Sirius listening on vintage radio

[darklife]

Originally Posted by fifties

After moving far enough away from any AM transmitter the signal will fade, and in some instances heterodyne with one or more stations on the same frequency.

I have never heard, however, of an FM transmission being reproduced on an AM receiver; it's just not possible.

FM stands for frequency modulation remind you. When the modulated audio of the talking house transmitter is excited it tends to drag the frequency of the oscillator with it thus causing FM. I know it's built on PLL circuitry but for some reason it still seems to drift with its own modulation. Though it's only a few tens to hundred hertz it's noticeable enough to cause a wobbling sound in the audio program material when hetrodyning with another locked carrier that is solid on frequency.
I believe this is caused by the lack of good power supply capacitors in the talking house transmitter, or just an inadequate power supply.
When pushing an AM transmitter to it's fullest modulation it causes a large power spike on the positive amplitudes because of the required output power to the RF stages. If a power supply is not used to support this varying power fluctuation it can cause FM to ride the AM signal which is what I have heard numerous times in these transmitters.
I believe in the professional world of AM transmitters they call this undesirable effect "carrier shift".
Most tiny part 15 transmitters tend to be built with underrated power supplies causing this effect.
In reality it won't matter for the average person who wants to just hear audio a few hundred feet away from the transmitter. OTOH if trying to hear their signal outside of the perimeter of the average range they will notice the effect especially at night when the AM dial becomes cluttered with distant stations.
Hope what I am getting at makes more sense now.