radio tuning quesiton

[pc88]

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!

 

[Hero999]

The copper plate changes the load on the transmitter which detunes the transmitter. This is because the aerial is driven directly from the oscillator. A real transmitter doesn't suffer from this because it has a buffer before the aerial to isolate the oscillator from changes to the load.

As far as picking up other stations is concerned, I don't know what's causing that. Perhaps the non-linear characteristics of the transistor in the oscillator are causing demodulation of the radio station which is immediately remodulated by the oscillator and thus broadcast on a different frequency.

 

[pc88]

well, I found this tidbit about images being created in the presence of strong signals:

http://cappels.org/dproj/FMXMTR/fmxmtr.htm

(see "A note about FM receivers" at the bottom of the page)

The receiver I'm using is a 20-year old Radio Shack combination portable radio/tape player, but perhaps the same thing is going on (?)

 

[Hero999]

That makes sense.

What type radio did you use to listen to your transmitter on?

To stop the frequency from drifting too much you should build a transmitter with an output amplier like the circuit in the link you posted.