This simple transmitter allows you to broadcast on FM radio band (VHF) 87.5 - 108 MHz.
It consists of a simple oscillator with silicon planar RF PNP transistor.
Directly to the oscillator an antenna is connected. Due to the large amplitude of RF voltage is sufficient antenna
length of about 5-10 cm. I used insulated 7cm long copper wire 1mm diameter.
I eliminated the tuning capacitor, which is usual for most bugs
and miniature transmitters, because this greatly complicates the tuning. From my own experience I know that if you get closer to such
capacitor, the operating frequency is changed.
That's why I chose to use the voltage tuning using the Voltage Controlled Oscillator (VCO).
Instead of tuning capacitor the varicap (capacitance diode) is used, which changes its capacity
by changing the reverse DC voltage. We can tune the operating frequency by changing the DC
voltage using the trimmer P1. Varicap also provides frequency modulation.
Tuning: Set P1 to the center. Turn on the FM radio and tune it to an unoccupied frequency in the 87.5 - 108 MHz band. You will hear a noise. Turn on the transmitter and the first tune the operating frequency of roughly by stretching turns in the coil L1. Then fine-tune the frequency using P1. Proper tuning is indicated by the radio getting silent. You can then connect audio source to the input (such as cassette player, CD or MP3 player, record player, audio output of PC or laptop, etc.). It is also possible to tune while already connected to the signal source. The circuit can be powered from 5V USB port available on your PC or laptop.
Inductor L1 is airborne and has six turns of 0.5 mm diameter wire wound on 3 mm diameter. Varicap is arbitrary, which covers the range of about 5-20pF, such as BB105, KB105, KB109. I used the varicap KB109G made by Tesla with yellow paint on the cathode. The transistor is a high-frequency planar PNP type, for example, BF970, BF979, or similar. You can also use a transistor with different type of case. The disadvantage of the circuit is sensitivy to changes in supply voltage (it is changing the varicap voltage and thus the operating frequency). The antenna is connected directly to the oscillator, so if you touch it or placing it near the conductive object, the frequency shifts. At its simplicity, however, the circuit works surprisingly well and the range is about 20 to 100 meters. You can use power supply of 5-12V or a battery. There should be no ripple in the supply voltage, otherwise it may be heard in the receiver.