18W FM Transmitter

 

 

 

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 )