The FM transmitter shown below can transmit signals to a range of two kilometers. The transistor marked Q1 and Q2 forms a standard high sensitive preamplifier stage. The audio signal to be conveyed is joined to Q1 base through capacitor C2. The biasing resistors for this preamplifier stage that comprise of Q1 and Q2 are R1, R4, R3, R5, R6 and R9.
Transistor Q3 will perform the collective job of mixer, oscillator and final power amplifier. The tank circuit that is important for creating oscillations is formed at C9 and L1. Inductor L2 is responsible for coupling the Frequency Meter signal to the antenna.
Below is the circuit diagram.
Points to note
- The circuit should be assembled on a good quality PCB.
- Powering the circuit can be done from as low as nine to 24V DC.
- VK220J type RFC can be used as Inductor L3.
- Three turns of one mm enameled copper wire coupled on a ten mm diameter plastic former can be used for L1. To make L2 two turns of one mm enameled copper wire made close to L1 is used.
- Varying C9 can be used to adjust the frequency.
- R9 can on the other hand be used in adjusting the gain.
- The value of C8 should also be adjusted for optimum performance.
- In case you want to reduce the noise, it is advisable to use battery for powering.
- With creativity, you can also make a long range remote control.
- Frequency of input signal must be less than a half of the central frequency of your transmitter.
- Transmitting power highly depends on the sort of antenna used.
- Assembling the 2KM FM transmitter can be a bit challenging for beginners. They are therefore advisable for them to familiarize themselves with transistors in simpler apps.