I've built Harry's (SM0VPO) FM microphone:
**broken link removed**
It doesn't have the greatest frequency stability, but that's not what I'm concerned about right now. What I'm interested in is understanding the following phenomenon:
I turn on the transmitter and a nearby radio. I tune the radio until I get to an unusually quiet frequency. I verify I'm at the transmitter's frequency by turning off the transmitter, and usually I'll hear static. I have the microphone disconnected so only the carrier frequency is being transmitted.
With the transmitter on, I place a small (1" x 1") piece of copper clad near the transmitter's pcb coil. This no doubt changes the transmitter's frequency. By varying the distance between the coil and copper clad I can change what I hear on the radio. At certain distances I can tune in a station; at others I'll get static; too far away and I'll get silence.
So what's going on? Is there any frequency mixing involved? Is the transmitter amplifying the broadcast signal so that the radio picks it up even though it's tuned to a different frequency? I'd really like to understand what's happening!