This long range FM transmitter circuit is like a tiny radio station in a box.
It can take your music or voice from a microphone and send it out over the airwaves much farther than a regular bluetooth speaker.
This makes it great for things like community radio stations, wireless speakers for big backyards or even just sharing your tunes with the neighborhood.
In some areas operating a transmitter might require a license.
Always check the regulations in your area before using one.
Circuit Working:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors | R1 10k | 1 |
| R2 1k | 1 | |
| R3 22k | 1 | |
| R4 2.2k | 1 | |
| R5 68Ω | 1 | |
| R6 68k | 1 | |
| R7 18Ω | 1 | |
| R8 2.2k | 1 | |
| R9 1k | 1 | |
| Capacitors | Ceramic C2 0.01µF | 1 |
| Ceramic C3 1nF | 1 | |
| Ceramic C4 8.2pF | 1 | |
| Ceramic C5 0.1µF | 1 | |
| Ceramic C6 0.1µF | 1 | |
| Ceramic C7 1nF | 1 | |
| Ceramic C8 1nF | 1 | |
| Ceramic C9 1nF | 1 | |
| Ceramic C10 0.01µF | 1 | |
| Electrolytic C1 2.2µF 25V | 1 | |
| Electrolytic C11 100µF 25V | 1 | |
| Trimmers | Trimmer VC1 22p | 1 |
| Trimmer VC2 22p | 1 | |
| Trimmer VC3 22p | 1 | |
| Trimmer VC4 22p | 1 | |
| Trimmer VC5 22p | 1 | |
| Trimmer VC6 40p | 1 | |
| Trimmer VC7 40p | 1 | |
| Semiconductors | IC 7809 | 1 |
| Transistor T1 BF494 | 1 | |
| Transistor T2 BF200 | 1 | |
| Transistor T3 2N2219 | 1 | |
| Transistor T4 2N3866 | 1 | |
| Others | Electret MIC | 1 |
| Coil L1 4 turns | 1 | |
| Coil L2 6 turns | 1 | |
| Coil L3 6 turns | 1 | |
| Coil L4 5 turns | 1 | |
| Coil L5 7 turns | 1 | |
| Load Antenna 50Ω | 1 |
This A long range FM transmitter circuit comprises four stages dedicated to radio frequency operation.
It begins with a VHF oscillator centered on transistor BF494 T1, followed by a preamplifier with transistor BF200 T2, a driver employing transistor 2N2219 T3 and a power amplifier using transistor 2N3866 T4.
At the input of the oscillator a condenser microphone is connected.
The functionality of the 2 Watt transmitter is basic: when speaking near the microphone frequency modulated signals are produced at the collector of oscillator transistor T1.
These FM signals undergo amplification through the VHF preamplifier and the pre driver stage.
An alternative to 2N2219 is the use of transistor 2N5109.
The preamplifier operates as a tuned class A RF amplifier while the driver functions as a class C amplifier.
The signals are then directed to the class C RF power amplifier which supplies RF power to a 50 ohm horizontal dipole or ground plane antenna.
For efficient heat dissipation, a heat sink should be used with transistor 2N3866.
Note: 2N4427 can be substituted for better performance at 12V delivering up to 1 watt RF power.
Trimmer VC1 connected across L1 must be carefully adjusted to generate a frequency within the range of 88 to 108MHz.
Additionally, trimmers VC2 through VC7 should be adjusted for maximum output and range.
A stable 9V supply is provided to the oscillator by regulator IC 78C09 ensuring that frequency generation remains unaffected by supply voltage variations.
A 12V battery can also be used to power the circuit.
Assembly of the circuit should be done on a general purpose PCB, and proper installation of the antenna is crucial for achieving maximum range.
Coils L1 through L5 are wound using 20 SWG copper enameled wire over air cores with a diameter of 8mm.
They consist of 4, 6, 6, 5 and 7 turns of wire respectively.
Electrical Characteristic of transistor 2N3866:
| Symbol | Test Conditions | Value | Min | Typ | Max | Unit |
|---|---|---|---|---|---|---|
| BVCER | Collector Emitter Breakdown Voltage | 55 | Vdc | |||
| BVCEO | Collector Emitter Sustaining Voltage | 30 | Vdc | |||
| BVCBO | Collector Base Breakdown Voltage | 55 | Vdc | |||
| BVEBO | Emitter Base Breakdown Voltage | 3.5 | Vdc | |||
| ICEO | Collector Cutoff Current | 20 | μA | |||
| ICEX | Collector Cutoff Current | 100 | μA | |||
| HFE | DC Current Gain | (IC=360mA, VCE = 5.0 Vdc) | Both | 5.0 | – | |
| (IC=50mA, VCE = 5.0 Vdc) | 10 | 200 | – | |||
| (IC=50mA, VCE = 5.0 Vdc) | 2N3866 | 25 | 200 | – | ||
| VCE(sat) | Collector Emitter Saturation Voltage | 1.0 | Vdc |
How to Build:
To build a Long Range FM Transmitter Circuit you need to follow the below mentioned steps:
Circuit Assembly:
- Refer to the circuit diagram and layout the components on the PCB.
- Start by soldering the resistors, capacitors, and other small components.
- Install the transistors BF494, BF200, 2N2219, and 2N3866 in their respective places on the PCB.
- Connect the condenser microphone at the input of the oscillator transistor BF494.
- Connect the coils L1 to L5 according to their specifications number of turns, wire diameter, etc.
Power Supply and Regulation:
- Connect the power supply 9V or 12V battery to the circuit.
- Use the regulator IC 78C09 to provide a stable 9V supply to the oscillator.
Testing and Adjustment:
- Carefully adjust trimmer VC1 connected across L1 to generate a frequency within 88 to 108MHz.
- Adjust trimmers VC2 through VC7 to get maximum output at the desired range.
Finalization:
- Install a heat sink with transistor 2N3866 or 2N4427 for heat dissipation.
- Properly install the antenna 50 ohm horizontal dipole or ground plane antenna for maximum range.
Note:
- This transmitter is for educational purposes only and should not be used with an outdoor antenna as it may be illegal in many places.
- Remember to take necessary precautions while working with electronic circuits such as ensuring correct polarity, avoiding short circuits, and following safety guidelines for soldering and handling components.
Conclusion:
A long range FM transmitter circuit is designed for transmitting FM signals over extended distances often several kilometers.
It features high power output, high quality modulation, efficient antenna design, frequency stability and regulatory compliance.
Building such a circuit requires careful design and calibration to ensure optimal performance and legal compliance.