This circuit is based on two parts, a common emitter that provides audio signal input amplification and a Hartley oscillator whose base transistor is modulated by the output of first stage. This results in a audio-frequency modulation in the range 88-108MHz. The output power is 1W average and the signal can be received for 200-300 mq.

Frequency can be selected using the trimmer 2-22pF capacitor. Th switch S can be used closed with the microphone or open, replacing the microphone with any audio output source, an mp3 player, cd player...

Inductor is the most critical component, and shuld be choosen by emphirical method. I have tried with 4-5 turn on 4mm diameter and with a common FM radio receiver as metter, I have tuned my transmitter stratching more or less the inductor. The AF output (where the antenna must be connected) is at the end of first-second turn of inductor, neer the V+ supply. The power supply can be 9-12V.

This output can be also connected to an RF amplifier to put the RF power trasmission to 20W: the schematic below is a proposal.

The power supply voltage is 12V and current available is related to resistor R and transistor used. For input of 100mW, choosing 2N4427 transistor and R=12ohm the output power will be 2W; changing the 18ohm resistor to 5R6, with R shorted, replacing the transistor with a BLW90 and providing at least 1.5A power supply current, the power trasmission will be 20W.

Inductor indication:

L1=4 turns 22SWG of wire on 5mm di diameter in air

L3=3 turns 22SWG of wire on 5mm di diameter in air

L2=4 turns of thin wire in ferrite choke

L4=3o4 turns of 22SWG of wire on two adiacent ferrite pipes

L5=RF choke made of da 20 turns of 22SWG wire on ferrite ring.

Imperfections of inductor can be repaired by capacitive trimmer.

With this RF power amplifier at least 2W are provided, for a 14dB gain and a double coverage range. I suggest to check your local laws and regulamentations relating to RF trasmission before to increase the transmitter power.