Description

Here is a well designed FM transmitter circuit that works effectively in 1 - 2 km range.

Input source is an electret condenser microphone and signal gain can be adjusted by using the potentiometer P1. You can easily prepare the PCB by using any PCB editor and apply it to the board by using the ironing method. The coils and the antenna can be designed as a part of the PCB made up of the copper area.

Parts List

Description

Features of the transmitter:
Supply Voltage:+3V DC
Current: 21mA
Output Frequency: 88MHz - 108MHz
Modulation: FM
Frequency Shift: ±30KHz
Input: Electret microphone or other input audio source

The important part of the circuit is formed of the Colpitts type oscillator. C3,C4,C5,C6,CD1-CD2 and L1 determines the frequency. BF982 and dual gate MOSFET are active parts in the oscillator. When the input impedance of the MOSFET gate inputs are high, LC tank is not affected. However transistors force the LC tank and cause phase shift.

Two driver stages are added to isolate the antenna from oscillator. First stage (BF199) amplifies the low signal of the oscillator and works as a constant load. The second stage (BFR90) amplifies the signal going through the antenna some more. A short copper wire can be used as an antenna here. Attaching a large antenna to this circuit is unnecessary because the output power is low.

Notes

1. Coil 1: Winding wire must be 1 mm thick and isolated. Number of turns: 3.5 .  5mm core must be used and the distance between each turn must be 1mm.
2. You can use BF199 instead of BFR90.
3. If you can't find the varicap diode, you can use two BB405 instead.