FM Transmitter Mod 2

An low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier.

L1 is 0.112uH (this tunes to the middle of the FM band, 98 MHz, with VC1 at its centre value of 33pF).L1 is 5 turns of 22 swg enamelled copper wire close-wound on a 5mm (3/16") diameter former. Alternatively, you can have a fixed 33pF cap instead of VC1 and have L1 as an adjustable molded coil (eg UF64U from Maplin). VC1 will give you a tuning range of 85 - 125 MHz, and a possible choice is the Philips type polypropylene film trimmer (Maplin code WL72P).Two sets of oscillator bias resistors are given, the ones in the brackets give about 20% more RF power.Mike is our favourite Omnidirectional sub-mini electret (Maplin code FS43W). Ant is a (lambda / 4) whip monopole (eg 76 cms of 22 swg copper wire).Q1 is configured as a Clapp oscillator. Frequency modulation results from the audio voltage changing the transistor's base-emitter capacitance.

R1=4K7, R4 = 150K, R7=3K9 (2K7), R2 = 4K7, R5 = 220R, R8 120R (82R), R3 = 4K7, R6 = 4K7. All resistors except R8 are at least 0.25W rated. R8 is at least 0.5W rated(the 0.6W metal film M-series from Maplin can be used for R1-R8). C1 = 1n, C4 = 22uF, C7 = 10n, C10 = 1n, C2 = 4u7, C5 = 1n, C8 = 1n, C3 = 1n, C6 = 10n, C9 = 33pF, VC1 = 5-60pF, IC1 = LM358, Q1 = ZTX108.