Here Is a low-cost, good-stability RF / Am transmitter which uses only one FET.
The heart of this circuit is a FET BFW10 (T1) which works as a feedback oscillator. The frequency of the RF signal can be determined by the oscillator coil L1 and variable capacitor VC1.
The audio signal from an amplifier or any audio device such as cassette player, organ, TV etc whose output impedance is 4Ω to 16Ω is fed to the source of FET.
The amplitude of the RF wave can be varied by 1 KΩ preset VR1. The frequency adjustments are made through the coil and capacitor C4 and C5. The output of the transmitter is taken through trimmer VC3 to a telescopic or indoor wire antenna.
Turn the radio receiver to the transmitter carrier frequency and adjust the volume control of audio input device, VR1 and VC3 to get clear undistorted sound in radio receiver as well to achieve better modulation percentage.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 100 KΩ
R2 = 320Ω
VR1 = 1 KΩ
C1 = 100 pF
C2 = 100 µF/16V
C3 = 0.01 µF
VC1 = 365 2j
VC2 = 22p
VC3 = 70p
T1 = BFW10
L1 = MW or SW osc. Coil