I found this simple FM transmitter circuit:
I built it an it works rather nicely. There are many simple FM transmitter circuits on Google, most of which look basically like this:
These work by modulating the base-collector capacitance of the BJT using the sound signal, which appears in parallel with the L1-VC1 resonant circuit, whose frequency changes with the capacitance.
The one I built however seems different (the top one):
1) There the Rf OUT is taken from the emitter of the BJT, while in the ones resembling the bottom picture, output is taken from the collector. The circuit on the bottom Picture is operating in common-base mode, so taking the output from the collector makes sense. But what amplifier configuration is used in the first Picture? Only time I know when the output is taken from the emitter is the common collector configuration, but that one has no voltage amplification so it seems strange that it would be used here.
2) In the bottom one, C1 and C3 are huge capacitors for the high frequency RF signal, and therefore in essence short the base of the BJT to supply, and so the base-collector capacitance appears in parallel to L1 and VC1. But what is happening in the top circuit? My quess is that C3 in the top one does the same as C3 in the bottom one, as it sits parallel to the battery (one side grounded, one side to +9V). But in the top one, we have C4 connected to ground from the collector, instead of being connected in parallel to L1. Which components form the resonant circuit in the top circuit?