FM Transmitters are always one of most popular circuits for hobbyists. Transmitting audio without wire is really exciting to make and use. I particularly know this because I am hobbyist. I am not an Electronics Professional. My major was not Electronics. I started my journey with Electronics as a hobbyist who loves to make every hobby circuit available on the web. After gaining much experience on which circuits to make and which projects are fake, I started writing this blog. I personally test each of the circuits, which are posted in my electronics hobby section. I’ve been playing around with this long range high power FM Transmitter circuit for quite a while and finally got a chance to make and test it personally last week.
As I mentioned here (and probably you knew) that I am not a Electronics Expert. So I won’t explain the circuit in detail. This is a pretty common FM Transmitter circuit for Long Range Transmission. You can search Google and find plenty of blogs explaining how these circuits work. But none of them elaborated the common mistakes and key points to take care of while making such FM Transmitter Circuits.
FM Transmitter Project : DOs and DON’Ts
- Do not assemble on Breadboard – These FM Transmitter circuits won’t work if assembled on a Breadboard. Breadboards do not have the necessary fineness which is required for such high frequency and high power circuits. Assemble on a Veroboard (Perforated Stripboard) or etch the PCB Layout on a copper clad board and then assemble the circuit on it.
- Use Old day’s Metal Casing Transistors if Available – FM Transmitter circuits available on the web often uses transistors which are available in both plastic and metal package. Prefer to use the metal package transistors if possible.
- Understand the circuit to find alternative/replacement components – Often understanding the circuit first is inevitable. When you understand the circuit properly, you can identify silly mistakes made in circuit diagram and find suitable alternatives for components which are rare/unavailable. With experience, you will do better on this point.
- Looks for Inductance in description, when making the coils – Often the blogs publishing a FM Transmitter circuit doesn’t mention inductance of the coils used. They mention wire Gauge along with diameter and number of turns for the coil. Use the formula given below to determine the required inductance of the coil. If you do not have the exact Gauge of wire handy, use the same formula to determine the dimension and turns of your coil with the available wire.
Relative permeability of the wire-material (often copper) can be found on the web, e.g. for copper it is 0.999994
- Trimmer/Variable Capacitor is Rare and Not Mandatory – Often for the purpose of testing, you don’t need a variable capacitor for your FM Transmitter. You can tune your FM Receiver (which is a FM Radio) to match the transmission frequency. Trimmer Capacitors are rare these days. So, you can easily avoid using them. If a trimmer cap is used in circuit which is rated 0-40pF for example, use a value from the middle of the range, e.g. 18pF ceramic capacitor (fixed value). Use the formula given below to calculate the frequency of the transmitter (LC Tank Circuit) for the capacitance and inductance of the used capacitor and inductor.F = sqrt(L * C) * 6.28318
- Understand the antenna – Antenna often plays a crucial role in transmitters, specially when you are making a powerful one. Understand the right antenna applicable for the circuit and try to use the use the right one for best results.
- Don’t easily believe the ‘Long Range’ myth – I am not saying that most of the blogs claiming a FM Transmitter circuit to cover long range is fake. Rather I am requesting you to understanding the operating conditions to get coverage of that range. Most of the circuits covers the claimed area without any obstacle and under clear weather conditions. Do not expect to get the same range when testing in the middle of a city.
- Do not expect same audio quality as Radio Stations – Transmitter circuits which are available on the web are pretty basic, even if they cover long range. Do not expect crystal clear and noise free transmission of audio unless you spend plenty of effort to enhance or atleast maintain the input audio quality.
- These FM Transmitter are not precise – It is expected to hear your transmitted audio at a wide range of frequency in your radio. Do not get confused. That’s expected. To precisely maintain audio frequency complex circuitry is required, which is not worth looking for, when working on a hobby project. Tune your radio to match the frequency where the sound quality is the best.
- Prefer an analog receiver for testing, over a digital FM Radio – Now a days inductor-less Digital FM Radio boards are quite popular. Avoid using one of them while testing your circuit. You won’t get expected results. Use an old day radio where traditional FM Receiver IC, RFC Coils and Trimmer Capacitors are used.
- Power up the transmitter from Battery, always avoid Power Supplies/Adopters – If you use a power supply where AC Mains is converter to suitable DC voltage, you should expect more noise in your transmitted audio. The FM Transmitter should be powered from a battery to avoid those unwanted noises, injected by your power supply. Or atleast use large value electrolytic capacitors to filter the DC line as much as possible.
- Read the assembly instructions, minute details are big factors – Take your time and read the blogs in detail. Read every user-comments and assembly instructions. They contain critical information, tips and tricks which will help you to succeed. e.g. Often your will read that the coil should be soldered as close as possible to the Trimmer Capacitor for best performance. This is true and essential instruction.
Long Range FM Transmitter Circuit Diagram
Now lets see how the circuit looks like. The circuit below uses transistor 2N2219 for FM Modulation (VHF Oscillation and Mixing) and RF Power Amplification. Transistors BC107 and BC177 creates a reasonable audio pre-amplifier, which results in a decent quality audio transmission. There are 3 inductors used in the circuit. You need a dipole antenna for this circuit. Component list is given after the circuit diagram. Click on the image below to see a larger circuit diagram.
|ID||Component Part No/Value||Alternatives|
|Q1||BC109 TO-18 Package||BC327, BD139, TIP41|
|Q2||BC177 TO-18 Package||BC328, BD140, TIP42|
|Q3||2N2219 TO-39 or TO-5 Package with Heat sink||2N3553|
|R1, R10, R11, R6||10KΩ 0.25W Metal Film|
|R2||150KΩ 0.25W Metal Film|
|R9||4.7KΩ Preset (Variable Resistor)|
|R12||56Ω 5 W||2 pcs x 100R 2W in parallel|
|C1, C3||22uF 50v Electrolytic|
|C4||47uF 50v Electrolytic|
|C6, C7||1nF Ceramic|
|C8||15pF Trimmer/Variable Capacitor||5pF or 10pF Ceramic Cap|
|C9||40pF Trimmer/Variable Capacitor|
|L1||3 turns 18 SWG wire 1cm diameter air core|
|L2||2 turns 18 SWG wire 1cm diameter air core|
|L3||220uH RFC Choke||250 turns 28 SWG wire on 6mm former 5mm width honeycomb winding|
Long Range FM Transmitter PCB Layout
Following is the PCB layout of the Long Range FM Transmitter Circuit. Click on the images to download the actual size PCB layouts. You can print these circuits as-is and fabricate PCBs if you know how to do it.
Top Side of PCB Layout
Copper Side of PCB Layout
Alternatively, you can download the PCB layout in PDF format and print them easily. Click on the links below to download:
Long Range FM Transmitter PCB Layout Top Side
Long Range FM Transmitter PCB Layout Copper Side
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