The purpose of this 1.5V FM broadcast transmitter circuit is to provide a simple, appliance-like low-power transmitter to rebroadcast audio. It can be used to rebroadcast the output of a CD player, television receiver, or radio receiver. This transmitter does not have pre-emphesies, so it is not high-fidelity. It has sort of an “AM” sound for music and is fine for speech. Since there is no audio level control on the input, the audio level out of the CD player (or whatever you are driving it with) needs to be adjusted. Or, you can just add a 10k to 50k as an input level control, its no big deal. The FM transmitter range is about 10 up to 20 meters. Your mileage may vary.
The transmitter, as built is not tunable once assembled, the coil tweaked to the desired frequency, and everything glued down. If you want to make one that’s tunable, it might be easiest to reduce the 18 pf capacitor and put a small trimmer capacitor in parallel with the inductor (across the reduced value capacitor). Voltage variable capacitors would be a nice alternative to a mechanical variable capacitor but they don’t offer much tuning range with only a 1.5V power supply.
Many kinds of transistors will work fine in this application. After all, its only an oscillator (frequency modulation is obtained my modulating the base-collector voltage, thereby modulating the depth of the depletion layer of the reverse-biased base-collector junction, which results in a change in capacitance at the collector, which results in a change the resonant frequency of the collector circuit.). This circuit uses transistor 2N4401 but you can uses other transistor like 2N3904 or MPSH34.
Tips to get it working: Wind the coil on a 4 or 5 mm diameter Philips blade screwdriver or similar form then slip it off. I used some vinyl insulated #24 hookup wire as well as #30 enameled wire. In both cases, I played with the length of the coil to tune the transmitter to a dead spot on the FM band. The coil is held in place with hot melt glue. If you don’t have a spectrum analyzer or frequency meter, use a good-quality FM receiver to make sure it’s tuned where you think it is. While adjusting the coil, keep in mind that all super heterodyne receivers have images. If you find that two or more adjustments make the transmitter show up on the same spot on the receiver, it might be necessary to take a short walk and find out which adjustment drops out first -this would be the image, because the receiver’s front end (if it has a tuned front end) will reduce its sensitivity to the image.