I built a transmitter based on this
http://www.sentex.ca/~mec1995/circ/fmt2.htmThis is the actual circuit I built.
It's oscillating because I can pick sound when I put it a couple inches from an old fm receiver with analog tunner. So I think it is out of frequency and it is picking harmonics, but I placed it a meter far from the receiver I poke the coil stretch it tighten it, and can't get it in frequency.
I tried soldering an output cap in the antenna of 24pF (I don't know how to calculate this, just thought it would be enough), or removing the antenna completely. Tried winding different coils, with 4 5 and 6 turns.
I'm using a couple AA batteries to power it (3v). The inductor I used I think AWG23 copper wire from a CAT5 cable, wound on a 3.5mm jack.
I know this kind of free running transmitters are not good, but when I was a kid I had one transmitter that had a single transistor, and a PCB spiral coil and a condenser mic and worked out of a single 1.5v battery, and the stability and range was fairly good, unfortunately I don't have it or its schematic anymore.
some other inductors I wound and tried.