 10 KPF
C8 = 3.3 PF
VC1 = 22 PF
T1 = BF194B
MIC1 = Condenser mike
L1, L2 = 3 turns of 22 SWG wire around any thin pencil
L3 = 2 turns of 22 SWG wire around any thin pencil.
5 Volts, USB FM Transmitter using IC MAX2606
Here is a simple USB FM transmitter that could be used to play audio files from an MP3 player or computer on a standard VHF FM radio by connecting it to an USB port. The circuit use no coils that have to be wound. This USB transmitter can be used to listen to your own music throughout your home. To keep the fm transmitter circuit simple as well as compact, it was decided to use a chip made by Maxim Integrated Products, the MAX2606.
MP3 FM Transmitter Parts List
Resistors (all SMD 0805)
R1,R2 = 22kΩ
R3 = 4kΩ7
R4,R5 = 1kΩ
R6 = 270Ω
P1 = 10kΩ preset
P2 = 100kΩ preset
Capacitors (all SMD 0805)
C1,C2,C5 = 4μF7 10V
C3,C8 = 100nF
C4,C7 = 2nF2
C6 = 470nF
L1 = 390nF, SMD 1206
L2 = 2200Ω @ 100MHz, SMD, common-mode choke
IC1 = MAX2606EUT+, SMD SOT23-6
K1 = 3.5mm stereo audio jack
K2 = 5-pin header (only required in combination with pre-emphasis circuit)
K3 = USB connector type A
This IC from the MAX2605-MAX2609 series has been specifically designed for low-noise RF applications with a fixed frequency. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. The variable-capacitance (varicap) diode and feedback capacitors for the tuning have also been integrated on this chip, so that you only need an external inductor to fix the central oscillator frequency.
It is possible to fine-tune the frequency by varying the voltage to the varicap. Not much is demanded of the inductor, a type with a relatively low Q factor (35 to 40) is sufficient according to Maxim. The supply voltage to the IC should be between 2.7 and 5.5 V, the current consumption is between 2 and 4 mA. With values like these it seemed a good idea to supply the circuit with power from a USB port.
A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply. There is not much else to the circuit. The stereo signal connected to K1 is combined via R1 and R2 and is then passed via volume control P1 to the Tune input of IC1, where it causes the carrier wave to be frequency modulated. Filter R6/C7 is used to restrict the bandwidth of the audio signal. The setting of the frequency (across the whole VHF FM broadcast band) is done with P2, which is connected to the 5 V supply voltage.
USB FM transmitter PCB Layout
The PCB designed uses resistors and capacitors with 0805 SMD packaging. The size of the board is only 41.2 x 17.9 mm, which is practically dongle-sized. For the aerial an almost straight copper track has been placed at the edge of the board. In practice we achieved a range of about 6 metres (18 feet) with this fm transmitter usb. There is also room for a 5-way SIL header on the board. Here we find the inputs to the 3.5 mm jack plug, the input to P1 and the supply voltage. The latter permits the circuit to be powered independently from the mains supply, via for example three AA batteries or a Lithium button cell. Inductor L1 in the prototype is a type made by Murata that has a fairly high Q factor: minimum 60 at 100 MHz.
Take care when you solder filter choke L2, since the connections on both sides are very close together. The supply voltage is connected to this, so make sure that you don’t short out the USB supply! Use a resistance meter to check that there is no short between the two supply connectors before connecting the circuit to a USB port on a computer or to the batteries.
P1 has the opposite effect to what you would expect (clockwise reduces the volume), because this made the board layout much easier. The deviation and audio bandwidth varies with the setting of P1. The maximum sensitivity of the audio input is fairly large. With P1 set to its maximum level, a stereo input of 10 mVrms is sufficient for the sound on the radio to remain clear. This also depends on the setting of the VCO. With a higher tuning voltage the input signal may be almost twice as large (see VCO tuning curve in the data sheet). Above that level some audible distortion becomes apparent. If the attenuation can’t be easily set by P1, you can increase the values of R1 and R2 without any problems.
Measurements with an RF analyzer showed that the third harmonic had a strong presence in the transmitted spectrum (about 10 dB below the fundamental frequency). This should really have been much lower. With a low-impedance source connected to both inputs the bandwidth varies from 13.1 kHz (P1 at maximum) to 57 kHz (with the wiper of P1 set to 1/10).
In this usb fm transmitter circuit the pre-emphasis of the input is missing. Radios in Europe have a built-in de-emphasis network of 50 μs (75 μs in the US). The sound from the radio will therefore sound noticeably muffled. To correct this, and also to stop a stereo receiver from mistakenly reacting to a 19 kHz component in the audio signal, an enhancement circuit Is published elsewhere in this issue (Pre-emphasis for FM Transmitter, also with a PCB).
5 Volts, USB FM Transmitter using IC BH1417
Here’s BH1417 USB FM Transmitter with built-in PLL circuit. Its low-frequency signal is converted into high-frequency, which can take any audio device with FM radio (stereo, car CD, MP3, DVD player, etc.), as a normal radio station. Transmitter power is sufficient for reliable reception of its signal within a few tens of meters. The basis of the device is a chip BH1417F, included in a typical scheme. This device contains all the necessary circuitry to generate a composite stereo signal c of the pilot tone, the RF generator with PLL and power amplifier. A detailed description is given in.
BH1417 specs and features chips:
– Supply voltage 4 … 6 volt;
– Current consumption 20 mA;
– Operating temperature range -40 … +85 ° C;
– A range of audio frequencies 20Hz … 15kHz;
– 40 dB channel separation;
– THD 0.1%.
Formation of carrier frequencies in the broadcast signal chip carries a stabilized quartz resonator Z1 (7.6MHz) PLL-synthesizer with 4-bit parallel interface control. Changing the position of switches SA1 … SA4 can get 14 discrete frequencies. Their values are listed in the table.
This device PLL is stable only works in one of the sub-bands (87.7MHz … 88.9MHz or 106.7MHz … 107.9MHz) without having to reconfigure L1. Structurally, the device is on the circuit board 50 x 23mm and is equipped with a USB connector, through which the supplied 5V. The photographs shows the appearance of printed circuit board unit.
The PCB is made of fiberglass-sided 1.0 mm thick. All resistors, inductor L2 and Ceramic Capacitors – SMD, 0805. As the VT1, VT2, you can use npn transistors for surface mounting with a static gain of at least 100. Crystal at 7.6 MHz can be purchased at the store “Quartz” 50m from the metro station “road enthusiasts” (Sh Enthusiasts, 31). You can also use less scarce, the frequency of 7.68 MHz. The coil loop oscillator L1 is wound on a frame of 4 mm diameter tinned copper wire with a diameter of 0.6 mm has 4.5 turns in the winding length of 6 mm. Within the framework of core trimming high ferrite.
It should be noted that the independent production and installation of such a board problem is rather complicated. A simpler version of the printed circuit board can be found in . When first turned on and set power 5.1 V is supplied from a laboratory source. To get started, choose the sub-band switch SA4, which will operate the transmitter. Then, turning off the SA3 (on pin. 17 circuits DA1 to be a logical unit) and adding SA1 and SA2 (at the pin. 15 and 16 must be logic zeros) adjusting core coils L1, seeking to Pin 7 voltage 2.8 V. After that, the FM receiver check the stability of the PLL capture within the sub-band. To do this, changing the position of the switches SA1 … SA3, check for the receiver of the transmitted signal at all frequencies sub-band.
In conclusion, the maximum undistorted signal is selected value of resistors R1 and R2 (structurally they are located inside the input jack). The device is preferably placed in a screen of thin sheet metal, perhaps after this it is necessary to adjust the L1. As a piece of wire antenna and MGTF-0, 07 length of about 80cm. When choosing the operating frequency to avoid interference is desirable that it defended by no less than 200 kHz operating in your area FM radio stations and broadcast television channels.
5 Volts, FM/RF Transmitter Amplifier with BA1404 and UPC1651
BA1404 transmitter includes onboard RF amplifier for increased transmitting range. Operating voltage range is 1-3V, the circuit contains FM stereo mixer, 38KHZ oscillator, FM modulator and high-frequency amplifier monolithic integrated circuit. As the “electronic newspaper” BBS there are many users requiring detailed information on the FM stereo transmitter, so I re-collect the relevant information on the simple discrete, merge, integrated FM stereo transmitter experiment, that BA1404 with μpc1651 mix of the most easy to make and debug, and very high frequency stability (relative to the previous circuit BA1404), transmission power is increased by UPC1651RF amplifier.
Works (see Figure I):
Stereo audio signal and by increasing the input matching network consists of 1,2 feet, the amplified FM stereo into the mixer, to produce a main signal from the L + R and LR stereo sub-signal composite signal composed of amplified by the buffer from 14 feet out (16,17 feet to adjust the parameters of the composite signal, can control the left and right balance).
4,5,6 pin external discrete components and internal oscillator circuit 38KHZ 38KHZ signal by the buffer supply of mixers and amplification, respectively, 1 / 2 divider, 38KHZ 19KHZ signal by the divider to get a pilot signal from 13 feet out.
13-14 feet out from the composite signal and the pilot signal by the matching network consists of 11 feet into the FM Modulator (9,10 feet of production sub-component to determine the external oscillation frequency) to produce a FM signal, the amplified output from 7 feet.
2 feet for the AF offset 3 feet for the AF ground point,
8 feet is the RF connection location,
15 feet for the positive power supply. (Note: a reference voltage output pin 11 to facilitate external discrete components to control the oscillation frequency, where not used.)
7 foot by μpc1651 final amplified output signal.
13,14 feet, only the reference value of resistance, because of its value on the stereo separation are related, according to the actual situation given value.
L for the iron shell variable inductance, its parameters and frequency-related, here is the critical frequency stabilization, be careful.
If you want electricity supply, pay attention to power, the current small, discrete filter can be regulated power; μpc1651 the working voltage is 5V, not exceed, easy to burn.
6-9 Volts, 90 meters FM Broadcast Transmitter
This FM Broadcast Transmitter circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards (90m).
A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency. Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery. Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor.
The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then streached to about a 3/8 inch length and tapped near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz).
The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pullup resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar.
9 Volts, 50m, Mini FM Radio Transmitter with TR BF199
This small FM transmitter with a range of about 50 meters designed for hoby. With lots of mini-transmitters then you have a comprehensive, action-packed radio program. Due to the power supply via the USB port of a high frequency stability is achieved. Alternatively, the receiver, a battery 5 to 12 volts to operate.
Mini FM Radio Transmitter circuit with BF199:
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3-9 Volts, 76-110 MHz FM Radio Transmitter TR 9018
Here is a simple 76-110MHz FM transmitter that can transmit your voice or audio over an ordinary FM radio within the FM broadcast band. It can transmit both voice using microphone and music from any music player. Frequency is changed by adjusting 5.5 turn inductor coil. Transmitter is powered by 9V battery or 3V-9V power adapter. Transmission range is 100 meters but can be increased with better antenna or RF amplifier.
Transmission range is 100 meters in open area. The transmission depends upon the length of antenna used. I used just a simple 10cm long wire. The signal can be transmitted even further with longer antenna of length around 50cm and 9V power supply.
Electret microphone (MIC) converts the natural sound signal into the electrical signal. The capacitor C2 is coupled to the base of transistor 2SC9018 and whenever there is signal from the microphone, the junction capacitance of the transistor changes that contributes to change in oscillation frequency.
Resistor R1 2.2K is a microphone MIC bias point resistance, usually value between 2-5.6K is preferred, R2 provides biasing to the base of transistor. C4 and inductor L 5.5 turns coil make up the oscillator tuning circuit, you can change the value of C4 and L to change the transmission frequency.
Use FM radio, turn on the power and volume, adjust the frequency around 88 MHz. Connect transmitter to the power supply, align towards the radio and use a screwdriver to adjust the coil until the radio catches the signal. Then slowly increase the distance between the microphone and radio, while properly adjusting the radio (or handset) volume, tuning knob until the clearest sound is heard.
9 Volts, Sensitive FM Transmitter Bug with 2N2222
This easy to build FM transmitter bug can transmit voice to exceptionally good range. Tune trimmer to hear the signal to your near radio. Transmitter frequency range is 88-108 MHz. Max current consumption is 30mA. You can power the fm transmitter bug with a 9Volt Battery, or you can plug a power supply to feed in 9-12 Volts. That bug will pick even a low whisper or even the sound of a breath well far from the microphone. Great spy transmitter equipment.
9 Volts, Mini FM Transmitter TR BC547
Here’s how to build your own mini FM transmitter. It transmits FM waves so you could easily receive the signals on your mobile phone, radios, etc. As the name and the picture indicates it is very small and is approximately the size of a 9v battery clip. With this FM transmitter you could start your own mini FM station. The circuit uses BC547 transistor to amplify the signal and then frequency modulate it. It uses “frequency modulation” most commonly known as FM, the same principal to transmit audio signals captured by the microphone.
Lets start with getting all the parts.
A variable capacitor 47pf
An Inductor (see steps for description)
1n capacitor (102)
An FM receiver (any mobile phone)
I got almost all of the components from a pile of old PCBs I had in an old forgotten box. All I had to get was the BC547 and the electret microphone. Actually I did find the BC547 in an old PCB but i was not sure if it would work. It looked quite burnt to me. The old PCBs had many components resistors, crystals, diodes, etc. I may use them some day and for know back in the box.
I had to de-solder the parts of the old board, for those who don’t know how to solder and de-solder there is a bunch of instructables that describe how to do this and learning to solder is not a hard task.
First of all lets start with cutting up a PCB to the required size. The size to compare is a 9v battery clip, it might look quite small in the beginning but don’t worry it would hold all the components just fine. Use a sand paper for smoothing the sides of the PCB and to clear out any rough edges.
Make sure to get a PCB with big holes as the variable capacitor pins won’t go in the standard size holes. You can get the microphone at a local hardware store. And be sure to get some male pins to hold the microphone in place refer the picture as to how to solder the microphone in place.
Why not use some wires to hold the microphone?
I would not suggest wires as when you tape the circuit if the last few steps you would not get a clear audio. I tried it and got a lot of noise. I got lesser noise when I used the male pins soldered to the microphone.
Once you’re done with the PCB and know where and how to solder the microphone now it’s time to complete the rest of the circuit. Follow the circuit above and solder all of the components. Make sure not to leave any space between any of the components if you need to get the circuit small. For the inductor use 0.5mm wire and 8 turns, with each turn with a diameter of 6mm.
And for the antenna just use a thin 5cm long wire. For more stability you could center tap the coil and solder the antenna to the center tap. Also If you notice the circuit has a LED in it, it is used to show when the circuit is functional. I did not add the LED in my circuit because it was draining my battery faster.
Once you got the circuit like the one in the above picture, it’s time to cover it with tape. I used wiring tape to cover the whole circuit except the microphone and the variable capacitor. This is an important step as when you proceed to the next step, where you tune to the required bandwidth. Touching the circuit (mainly the coil) with your fingers would lead to severe noise.
You could also use a heat sink instead of tape, I used tape because I wanted to experiment withe circuit so I did not want it to be permanent.
Now it’s time to tune the circuit to a required bandwidth, you could do this in two ways.
Use your mobile phone to find the signal.
Manually tune the variable capacitor to match a frequency
The first step is recommended all you have to do is power the circuit and turn on auto find bands on your mobile. Your mobile would scan for channels and all you have to do is look for your transmitter (play some music in front of the transmitter) on that list.
The second method is time consuming, in this method you have to turn on your radio and the circuit. Keep the radio at a specific channel, and then tune the variable capacitor extremely slowly. When you hear stuff on the radio maybe a song that you are playing stop and the bandwidth on the radio is the required bandwidth.
After using the 9v battery circuit for some time I thought of replacing the battery with rechargeable Li-ion batteries. If you have viewed my previous instructables you would have seen that I use these batteries a lot.
The rechargeable batteries provide longer transmission than the common 9v battery.
9 Volts, Portable FM Transmitter TR BC548
This portable FM Transmitter is easy to build. I have used a pair of BC548 transistors in this circuit. Although not strictly RF transistors, they still give good range. Transmitter is powered by 9V battery. The coil L1 consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter.
Actual range on my prototype tuned from 70MHz to around 120MHz. The aerial is a few inches of wire. Lengths of antenna wire should be 1 – 2 feet. The circuit is basically a radio frequency (RF) oscillator that operates around 70-120 MHz. Audio from audio jack is fed into the audio amplifier stage built around the first transistor.
Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor. The tank circuit is connected in a Colpitts oscillator circuit.
One final point, don’t hold the circuit in your hand and try to speak. Body capacitance is equivalent to a 200pF capacitor shunted to earth, damping all oscillations.
9 Volts, 100-150m, iPod FM Transmitter using TR BC548
Here are instructions for building your own ipod FM radio transmitter. It works quite easy, there is a power switch on the bottom to turn it on and tune your radio and transmitter to the right frequency. For the antenna you can use a copper wire of 70 cm. The range of this FM transmitter is about 100 to 150 meters (500 feet). With R5 you can adjust the input signal and with C6 you can tune your frequency. Transmitter is supplied by 9V battery.
iPod FM Transmitter parts list:
C3 ………………………….1 μF/16 V
C4,C5 …………………….1 nF ker.
C6 trimmer capacitor…4-40 pF
C7 ………………………….10 pF ker.
R5 Trimmer……………..10 kΩ
R6 ………………………… 10 kΩ
R7 ………………………….2,7 kΩ
Use a 10 cm long Silver wire.
The diameter inside the spool should be 3 mm.
7 winds total
Length of the spool should be 15 mm (f-g)
The antenna is connected to the spool at 3mm from f
a: + 9volt
c: audio in
Ground your audio input to b
For the antenna you can use a copper wire (70 cm)
With R5 you can adjust the input signal and with C6 you can tune your frequency.
9 Volts, Low Power FM Transmitter TR BC549
This low power fm transmitter is designed to use an input from another sound source and transmits on the commercial FM band. This low power fm radio transmitter it is actually quite powerful. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise is removed.
When assembling the fm transmitter circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver – this will not alter the frequency. Q1 is a conventional Colpitts oscillator design. The audio signal applied to the base of Q1 causes the frequency to change, as the transistor’s collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver.
Small Power FM Transmitter PCB Layout
The inductors are 9.5 turns of 1mm diameter enamelled copper wire. They are close wound on a 3mm diameter former, which is removed after the coils are wound.
The output is a low power of 100 mW, but for some of you this fm rf transmitter can delivers the desired power for broadcasting on your street or with a proper antenna you can cover a small neighborhood. If you need a power wireless fm transmitter use the above menu, you can find transmitters starting with low fm power up to high power fm transmitters.
9 Volts, Simple FM Transmitter TR BC549
This is simple FM transmitter for FM broadcast band in 88-108 MHz. BC 549 is small signal transistor for wide applications, but usually for AF. You can build simple FM transmitter with one BC549 transistor and several other component parts. Simple FM transmitter with only one transistor is often called “bug”. This project is suitable for beginners in radio amateur, education, or hobbies. As an antenna you can connect 150cm of copper wire.
The input can be replaced with any sound source, like ipod, Mp4 player, laptop, or TV. Transistor circuit outputs 1-5 mW RF signal that can travel around 30 meters, due to limited power supply voltage, limited modulation, very loose coupling with the Antenna. The Antenna has to be connected either directly to the tank circuit or via a small capacitor. The Antenna now forms part of the tuning circuit. If you approach the antenna, the frequency of the oscillator may shift slightly. This effect is called “Frequency Pulling”. The frequency of operation shifts as the battery runs down. This effect is called “Frequency Pushing”. The internal capacitance of the transistor also changes with the temperature of the transistor. The tuning capacitor also changes values slightly with temperature. So one experiences a slow frequency drift till the transmitter reaches thermal equilibrium with it’s surroundings.
9 Volts, Mini FM Broadcast Transmitter TR 2N3904
Build your own simple mini FM transmitter. This fun project will show you how to build a mini broadcasting transmitter that can transmit an audio signal up to a quarter mile to any FM receiver. It’s easy to build and a good learning experience. It serves as a hands-on learning tool for students or anybody interested in electronics. Having a range of up to a quarter mile, it’s great for a house security system, baby monitoring device or simply a listening gadget that you can place anywhere!
C4 is a small, screw-adjustable, trimmer capacitor. Set your FM receiver for a clear, blank space. Then, with a non-conductive tool, adjust this capacitor for the clearest reception. Although this transmitter is designed for the FM broadcast band, it can be tuned to 2 meters, and other VHF bands by changing values of C4 and L1.
L1 is 9 turns of #22 gauge solid wire (air-wound) 1/4 inch diameter coil. Use a 1/4 inch diameter bolt and wrap the wire in the threads. After mounting the coil, back out the bolt.
C1, C2, C3, and C5 are ceramic type capacitors, preferably npo (low noise) or equivalent. However, you can use any type you have around, but do not use electrolytic or tantalum capacitors.
A 2N3904 transistor was used for Q1 and Q2. The 2N3904 is a general purpose silicon NPN bipolar transistor used for switching and amplifier applications. However, you may substitute the 2N3904 with a 2N2222 or a 2N3906, these are also general purpose transistors.
The antenna is 8” to 18” of any type wire.
Try to keep all leads as short as possible to prevent stray capacitance.
R1, R4, R6 10K resistor
R2 1Meg resistor
R3 100K resistor
R5 100 ohm resistor
R7 1K resistor
C1, C2 0.1uf capacitor
C3 0.01uf capacitor
C4 5 – 30pf variable capacitor
C5 4.7pf capacitor
Q1, Q2 2N3904 transistor
L1 9 turns of #22 gauge
Microphone Electret Mic
FM Transmitter Circuit Board
9 Volts, Super Simple FM Transmitter TR3904
FM transmitters can be complicated to build, but not this one. It’s about the easiest you can possibly make. And though the science of radio is well understood, there’s a magical, emotional quality about it that we don’t often stop to appreciate. You will not forget the first time you pick up a transmission broadcast from a device you soldered together, yourself, from a few bits of copper, carbon, plastic, and wire.
The circuit itself is a simplest FM transmitter design, and the method of building it is sometimes referred to as “Manhattan style.” It uses a piece of copper-clad circuit board but, rather than etching the circuit traces through the copper layer, a large piece of continuously-plated board is used to make all the circuit’s ground connections, and small sections of plated board are glued to the surface to form nodes or “pads” that are insulated from ground. Besides being a convenient way to assemble circuits using minimal tools, this building method encourages you to think about circuits in an interesting way as groups of connections that are either grounded or “floating above” ground.
This transmitter uses ten on-board components and will transmit a monaural audio signal about 30 feet. It is possible to extend that range by adding 10″ wire antenna.
Depending on where you live, operating an FM transmitter — even a very short-range one like this — may be illegal without a license. Unless you attach an antenna, it’s very unlikely that anyone will notice or complain about any transmissions you may make with this device. On the other hand, it’s very difficult to predict, before construction is complete, just where on the FM band this transmitter will broadcast. Use due caution during testing, and make sure you understand the law in your area before attaching the battery.
9 Volts, Simple Coilless FM Transmitter using IC CD4069
The RF oscillator using the inverter N2 and 10.7Mhz ceramic filter is driving the parallel combination of N4 to N6 through N3.Since these inverters are in parallel the output impedance will be low so that it can directly drive an aerial of 1/4th wavelength. Since the output of N4-N6 is square wave there will be a lot of harmonics in it. The 9th harmonics of 10.7Mhz (96.3Mhz) will hence be at the center of the FM band. N1 is working as an audio amplifier. The audio signals from the microphone are amplified and fed to the varicap diode. The signal varies the capacitance of the varicap and hence varies the oscillator frequency which produce Frequency Modulation.
9-12 Volts, Easy FM Transmitter TR3904
The figure shows a schematic of an easy to build FM transmitter circuit. Mostly all FM transmitter circuits you will find online or in books require some kind of hand build inductor/coil and after building the transmitter you have to adjust that coil and trimmer capacitor a little to adjust the transmitter to transmit on your desired frequency. If you are looking for an easy or simple FM transmitter circuit in which you don’t have to make a coil with your hand then the circuit given here is ideal for you. The circuit is using a ready made 1uH inductor which can be purchased from an electronic components store.
These inductors are mostly look like resistors. The circuit also does not require a trimmer capacitor, because we have used a fixed value of 39pF capacitor in the place of trimmer capacitor. We have already calculated and used the values of coil and capacitors of oscillator to broadcast on FM band, so you don’t have to do any further adjustments and tuning after building the circuit. The circuit can be operated with 9 to 12 volt DC. For antenna use a 12 inch wire or for maximum range use a 30 inch wire and make it vertical.
Note: If the circuit is not broadcasting on your desired frequency then you can change the value of 39pF capacitor a little bit higher or lower to change the frequency.
9-16 Volts, 1 Watt Veronica FM Transmitter TR 2N4427
Veronica 1W FM transmitter is an easy to build transmitter. Veronica is also known for frequency stability, clean FM signal and uses no integrated circuit. The Veronica oscillator is actually formed from 2 oscillators which operates somewhere around 50 MHz in anti phase and the 2 signals are combined to form 100MHz FM radio signal. This kind of circuit design is stable and is amplified up to 1W by 2n4427 transistor. Veronica transmitter is equipped with a mini-mixer and so you may forget an external mixer. This consist from T1 transistor which amplifies the microphone signal before it is combined with CD player audio or PC signal. R1 and R2 are potentiometers (variable resistors) used to adjust the audio level. The component between R8 and C21 represents the oscillator which generates radio signal. D1 is a varicap diode (like a variable capacitor or trimmer) controlled by audio signal. C12, C13 and L1 determines the frequency.
Veronica components list :
C1, 2, 7, 16, 17, 19,
24, 29 & 31 1n
C3-5 & 8 10u elect.
C6, 18 & 30 220u elect.
C9, 10 & 20 10n
C13 22p trimmer
C14 & 15 15p*
C21, C25 & 26 65p trimmer
C27 & 28 1.8p
L1 – 6 coils / 2 turns, 5mm diam si 5mm length
L2 – 3 turns, 7mm diam si 7mm length
L3 – 4 turns, 5mm diam si 7mm length
L4 – 6 turns, 5mm diam si 10mm length
D4 Standard LED
The transmitter must be mounted in an aluminium carcase connected to ground. The power supply voltage is between 9 and 16V, at 16V the rf output power is 1W and at 12V is 600mW and at 9V is 200mW.
9-12 Volts, 2km Long Range FM Transmitter TR 2N3866
The power output of many transmitter circuits are very low because no power amplifier stages are incorporated. The transmitter circuit described here has an extra RF power amplifier stage using 2N3866 RF power transistor after the oscillator stage to increase output power to 250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element Yagi antenna, this transmitter can provide reasonably good signal strength up to a distance of about 2 kilometers.
Transmitter’s oscillator is built around BF494 transistor T1. It is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to tune the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. 2N3866 transistor T2 forms a VHF-class A power amplifier. It boosts the oscillator signal power four to five times. Thus 250mW of power is generated at the collector of transistor T2.
Transmitter Assembly Notes
For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below:
L1 – 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 turns of 24 SWG wire near top end of L1.
(Note: No core (i.e. air core) is used for the above coils)
L3 – 7 turns of 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 turns of 24 SWG wire-wound on a ferrite bead (as choke)
Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.
This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal.
11-18 Volts, 40mW, Solar Powered Long Range FM Transmitter TR 2N2222
There are many miniature FM transmitter bug circuits online, this one is unique in that it runs completely on solar power. No battery is required. As long as the sun is shining on the PV panel, the transmitter will transmit. The transmitter bug is useful as a “remote ear”, and can be used for anything from listening birds to surveillance work. The mic preamp and oscillator circuits were borrowed from a common circuit found around the Internet, a regulated solar power supply and an RF amp that extends the range of transmitter and improves frequency stability were added.
1X GM 684 60 mA 18V PV panel (available from Electronix Express) or equivalent
1X 78L09 voltage regulator IC
1X 1N4001 diode
1X 2N3904A transistor
2X 2N2222A transistors
1X 1000uF 25V electrolytic capacitor
1X Electret microphone
4X 100nF capacitors
2X 22nF capacitor
1X 1nF capacitor
1X 3pF silver mica capacitor
1X 6pF silver mica capacitor
1X 10pF silver mica capacitor
1X 20pF ceramic disk capacitor
1X 27pF ceramic disk capacitor
2X 5-20pF (or similar) miniature variable capacitor
1X six hole ferrite choke or equivalent
1X 100 ohm 1/4W resistor
1X 470 ohm 1/4W resistor
1X 10K 1/4W resistor
1X 20K 1/4W resistor
1X 33K 1/4W resistor
1X 47K 1/4W resistor
1X 1M 1/4W resistor
1X 1-3/4″x3″ copper plated blank printed circuit board
1′ length of #20 tinned copper hookup wire for making two coils
1X weatherproof plastic box (recommended)
The solar power supply consists of a small 18V PV panel which charges a 1000uF electrolytic capacitor. The capacitor keeps the circuit running during brief interruptions of light, such as a bird flying over the PV panel. The 18V is regulated down to 9V with the 78L09 regulator IC to provide a steady 9V supply for the rest of the circuitry. With the PV panel shown above, the circuit will only work when direct sunlight is shining on the panel. A larger panel that can provide 22mA at 12V during cloudy conditions would extend the circuit’s operating conditions.
The Electret microphone is biased with a 33K resistor, the resistor value can be changed to vary the amount of modulation and optimize the performance of specific microphones. The microphone signal is amplified by a 2N3904 audio amplifier. This signal is sent to the 2N2222A oscillator stage where it changes the oscillator’s frequency (FM). The oscillator’s operating frequency is set by L1, the 6pF capacitor and the 5-20pF variable capacitor. With L1 wound as specified on the schematic, the circuit will operate near the low end (88Mhz) of the FM broadcast band.
The output of the oscillator circuit is taken from a tap on the oscillator coil L1 and fed to the RF amplifier 2N2222A transistor. The output of the RF amp is run through a low pass PI filter to remove unwanted RF harmonics before the signal is sent to the antenna.
Output Frequency: 88Mhz nominal, can cover 88-108Mhz with coil adjustments
Input voltage: 11-18VDC
Operating current: 22mA @18VDC
DC input to RF amp: 81mW
RF output power: 40mW (approx.)
The prototype circuit shown was built using the “dead bug” construction method, it was laid out as the circuit was designed. A second-generation version of the circuit was built using a home-made printed circuit board, this is shown in the second photo. The frequency stability of the transmitter was greatly improved when it was built with the circuit board. Artwork for the PCB is available at the end of this page.
It important to mount the oscillator components solidly so that they don’t move around and cause unwanted frequency shift. The component leads for all of the RF wiring should be kept short. The coils were wound on a #2 Philips screwdriver shaft and stretched out a bit. To improve the circuit’s frequency stability, wind the oscillator coil on a 1/4″ form, then heat the coil in an oven at to anneal the metal. A layer of polystyrene “Q dope” can be painted onto the coil to further improve the stability.
Another trick that will improve the transmitter’s frequency stability is to build it into a metal box that is surrounded by an insulating material such as styrofoam or bubble-wrap. If the transmitter box is mounted in the shade, it will be less likely to change frequency due to solar heating and cloud shading.
This circuit will work with a variety of antennas. An adequate short-range antenna can be as simple as a 1′ to 2′ wire connected directly to the circuit. A resonant antenna such as a tuned dipole or a vertical antenna will greatly extend the range of the transmitter.
A resonant half-wave diple antenna for 90Mhz can be made with two 2.6 foot pieces of wire fed in the middle, using the classic dipole formula: quarter wave length (feet) = 234 / frequency (Mhz). the PV panel and wiring should be kept away from the antenna, or in the case of a short whip antenna, the PV wiring can be run in the opposite direction as the antenna to act as the other half (counterpoise) of a dipole.
The circuit can be aligned in the laboratory by putting 12V to 18V DC across the PV panel to power the regulator. Tune your receiver to a blank spot on the lower end of the FM band and adjust the frequency calibration trimmer until you hear the microphone signal. Turn the trimmer very slowly, alignment takes a light touch. Don’t turn the receiver volume up too much or you will get audio feedback. A frequency counter may be useful for setting the output frequency. It may be necessary to retune the frequency a bit after the circuit has warmed up in the sun.
The output capacitor should be tuned for the maximum transmitted signal, this setting varies with different antennas. The best way to do this is to connect the antenna to the transmitter and monitor the signal with an oscilloscope (100 Mhz bandwidth) connected to a nearby antenna. Adjust the control for the highest signal. If you have a receiver with a signal strength indicator, that can also be used for monitoring the transmitter’s output level. Adjustment of the output capacitor will pull the oscillator frequency a bit, it will be necessary to alternate between oscillator and output adjustments to fully align the circuit.
Place the PV panel in the sun and tune your receiver to the bug’s signal, listen to the world outdoors. An analog receiver is best for picking up the signal since, unlike a digital receiver, it can be fine tuned to track the signal. I use a 1970s vintage Pioneer receiver to good effect. Once the bug’s temperature has stabilized, its frequency should not drift very much.
The microphone enclosure and placement can be tuned to optimize sound reception in a particular direction. A good directional microphone can be made by putting the mic element into one end of a short piece of PVC pipe. Inserting a thin tube of porous foam into the pipe can lower the resonant nature of the cylinder.
9 Volts, 200mW, FM Broadcast Transmitter using TR BFR96
A simple 200mW FM Transmitter circuit which covers frequencies from 88 to 108 MHz. It is built with 3 transistors: BC109, BFR91A and BFR96S. It is quite stable and the output power is around 200mW. The first stage of transmitter is a mic amplifier but if you connect this radio transmitter directly to an audio source you can remove this stage and connect the audio signal to R5. U1, 1PH51C can be replaced with LM7805. You must use a stabilized power source for oscillator stage to prevent frequency variation. You can remove C7 and use a linear potentiometer instead of R6 with the median connector to C4, one pin to ground and the other one to +. FM Transmitter uses MV2109 varicap diode and C7 for frequency tuning.
All coils must be perpendicular one to the other, especially L2 and L3. The oscillator stage must be encased with a copper 1mm foil. If you use an external antenna, like fm dipole antenna connect it with a good coaxial cable because the power is low and you want to use most of it. If you don’t want to loose the RF power connect the radio transmitter close to the antenna (2 meter coaxial cable) and use a longer cable for the audio signal (coaxial). If you connect the antenna as I told you before you can cover larger diameter area.
12,5 Volts, 300 mW, FM Transmitter using TR 2N3866 or 2N4427
Here’s a long range 300mW FM Transmitter for the 88MHz to 108MHz band. This particular TX is of special interest to those wishing to build low power Power Amplifiers for the VHF bands since it used impedance matching, power amplifier and antenna filtering, all of which should be used by radio constructors, whether it be for amateur radio or any other form of radio.
The features of this project are:
Higher output power – 150mW min (at 9v) and 300mW+ (at 12.5v). Very pure output signal due to careful design and filtering. VARICAP modulation – possibility to add a synthesizer. Single sided Printed Circuit Board, only 40mm x 72mm. Covers the domestic FM band – 88MHz to 108MHz. Easy to build, but coil winding experience is required.
Ok, so I have done a lot of playing around with the domestic FM band. This will probably be one of the last transmitters for the 88MHz to 108MHz band. This particular TX is of special interest to those wishing to build low power Power Amplifiers for the VHF bands since it used impedance matching, power amplifier and antenna filtering, all of which should be used by radio constructors, whether it be for amateur radio or any other form of radio.
The features of this project are:
Higher output power – 150mW min (at 9v) and 300mW+ (at 12.5v)
Very pure output signal due to careful design and filtering
VARICAP modulation – possibility to add a synthesizer
Single sided Printed Circuit Board, only 40mm x 72mm
Covers the domestic FM band – 88MHz to 108MHz
Easy to build, but coil winding experience IS required
NOTE – This project is illegal to use or build in many countries. I accept no responsibility what-so-ever for any illegal use. This circuit is provided solely as an educational project. Now I have got that off my chest, let me get on with the project.
The circuit itself is fairly conventional, with a couple of small refinements. It all begins with TR1 (BC547) in an inverted Hartley oscillator configuration. The feedback to the Base of TR1 is via a small 4.7pf capacitor to help keep the oscillations as weak as possible whilst allowing the oscillator to be a reliable starter. The frequency of the oscillator is determined by L1 and the 22pf trimmer capacitor and functions in the range of about 76MHz to 119MHz using the PCB I have made.
TR1 = BC547
TR2 = BC547
TR3 = 2N3866 or 2N4427
The 15pf capacitor couples the top of L1 to the varicap diode which serves to add more capacitance to the tuned circuit to alter the frequency. R1 adds the supply voltage to the varicap, with a little noise decoupling (the 0.1uf capacitor). If you are to use synthesizer control then it is important to remove R1 from the circuit, then connect the synthesizer loop filter output to the terminal marked “Ctrl”. Audio is coupled to the BB105 via a 47K resistor. There is only 47pf of decoupling in order not to restrict the AF bandwidth of the complete transmitter. The AF bandwidth is flat from 3Hz to about 72KHz, but if we look beyond these limits, there is an increase of +6dB at DC. This is because the two 47K resistors divide the AF input voltage by 2, but at DC the 0.1uf capacitor has time to charge, the two 47K resistors do not therefore divide.
TR2 (BC547) is both biased and directly connected to the Emitter of TR1, which is a little unconventional in a VHF circuit. I needed to get a good input to TR2 and cut down on components. There are already far too many coils as it is in this circuit. Remember that the BC547 is an audio transistor but works well at VHF. The inductor in the Emitter of TR2 helps to extend the response a little to give a bit more signal to drive the final power amplifier transistor (TR3). TR2 gives no voltage gain; it is current we need to drive TR3. We already have enough volts from the oscillator.
22pf and L3 couple TR2 output into the Base of TR3. These components match the impedance so we get the maximum power possible into TR3 Base. The signal level, however is still quite low, so some DC biasing has been added to turn TR3 ON a bit. The transistor should draw about 5mA with no signal. This is not enough to make it become linear, but it is operating around class “B”. This would make a very poor frequency multiplier, so harmonics are also reduced a little by the DC bias. Note that NO emitter resistor has been used. The prototype units all worked well without one and the drive level is not enough to cause the transistor to conduct very much. The small standing DC bias of 5mA doesn’t even “tickle” TR3. In operation the DC voltage on the Base of TR1 will be negative due to the drive level, conduction of TR3 Base/Emitter junction and the 22pf capacitor. TR3 does NOT need a heat-sink.
If you want to use a different transistor in place of TR3 then I suggest you remove L3 and substitute a current meter in place of L4. Apply volts to the transmitter. The current should be about 5mA. Select the value of the 47K resistor if required. Any current reading between about 2mA to 8mA “will do nicely sir” (even without your American Express card!)
The collector of TR3 (2N4427) has a big (by QRP standards) choke to pass the supply DC, but presents a high impedance to RF. The RF signal is then matched to 50-Ohms with the 15p, L5 and 56p. The 1nf cap simply blocks the supply voltage that would otherwise pass to the antenna. L5 and 56pf form a low-pass filter that helps to block harmonics present in the output signal. L6 and 47pf are added to further reduce the harmonic levels. This filter is an absolute MUST for all transmitters if one does not wish to offend every other user of the radio spectrum. L5 and L6 have also been positioned on the PCB so that there is a little coupling between them. This coupling serves to cancel out any residual signals, not within the pass band of the filter, that may be present at the input to L5. It is this effect that was responsible for the unexpected cleanliness of the first prototype, and a little layout experimenting has now reduced the 2nd and 3rd harmonics to -60dBc at all supply voltages. With 150mW output, this corresponds to 3rd harmonic of 150 nano-watts and a 2nd harmonic level of just 50 nano-watts.
I have “played around” with the values and taken a few liberties. If you want to try adjusting the coils then then will be able to get another 2 to 3dB out of the TX. I have deliberately mis-tuned a couple of times in order that impedance and resonances will improve at the edges of the band. The result is that the performance of the transmitter does NOT vary (much) no-matter which end of the band you are operating at.
As you will appreciate, L1 and the tuning capacitors all affect the frequency of the complete transmitter. Winding L1 has therefore NOT been considered. This would result in a spring-like affair that would cause instability, or more precisely, “microphony”. This is an effect where the coil wobbles about with very small mechanical movements. In severe cases you can even talk to the the circuit, as any owner of a Marconi TF995 signal generator will testify. By using a coil etched on the PCB you will find that microphony has been eliminated. So has coil expansion with temperature. It must be remembered, however, that this circuit is STILL based upon an LC circuit and therefore subject to changes of frequency with changes of supply voltage and “hand capacitance”, etc. I will cover the supply voltage changes shortly.
L2 is wound on small ferrite beads. L2 is placed in series with the emitter of the buffer transistor, TR2. In the interests of stability it is very important that this coil does NOT radiate like a loop antenna. It is composed of 4 turns of 0.15mm Dia. enameled wire (magnet wire in the US). The grade of ferrite is unimportant, as long as it is a grey one. One complete turn is formed when the wire passes through the hole in the middle once. The ferrite is mounted vertically in the same manner as a resistor.
The ferrite is 3mm outside diameter and the assembled coil looks like this:
L3 is wound using 3 turns of 0.8mm Dia enameled copper wire with a 6mm inside diameter. Wind it on a drill bit to get the inside diameter correct. The coil is close-wound, that is to say that the turns are just about touching and shall not be spaced. Form the ends of the coil, bending them out then down so that the leads are 5mm apart. The coil should look like above.
L4 comprises 6 turns of 0.15mm Dia. enameled wire (magnet wire in the US) wound on TWO ferrite beads, the same beads that were used for L2. The beads are placed side-by-side as a pair of biniculars. One complete turn is when the wire is threaded once though both beads.
You can get a closer view of L4 in this picture:
L5 is 5 turns of 0.8mm Dia. enameled copper wire with a 6mm inside diameter. Just as L3, wind it on a drill bit to get the inside diameter correct. The coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart.
L6 is 3 turns of 0.8mm Dia. enameled copper wire with a 6mm inside diameter. Just as L3 and L5 the coil is also close-wound. Form the ends of the coil, bending them out then down so that the leads are 5mm apart.
You can get a closer view of L5 and L6 in this picture:
The position of the capacitor between them is very important, equal distance from both coils keeps the 2nd harmonic at the lowest level.
NOTE – L5 and L6 MUST be wound in the same direction. If you try to wind one of them backwards or in the other direction then the spurious outputs will increase.
The transmitter is constructed on a single-sided printed circuit board. I will place the PCB foil pattern on my DOWNLOAD section of the homepages. The board is only 40mm x 72mm. If you use any other construction method then you will need to change a few component values. If you use veroboard, prototype board then the project will probably not work. The board has been designed to compensate for the lack of a second copper ground land and has also “thermal breaks” around some of the component connections. This makes it easier to solder for new beginners (for me too, but I should not admit that!). I will shortly add a “broadcast” version that does not need any PCB additions for use as a broadcast transmitter. 2x AF inputs, even a prescaler chip on the board!
There are two wire links on the board, fit these first. I try to make my links and components as neat as possible with as short leads as possible. Look at the photograph of the finished transmitter to see how they lay. Form the leads and use a bit of masking tape to hold them in position when soldering. The link wires are made using off-cuts from the resistors.
You will note that the 15pf capacitor coupling L1 to the BB105 varicap diode is laying on the board. It´s legs are so formed that it acts as a link. Assembly order is not particularly important, but it is easier if all horizontal components are mounted first, then the passive components (resistors/caps), transistors and the coils last. Neatness and attention to detail is particularly important. The vertically mounted resistors should all be mounted as shown on the component overlay. It DOES matter which way round they are. This is one of the prices for using a cheap single-sided board.
When all the components have been fitted, check your work thoroughly. I recommend you shine a strong lamp behind the board component side and compare the tracks with the PCB foil pattern. This will allow you to check for solder bridges between tracks. Assuming all is well, connect a 50-Ohm resistor to the antenna (ANT) terminals. Two 100-Ohms in parallel will be fine. Now connect the board to a 9v supply in series with a 12v 3W torch lamp. If the lamp glows brightly then switch OFF and check your wiring because you have a fault. If there is no fault then the lamp should only glow dimly, if it glows at all. The complete transmitter should draw less than 100mA.
If all is well, switch ON an FM radio set tuned to somewhere around 108MHz. Adjust the tuning capacitor on the board so the plates are at around minimum capacitance and you will hear the transmitter on the radio. With the capacitor plates near maximum capacitance you should be able to tune the transmitter to 88MHz.
Now couple the AF IN terminals of the transmitter to the LINE OUT of a stereo system, your computer, or even the headphone terminals of your Sony Walkman. I prefer to use headphone terminals since the volume control will give you some control over the modulation depth. You can set the modulation depth by comparing it with another radio channel. Set your transmitter A LITTLE LOWER IN VOLUME than other channels, unless you have access to a modulation meter. Note that you may have to use a capacitor in series with the AF input wire. See the application data further down.
If your transmitter is working then you can remove the test lamp and connect the battery supply directly to the transmitter. Check that nothing is burning. TR3 should get a little warm, but comfortable to the touch. All other components should remain stone cold. TR3 may get a little warmer if you increase the supply voltage to 13.8v but in this case the transmitter will be delivering almost half a watt of output power.
I think that here I should give a little information about the actual measured values of the prototype. The target was to achieve a clean 100mW of output power at 9v. I also indented the transmitter to be equally stable at 13.8v DC since this is what most constructors seem to want. The target was exceeded on all counts. There are no spurious outputs visible from 500MHz upwards, so this spectrum analyzer view is only from DC to 500MHz. It shows that there is a little 2nd and 3rd harmonic outputs, but the levels are so low that they are quite negligible. I could hardly believe my eyes when I built the first prototype, but after cleaning up the PCB the output was even better!
The vertical scale is 10dB per division and the horizontal scale is 50MHz per division:
As you can see, the worst case is the 3rd harmonic at -60dBc. The carrier level was +23dBm (200mW) at 10v supply. This falls off a little to about 160mW at the ends of the bands. Brief specifications are given below. I have not been all that meticulous with the figures. When I got a reading of 73mA I rounded it up to 75mA to keep the figures simple. The figures are only a guide anyway.
Parameter Supply=9v Supply=12.5v Supply=13.8v
Freq range 76 – 116MHz 77 – 119MHz 78 – 121MHz
Supply Current (98MHz) 75mA 85mA 95mA
Output power (88MHz) 160mW 310mW 370mW
Output power (98MHz) 180mW 360mW 420mW
Output power (108MHz) 165mW 320mW 380mW
Spurious Outputs (DC – 1GHz) -60dBc -60dBc -60dBc
RMS AF for +/-75KHz deviation 210mV 200mV 195mV
AF response +0/-3dB 3Hz – 70KHz 3Hz – 70KHz 3Hz – 70KHz
Adjust the variable capacitor to get the transmitter on the frequency you want. Is that simple enough?
There are several different applications since the unit will modulate from DC to several thousand kilohertz, the most obvious being music. It may be, however, that you wish to change the frequency (to somewhere legal?) and use FSK data for moving information between computers. Perhaps you even want to convey DC changes or just stabilize the TX frequency.
Let us now cover these items, beginning with a recap of the circuit diagram:
(Not to be used with synthesizer)
As you can see from the original circuit, the varicap voltage is kept high by R1. Without this resistor the DC voltage on the diode will be zero, causing the oscillator to stop. R1 shall be removed if using external synthesizer control. We can, however, use the CTRL terminal to have a preset “frequency” potentiometer on the outside of the box. All we need is a 500K Linear potentiometer. Nothing else! no capacitors, nothing.
This will give typically 10MHz tuning range
(Not to be used with synthesizer)
Given that R1 is connected directly to the battery supply voltage, if the battery voltage were to vary then so would the TX frequency. You should really be using a stabilized power supply, or a high-current battery that has a fairly constant supply voltage. If this is NOT the case then you can use the CTRL terminal to bypass R1, without making any modifications to the TX. All we need is an external zener diode and a 6K8 resistor. The Zener diode should be as high as possible. If you have a 12.5v supply, for example, then a 10v diode would be great. With a 9v battery then a 6v8 diode is about the maximum practical. 8v2 would be Ok until the battery voltage went down a bit. We will assume that +VE is a 9v battery.
If you are using DC modulation then the AF input will allow this. This can be used to give low frequency Frequency Shift Keying. This should only be done in conjunction with the voltage regulator above. If you are NOT wanting to have a DC shift, and your input source has a “DC Continuity” (resistance) then there is problem. If you were to connect a magnetic microphone or CD player, for example, to the AF input then the TX frequency would jump. You therefore need to add a capacitor to block the DC shift at the input. 10uF will do nicely. A 4K7 resistor should also be added to give a load to the audio source and to help prevent “hum” or “pickup” from 50Hz (60Hz) wiring. The value of the resistor should be selected to match the audio source impedance of the device. 4K7 is normal for computer and CD LINE-OUT signals.
Frequency Modulation Pre-emphasis
If you wished to use audio directly from a PC or CD-Player, then you should add some form of pre-emphasis. If you are using a stereo encoder or FSK applications then you should NOT use pre-emphasis. Pre-emphasis increases treble a little so that when it is received and turned down again, added noise is also turned down. Select Cx(nf) to be 10x the number of Microseconds of pre-emphasis you want. If you wanted 50uS pre-emphasis then Cx = 50 x 10 nf = 500nf.
The resistor R1 may be removed from the circuit and the loop voltage from a synthesizer added to “Ctrl” on the circuit board. This will give the synthesizer a 10MHz tuning range, the preset capacitor determining the center frequency. The loop frequency control voltage can be increased to about 20v but it should be maintained somewhere around 9v to get repeatable modulation characteristics.
The coil of the tuned circuit can be reduced and TR1/TR2 replaced for more suitable RF devices, then the TX can be increased to the 144MHz band and modulated with NBFM. In this situation one MUST use synthesizer control to stabilize the frequency.
The transmitter is clean enough to drive larger power amplifiers to get much higher powers. I will be working on both 5-Watts and 25-Watts amplifiers at some time in the future, but exactly when is another question. The more e-mail I get then the longer it will take. My work will be primarily for the 144-146MHz amateur radio band.
I hope that you learn a lot from this project. It demonstrates Varicap diode tuning, decoupling and control. It also shows how to amplify low level signals from the 10mW level to the 150mW level and how DC bias can be applied to make compensations for low signal levels. Have fun with the project. If you have any questions then please do NOT e-mail me. I have a message board where you may post questions to many, some of whom have far more experience than I.
9-12 Volts, 500mW, PLL FM Transmitter 88-108MHz using TR BRF 95A
This PLL transmitter is controlled and the frequency is very stable and can be programmed digitally. Transmitter will work 88-108 MHz and output power up to 500mW. With a small change can set the frequency of 50-150 MHz. The output power is often set to several watts with transistors. So therefore I decided to build a simple transmitter with great performances. The frequency of this transmitter can easily be changed by software and space / compress air coil. This transmitter is the oscillator colpitts. Oscillator is a VCO (voltage controlled oscillator) which is set by the PLL circuit and PIC micro controller. This oscillator is called the Colpitts oscillator and voltage controlled to achieve the FM (frequency modulation) and PLL control.
T1 must be HF transistors to work well, but in this case I use a cheap and common BC817 transistor. LC tank oscillator needs to oscillate properly. In this case the LC tank consists of L1 with the C1, C2, C3, and varicap BB139. Coil parallel to the C1 and C2 in series. The same with the varicap and C3. You may think that L is parallel to the [(C1 / / C2) + (Varicap / / C3)]. C3 will determine the value range VCO. Large value of C3 will be broader in the range VCO can be.
PLL and Microcontroller
Oscillator is made to work as a “Voltage Controlled Oscillator” VCO. To control the frequency synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil (L2) is connected to pin 6. This coil should be placed close to the coil L1 to take some of the energy oscillates. The LMX2306 PLL in to use this frequency to adjust and lock the VCO to the desired frequency. Systems also need to set the external reference crystal. In this case I use 12.8 MHz. Pin 2 of MX2306 you will find the PLL filter to form a VM that is set voltage of the VCO. The PLL tries to arrange so that the oscillator frequency Fout kept locked to the desired frequency. The desired frequency programmed into the PIC EEPROM and clocked into the synthesizer (LMX2306) at power up.
I will below explain how to program the EEPROM to different frequencies.
In the pin14 of your synthesizer control output. In this output you will find a reference frequency for testing. (I must warn you that the signal is not symmetrical in form. Pulsa positive only a few microseconds, so you will be hard to see on the oscilloscope.) I solved by connecting it to 74HC4020 (14-stage Binary Counter) to input pin 10 Hours. In Q0 (pin 9) you will have a symmetrical square wave with a frequency half since the circuit is a table. In Q1 pin 7 will be divided by 4, see data sheet for more information.
You want to send audio must be connected to the audio input (left schematic). Will affect the signal and thus modulate the FM varicap RF carrier frequency. A potentiometer P1 was added to adjust the depth of modulation (FM Wide or Narrow FM). You may have to play a bit with a value of P1 because it tends to modulate the lot. You may need to add the 500k – 1M potentiometer only. You test and find out for himself.
Here you find other HF transistors and work in the class C. Resistor R1 and resistor Re2 regulate the flow of DC. In this case I find that 9.1k will give a good output power and thus equal to 150. If you want to increase the power should be lower Re2. You can add another 150 ohm resistor in parallel. In the table below I’ll show the output power with different voltages and resistor values of Re2. I advice you to not run the transmitter with a high output power. Transistor I use is small and tends to be hot. I advice you to run the unit from the 0 – to 200mW. At the transistor will 500mW pain …* smiles *
At the output you will find a network T. This “filter” will match the transmitter to the antenna impedance output stage. You have two variables 60pF capacitors to tune the transmitter for best performance. The antenna I use I a 1 / 4 wave whip antenna (wire) about 75cm long. Smaller antenna types, but not so good performance as a dipole. With a dipole you will be more long distance transmitter.
How long can I pass?
It is a very difficult question because the environment affects the transmission distance is very much. In a city environment with concrete buildings transmitter will send maybe 200m. I will send a proposed open 2000m. I did the test and filed with 70mW output power into a “bad” whip antenna is placed in the room I can send 200-300m to a park without a problem.
12-15 Volts, 500mW Broadcast FM Transmitter, Class-D Amplifier with 4 Transistors
This little broadcast FM transmitter has 500mW of RF output power and runs of 12-15V battery or power supply. DC whose signal modulated by FM using four transistors. Transmitter includes four transmitter stages and draws around 100-150mA of current. Using the values of the circuit components, the frequency will be around 100 MHz but can be changed via coil. Through the 5 pF capacitor and 10K ohm resistor, the modulation of audio signal is supplied to the tank circuit. The amount of modulation is being managed by the 1N4002, a general purpose rectifier diode. FM Transmitter’s output stage is functioning as a class D amplifier where the output transistors act as a switch.
The circuit was designed to produce a 500mW output power transmitter whose signal modulated by FM using four transistors.
All resistors are 1/4 watt 5%
R1,R2,R8 = 1K C1 = 1uF/63V, electrolytic
R3 = 100K C2,C3 = 10nF, ceramic
R4 = 150K C4,C5,C9 = 4.7uF/63V, electrolytic
R5,R7 = 10K C6,C12,C13,C14 = 1nF, ceramic
R6 = 220 ohm C7,C8,C11 = 5pF, ceramic
R9 = 10 ohm C10 = 220uF/63V, electrolytic
P1 = 5K trimpot
Q1,Q2 = 2N3904 L1 = 3.9uH
Q3,Q4 = 7001, NTE123AP L2 = 1uH
D1 = 1N4002 L3 = aircoil, 8.5 turns air space
1/4 inch diameter
Frequency is around 100Mhz with values shown.
Transmitter – an electronic device that can produce or amplify a carrier wave signal, modulates it with a significant signal, and radiates the resulting signal from an antenna which are being utilized in television, telecommunications, and radio.
Frequency Modulation (FM) – transmits its signal or information over a carrier wave by changing its frequency but it can also be taken into account as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal.
2N3904 – NPN device designed for general purpose low power amplifier and switch applications; extends to 100 mA as a switch and to 100 MHz as an amplifier.
7001 (NTE123AP) – a small signal bipolar transistor having an N channel polarity; collector emitter voltage of 40 V; collector base voltage of 60 V; DC current gain minimum of 20; collector emitter breakdown voltage of 40 V, and continuous collector current.
Class D Amplifier – suitable for high power and portable applications because of its high efficiency.
Attached to the circuit is a preamplifier input microphone that is developed within the area of 2N3904 transistors having an audio gain that was preset to the threshold of the 5K ohm trim potentiometer. This potentiometer will supply a small percentage of modification which is often used with an uneven control. The oscillation of the circuit is being defined the parallel tuned tank circuit known as the colpitts oscillator which comprises of two voltage dividing ceramic capacitors (5 pF) in series and an inductor.
The capacitors have a common connection to the emitter circuit since it is a transistor version but for electron tube version, common connection to the cathode. This tank circuit controls the frequency of the oscillation. The output of the oscillator is supplied to the 3.9 uH inductor which will obtain high impedance when the circuit is tuned to RF frequencies.
Using the values of the circuit components, the frequency will be around 100 MHz. Through the 5 pF capacitor and 10K ohm resistor, the modulation of audio signal is supplied to the tank circuit. The amount of modulation is being managed by the 1N4002, a general purpose rectifier diode. The output stage is functioning as a class D amplifier where the output transistors act as a switch. There is no direct bias or DC voltage applied to the transistor to determine the desired operating point. On the contrary, the RF frequency acquired by the 3.9 uH inductor is just enough to energize this stage. To prevent instability and drastic thermal changes, the 7001 transistor utilizes its emitter resistance and the 1K ohm resistor.
The 2N3904 FET transistors were mainly used as switching transistors for use in pulse and square wave applications, as general purpose amplifier, in flyback converter for auxiliary power and charging applications, simple current limiting power supply, current sensing low side MOSFET driver, in high precision comparator using op-amp, protected high or low side MOSFET driver, and in small signal transistors. The 7001 or NTE123AP Silicon NPN transistor is used in audio amplifier and switch.
FM is commonly used at VHF radio frequencies for high fidelity broadcasts of music and speech. It is also used at intermediate frequencies by all analog VCR systems, including VHS, to record both the luminance and the chrominance portions of the video signal. It can also be used at audio frequencies to synthesize sound.
Class D amplifiers are widely used in audio amplifiers where a much higher frequency pulse modulated signal is achieved by converting analogue signals. Because of this, they are now being used in several audio appliances where quality is not a factor.
This circuit provides an FM modulated signal with an output power of around 500mW. The input microphone pre-amp is built around a couple of 2N3904 transistors (Q1/Q2), and audio gain is limited by the 5k preset trim potentiometer. The oscillator is a colpitt stage, frequency of oscillation governed by the tank circuit made from two 5pF ceramic capacitors and the L2 inductor.
The output stage operates as a ‘Class D’ amplifier, no direct bias is applied but the RF signal developed across the 3.9uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor prevent instability and thermal runaway in this stage.
Audio modulation is fed into the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the amount of modulation. The oscillator output is fed into the 3.9uH inductor (L1) which will have a high impedance at RF frequencies.
12 Volts, 500mW-1.2 Watt, PLL FM Broadcast Transmitter TR 2N4427
This is a 1 Watt PLL FM broadcast transmitter. The RF output varies from 500mW to about 1.2W depending on the frequency selected and RF output transistor used. Motorola 2N4427 always seems to work well. Transmitter uses CMOS PLL VCO that prevents the frequency drifts. The frequency is selected via DIP switches. The transmitter is supplied by 12V DC and can also be powered from the battery.
This is a rather crude PCB layout i did using MS Paint. Not the best but it should work . This shows the test point for the PLL lock voltage. Ideally, it should be set for 3Volts (in the middle) using the oscillator coil. If the voltage doesn’t change, oscillator may be at the end of its range in which case then c3 should be either increased slightly for a lower frequency and decreased for higher frequency. Try 22pf plus/minus 10pf
R1 – 10K
R2 – 10K
R3 – 10K
R4 – 10K
R5 – 10K
R6 – 100R
R7 – 1K
R8 – 10K
R9 – 470R
R10 – 10K
R11 – 470R
R12 – 100R
R13 – 470R
R14 – 10K
R15 – 10R
R16 – 47R
R17 – 10R
R18 – 100K
R19 – 1K
R20 – 10K
R21 – 10K
R22 – 1M
R23 – 100K
R24 – 100K
R25 – 100R
RP1 – 9 Pin 100K Commoned Resistor array
RP2 – 9 Pin 100K Commoned Resistor array
C1 – 1uF electrolytic
C2 – 27pF ceramic
C3 – 22pF ceramic (tuning cap- change to extend tuning range)
C4 – 27pF ceramic
C5 – 100pF ceramic (101)
C6 – 100pF ceramic (101)
C7 – 100pF ceramic (101)
C8 – 1000uF electrolytic
C9 – 100nF ceramic (104)
C10 – 100nF ceramic (104)
C11 – 100pF ceramic (101)
C12 – 4.7pF ceramic
C13 – 100nF ceramic (104)
C14 – 100nF ceramic (104)
C15 – 100nF ceramic (104)
C16 – 100nF ceramic (104)
C17 – 100nF ceramic (104)
C18 – 39pF ceramic
C19 – 39pF ceramic
C20 – 39pF ceramic
C21 – 39pF ceramic
C22 – 1nF ceramic (102)
C23 – 10nF ceramic (103)
C24 – 47uF electrolytic
C25 – 1uF electrolytic
C26 – 10uF electrolytic
C27 – 100nF ceramic (104)
C28 – 100nF ceramic (104)
C29 – 100nF ceramic (104)
C30 – 100nF ceramic (104)
C31 – 100pF ceramic (101)
C32 – 100nF ceramic (104)
C33 – 47uF electrolytic
C34 – 100nF ceramic (104)
VC1 – 2..22pF green trimmer
VC2 – 5..60pF yellow trimmer
VC3 – 2..22pF green trimmer
VC4 – 2..22pF green trimmer
L1 – Toko tuning coil with screening can 5.5 turns 100-075
L2 – Toko S18 Yellow
L3 – Toko S18 Yellow
L4 – Toko S18 Green
L5 – Toko S18 Green
L6 – Toko S18 Orange
L7 – Toko S18 Orange
L8 – RF choke 10uH-100uH
VD1 – Variable capacitance diode
VD2 – Variable capacitance diode
VD3 – Variable capacitance diode
D1 – 1N4841
ZD1 -BZX 6.2V Zener Diode
Q1 – BSX20
Q2 – BSX20
Q3 – BSX20 (or ztx313 on older boards)
Q4 – BSX20 (or ztx313 on older boards)
Q5 – 2N4427 with clip on heatsink (Motorola will give more power)
74HC4024 – Binary Counter
74HC4060 – Binary counter with oscillator
74HC4046 – Phase locked loop IC
74HC4059 – Divide by N counter
6.4 Mhz Quartz Crystal
DIP1 – 8 Way DIL DIP Switch
DIP2 – 8 Way DIL DIP Switch
Update: I finally got round to redrawing this in EAGLE. You can download the EAGLE PCB files below. Here are some 3D renders of the Gerber files.
1 Watt FM Transmitter Amplifier
This is a 1 Watt FM Transmitter amplifier with a good design that can be used to amplify a RF signal in the 88 – 108 MHz band. It is very sensitive if you use good RF power amplifier transistors, trimmers and coils. It has a power amplification factor of 9 to 12 dB (9 to 15 times).
At an input power of 0.1W the output will be 1W. You must choose T1 transistor depending on applied voltage. If you have a 12V power supply then use transistors like: 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87. At 18 to 24V power supply you must use transistors like: 2N3866, 2N3553, KT922A, BLY91, BLX92A. You may use 2N2219 at 12V but you will get an output power of 0.4W maximum.
Calibration of the 1 watt fm power amp:
Do not connect any RF source, just apply the power supply and measure the voltage at point 1. Adjust R3 until you measure 0.7V. Replace the antenna with 2 x 100 Ω 0.5W resistors in parallel at the RF output. Now connect the rf signal that you need to amplify and connect this RF Probe to the output.
Slowly adjust C1 in order to get the highest voltage value on the RF probe. Now adjust R3 again to get 0.7 V at point 1. Now adjust C5 and C6 for maximum output voltage (must be between 12V to 18V).
Check the temperature of T1′s heatsink, if it is ok turn off the power supply, disconnect the 2 resistors of 100 Ω and connect the antenna (keep the probe connected). Apply the power and now adjust again C1, C5 and C6 for maximum voltage indication on the probe.
You may use an ampermeter in order to check the current flow through T1. This must not exceed 150mA at 12V and 100mA at 24V or the transistor will burn. L2 and L3 coils must have an angle of 90 degrees between them. Don’t use the 1W rf fm amplifier if you find that you tv set is jammed and the laws of your country does not allow the use of FM transmitters.
R1 = 100Ω
R2 = 2.2KΩ at 12V and 4.7kΩ at 24V
R3 = 10KΩ
R4 = 100Ω
C1 = C5 = C6 = 10 – 60pF
C2 = C4 = 1nF
C3 = 10uF
D1 = 1N4148
L1 = 20 turns of 0.2mm EnCo* wire over R4
L2 = 7 turns of 0.8mm EnCo* wire with 6mm diameter on air
L3 = 4 turns of 0.8mm EnCo* wire with 7mm diameter on air
T1 = 2N4427, KT920A, KT934A, KT904, BLX65, 2SC1970, BLY87 (2N2219, output of 0.4W) at 12V
T1 = 2N3866, 2N3553, KT922A, BLY91, BLX92A at 24V
* EnCo = enamelled copper
12-16 Volts, 1 Watt 5km, Long Range FM Transmitter
Long range, very stable, harmonic free, FM transmitter circuit which can be used for FM frequencies between 88 and 108 MHz. With good antenna transmitter can cover 5km range. It has a very stable oscillator because it uses LM7809 voltage regulator which is a 9V stabilized power supply for T1 transistor. Frequency adjustment is achieved by using the 10K linear potentiometer. The output power of this long range RF transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970 or 2SC1971.
Long range FM transmitter circuit diagram:
T1 is used as an oscillator stage to deliver a low power stable frequency. To adjust the freq. use the 10k linear potentiometer like this: if you trim down, towards ground, the freq. will drop and if you adjust it toward + it will rise. Basically the potentiometer is used as a variable power supply for the two BB139 varicap diodes.
Those two diodes act like a variable capacitor when you adjust the pot. By varying the diode capacitance the L1 + diodes circuit makes a resonance circuit for T1. You can use transistors like BF199, BF214 but do not use BCs. At this moment you don’t have yet the long range fm transmitter because the power is quite low, no more than 0.5 mW.
How does long range FM transmitter works
Make sure to encase the oscillator stage in a metallic shield to prevent parasite frequencies destabilizing the oscillating stage.
T2 and T3 works as a buffer stage, T2 as a voltage amplifier and T3 as a current amp. This buffer stage is very important for freq stabilization because is a tampon circuit between the oscillator and the preamp and final amplifier. It is well known that poor transmitter designs tend to modify freq. as you adjust the final stage. With this T2, T3 stage this won’t happen anymore!
T4 is a preamplifier for the FM transmitter and is used as a voltage power RF amplifier and will deliver enough power to the final T5 transistor. As you can see T4 has a capacitor trimmer in its collector, this is used to make a resonance circuit that will force T4 to amplify better and get rid of those unwanted harmonics. L2 and L3 coils must be at 90 degrees angle one to another, this is to avoid frequency and parasite coupling.
The final stage of the long range RF transmitter is equipped with any RF power transistor that has at least 1 watt output power. Use transistors like 2N3866, 2N4427, 2N3553, BLX65, KT920A, 2N3375, BLY81, 2SC1970 or 2SC1971 if you want to have a pro FM transmitter with enough power to cover a long range area. If you use 2N2219 you will get no more than 400mW. Use a good heatsink for the T5 transistor as it gets a little hot. Use a good 12V/1Amp minimum stabilized power supply.
T1 = T2 = T3 = T4 = BF199
T5 = 2N3866, 2N4427 or 2SC1970 for 1Watt / 2SC1971, BLX65, BLY81, KT920A or 2N3553 for 1.5 to 2W power.
L1 = 5 turns / 0.6mm / 4mm silvered copper
L2 = 6 turns / 0.8mm / 6mm enameled copper
L3 = 3 turns / 1mm / 7mm silvered copper
L4 = 6 turns / 1mm / 6mm enameled copper
L5 = 4 turns / 1mm / 7mm silvered copper
Use silvered copper for L3 and L5 if you want to obtain better characteristics.
Adjustments of the long range transmitter
Start by construction the oscillator stage, solder a small wire to T1 10pF capacitor out and listening to a FM receiver, trim the 10k pot until you can “hear” a blank noise or if you plug in an audio source you can hear the music. With a 70cm wire you can cover a 2 – 3 meter area just with the oscillator stage.
Then continue to build the rest of the RF transmitter, use proper shielding as indicated in the circuit schematic. When you finished the transmitter construction connect the antenna or better a 50 or 75 Ω resistive load and use this RF probe, you can use 1N4148 diode instead of the probe diode.
Adjust again the 10k pot to desired freq. and then go to T4 stage and trim the first collector trimmer for maximum voltage indication on the multimeter. Then continue with the next trimmer and so on. Then go back to the first trimmer and readjust again until you obtain the highest voltage on the multimeter. For 1 watt RF power you can measure a 12 to 16 Voltage. The formula is P (in watt) is equal to U2 / Z, where Z is 150 for 75Ω resistor or 100 for 50Ω resistor, but you must remember that the real RF power is lower.
After those adjustment, if everything is going well connect the antenna, continue using the RF probe, readjust again all the trimmers starting from T3. Make sure you don’t have harmonics, check your TV and radio set to see if there is disturbance on the band. Check this in another room, far away from the FM transmitter or antenna.
This is my design for a long range transmitter and is working well. I used 2SC1971 which has a 12dB power gain in 88 … 108 MHz band, this is around 15 times RF amplification. As T4 delivers around 80 to 100 mW of RF power the final stage has enough power to deliver between 1 to 2W depending the transistor usage.
12 Volts, 3-3,5 Watts, FM Transmitter using TR 2N3553
This is the schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz. Stability of this transmitter is not bad and PLL circuit can be added on. This is a circuit that I’ve build a few years ago for a friend, who used it in combination with the BLY88 amplifier to obtain 20 W output power. From the notes that I made at the original schematic, it worked fine with a SWR of 1 : 1.05 (quite normal at my place with my antenna).
R1,R4,R14,R15 4 10K 1/4W Resistor
R2,R3 2 22K 1/4W Resistor
R5,R13 2 3.9K 1/4W Resistor
R6,R11 2 680 Ohm 1/4W Resistor
R7 1 150 Ohm 1/4W Resistor
R8,R12 2 100 Ohm 1/4W Resistor
R9 1 68 Ohm 1/4W Resistor
R10 1 6.8K 1/4W Resistor
C1 1 4.7pF Ceramic Disc Capacitor
C2,C3,C4,C5,C7,C11,C12 7 100nF Ceramic Disc Capacitor
C6,C9,C10 3 10nF Ceramic Disc Capacitor
C8,C14 2 60pF Trimmer Capacitor
C13 1 82pF Ceramic Disc Capacitor
C15 1 27pF Ceramic Disc Capacitor
C16 1 22pF Ceramic Disc Capacitor
C17 1 10uF 25V Electrolytic Capacitor
C18 1 33pF Ceramic Disc Capacitor
C19 1 18pF Ceramic Disc Capacitor
C20 1 12pF Ceramic Disc Capacitor
C21,C22,C23,C24 4 40pF Trimmer Capacitor
C25 1 5pF Ceramic Disc Capacitor
L1 1 5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm
L2,L3,L5,L7,L9 5 6-hole Ferroxcube Wide band HF Choke (5 WDG)
L4,L6,L8 3 1.5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm
L10 1 8 WDG, Dia 5 mm, 1 mm CuAg, Space 1 mm
D1 1 BB405 BB102 or equal (most varicaps with C = 2-20 pF [approx.] will do)
Q1 1 2N3866
Q2,Q4 2 2N2219A
Q3 1 BF115
Q5 1 2N3553
U1 1 7810 Regulator
MIC 1 Electret Microphone
MISC 1 PC Board, Wire For Antenna, Heatsinks
The circuit has been tested on a normal RF-testing breadboard (with one side copper). Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF rules of building.
Q1 and Q5 should be cooled with a heat sink. The case-pin of Q4 should be grounded.
C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.
Local laws in some states, provinces or countries may prohibit the operation of this transmitter. Check with the local authorities.
15 Volts, 6 Watts FM Transmitter 88-108 MHz TR 2SC1971
Presented FM transmitter is built around low power PLL transmitter and amplifier that boosts its signal all the way up to 6 Watts. The signal is amplified by three RF stages of amplification. In the first and second stages of the transmitter one of the best driver transistors were used 2SC2053.
You can use the other transistors but only up to 500mW of power. In the third stage 2SC1971 RF transistor was used to achieve 6W of power. For making any RF transmitter circuit at least two meters are necessary, one is frequency counter and the other is RF field strength meter for which the schematic is provided.
L3 will is 4 turns,
L6 will is 12 turns
Heat Sink is required for C1971 (2SC2053 can be used).
Simple field strength meter circuit:
14-18 Volts, 15-18 Watts FM Transmitter TR BLY88
Here’s FM transmitter for commercial FM band that provides 18 watts of power. Since the electronic diagram is too large we decided to divide it into two parts. The first part is the actual FM transmitter while the second part is 18W RF amplifier. The circuit should be built on an epoxy printed circuit board with the upper face components reserved for interconnecting tracks and the bottom solder to the ground plane.
If powered by 14V and 2.5A transmitter outputs 15W of power, whereas 18V and 3.5A will provide 18W. BB110 variable capacitor connected to the collector of transistor BF199 adjusts the transmission frequency of the circuit. 2K2 potentiometer serves as fine tuning. Once the output frequency is adjusted amplifier variable capacitors must be adjusted for maximum output power one stage at a time. All adjustments must be made with 50 Ohm dummy load connected to the output of transmitter.
Transistors 2N3924, 2N4427 and BLY88 must be mounted on star-shaped heatsinks. In the case of transistors 2Nxxxx the ideal size is 20mm in diameter and 10mm in height, while for the BLY88 must be 75mm diameter by 100mm tall. It is mandatory to use silicone grease to optimize the transfer of temperature of the transistors to their sinks. Remember that excessive heat is part of the output instability and can cause damage to components.
All coils should be made according to following table:
L1 ——- 3 Turns on ferrite of 5x10mm
L2 ——- 3 Turns on air 9mm ( 10mm long )
L3 ——- 1 Return on 12mm air
L4 ——- 4 turns on air 9mm ( 12mm long )
L5 ——- 2.5 laps on ferrite of 5x10mm
L6 ——- 1 Return on 12mm air
L7 ——- 2.5 laps on HF type ferrite 10x5mm
L8 ——- 3 Turns on air L8 9mm ( 8mm long )
L9 ——- 1 Return on 12mm air
L10 ——- 2.5 laps on ferrite of 5x10mm
L11 ——- 2.5 laps on ferrite of 5x10mm
L12 ——- 7 laps on air 9mm ( 19mm long )
L13 ——- 3 Turns on air L13 13mm ( 7mm long )
12-14 Volts, 4km FM Transmitter TR BFR96
This is a VCO FM Transmitter. With good antenna (dipole placed outdoor and high) the transmitter has very good coverage range about 500 meters, the maximal coverage range is up to 4 km. To calibrate for maximum power connect 6 V / 0,1 light bulb to the output and use R1 to tune the right frequency, adjust L1 coil if necesary. Then use C14 and C15 to adjust the highest power (the highest light of the bulb). Then you can connect antenna and audio signal. Adjust R2 until the audio sounds as loud as the other stations.
– Power supply: 12-14 V stab., 100 mA
– RF power: 400 mW
– Impedance: 50-75 ohm
– Frequency range: 87,5-108 MHz
– Modulation: wideband FM
1-1,5 Watt, QRP Power Transmitter (12-13,8 Volt)
The 1 watt 20 meter QRP transmitter with VXO. This is a nice QRP transmitter that can be used in combination of one of the simple receivers. Normally these designs have only two transistors: one is the X-tal oscillator and the second the final amplifier. A good example is my first QRP rig that is also described somewhere on this site. Here the VXO (Variabele X-tal Oscillator) has a tuning range of 16 kHz.
This VXO is buffered with an extra driver stage for a better frequency stability and a varicap diode is used instead of a variabele capacitor. An extra transistor is added for keying the transmitter with a low keying current. What you can do with such a simple 1 watt QRP power transmitter. This is a real low power transmitter, so do not expect that you can do everything with it but… When conditions are normal, you can easily make many QSO’s during one afternoon with stations with distances upto 2000 km with a simple inverted V wire dipole antenna! From Europe, I did even make QSO’s across the Ocean!
Here, a varicap diode is used for tuning the VXO, but of course you can replace it by a variable capacitor of 100-200 pF. The voltage stabilizer circuit with a 7808 can be deleted, the VXO is supplied from the 12V.
A parallel circuit of Lx (2,2 uH) and a trimmer Cx is added in series with the X-tal to increase the VXO range. Doing some experiments with Cx is very interesting: When starting at minimun capacitance, the frequency goes down when increasing the capacitance. At a certain moment the oscillator stops (resonance of Lx // Cx) or jumps a few MHz down, then it suddenly starts to oscillate at a frequency that is above the crystal frequency. When increasing the capacitance further the oscillator frequency goes down. The trimmer Cx is adjusted so that there is no gap between both frequency ranges (switchable with S1).
However, the VXO range is very dependent on the used crystal. I had good results with cheap computer crystals (even 150 kHz or so) and a very bad result with a high quality crystal (only 2 kHz). Stability reduces considerably when tuning further from the crystal frequency. At a few hundred kHz from the crystal frequency, the stability is more or less the same as that of a VFO… Many amateurs have very good results with VXO’s so do not give up if the first results are bad.
Buffering and driver
An extra driver/buffer stage is added between the VXO and the final stage. Due to this driver/buffer stage, the VXO stability is less influenced by feedback from the final amplifier when tuning antenna’s etc. When trying to make a VXO with a large frequency range with my first QRP transmitter without buffering, stations reported that my frequency was not stable.
The VXO is a low power oscillator to prevent high RF voltages across the varicap diode. This would also decrease the stability considerably. The driver/buffer stage has also the task to make sufficient driving power for the 1 watt final amplifier. The 12 ohm resistors are added to avoid parasitic oscillations.
Tuning ranges, crystal frequency 14060 kHz
Position | S1 | Min. Freq. (kHz) | Max. Freq. (kHz) | Range (kHz) :
Open | (Lx // Cx active) | 14047.0 | 14058.7 | 11.7
Closed | (Lx // Cx shortened) | 14058.7 | 14063.0 | 4.3
Total (open + closed) : 14047.0 | 14063.0 | 16.0
The 2N4427 transistor is doing the heavy work here but you can also use others like a 2N3553. The 1000 ohm resistor at the input of the low pass filter is added to avoid high peak voltages during mismatches of the antenna.
The simple QRP transceiver is used in combination with an old SSR1 shortwave receiver. It is not difficult to make QSO’s with it while using a simple inverted Vee dipole antenna with the center at a height of 4 meters.
This 1 watt QRP transmitter is all you need to be a radio amateur and to collect a shoe box with QSL cards! The total frequeny range is 16 kHz but even the small 4 kHz range without Lx//Cx is already more than enough for a lot of contacts. The RF output power is 1 watt at 12 VDC and 1.5 watt at 13.8 VDC.
FM/RF TRANSMITTERS AMPLIFIER:
25 WATTS, 12-13,8 Volts, FM Transmitter Amplifier TR 2SC 1946
40 WATTS, 28 Volts, FM Transmitter Amplifier TR 2N5643
Building two stage 40 Watt FM Transmitter Amplifier. RF input power should be between 0.5 and 1 watt. Amplifier is powered by 28V power supply. The diagram shows a 2N3375 driving a 2N5643 but there are many other transistors that will work.
I used these two transistors just because they were cheap at the time. If any of the variable capacitors are at full capacitance you can pad them out with a fixed ceramic capacitor of suitable value. Extra capacitance also might be needed on the base of the transistors (i had to add 3 100pF capacitors on the base of the 2N5643). The transistors are bolted to a piece of right angle aluminum which is fixed to the metal chassis to dissipate heat effectively.
60 WATTS, 12-13,8 Volts, FM Transmitter Amplifier TR 2SC2630
150-200 WATTS, 28 Volts, FM Transmitter Amplifier TR SD1407
This is 150W FM transmitter amplifier for 88-108MHz band. The amplifier has two stages using BLF244 mosfet transistor for the first stage which requires 0.5 – 1Watt of RF input to get about 20 watts to drive the final stage SD1407 which can push nearly 200 Watts on this design.
This design is more or less broadband however I added two variable capacitors after each stage for optimum matching and power output. Make sure the trimmer and the capacitors after the final stage SD1407 are a high voltage types with at least 200V rating. The power on this amplifier can be varied by adjusting the bias voltage using the white pot to the BLF244 mosfet. I added a zener diode onto the bias supply to protect the transistor from too much bias voltage.