Have my Whole House FM board on the wood work bench top temporarily wired with alligator clips and twisted power leads. Still not 100% committed to everything before liquid taping it.
Noticed a really crappy signal output earlier. As if the output strength had been reduced. Fussed with the long wire antenna and repositioned the sloping antenna. Discovered it seems easy with FM and wire antenna to make the signal directional.
Anyways, in course of debugging and experimenting the music stopped. All night I was catching some strange audio effect that was annoying me, but I wasn't sure where it was coming from or what exactly it was. At first while in the kitchen I thought it was just a fan running or the fridge compressor.
Well when the music stopped, I clearly heard hum. Much like the 60 cycle electric hum. Unsure if it was or wasn't.
Unplugged everything on the workbench and still humming.
Lifted the transmitter board up off the wood workbench and the hum was gone. Figure that one out. Not what I'd expect as source of the hum.
Also found a coil of headphone style cable feeding the audio to the transmitter can/will create hum.
Quite the experience and simple to introduce hum into transmitter.
Time to affix some standoffs and maybe a partial case to keep the unit elevated off the work bench (or this inevitably will happen again).
This is a common problem with FM and is likely due to two common causes: RF getting relflected back into the transmitter and RF getting into power supplies, including wall warts, in the vicinity of the transmitter. Moving the transmitter a small bit as you did changes the standing wave patterns in the vicinity of the transmitter and perhaps the hum vanished because you found a position where the standing wave pattern was a minimum at a source of the hum.
I went through this hum adventure and after trying different leads and positions finally solved it by enclosing the transmitter in a metal box and placing a clamp on ferrite choke on the DC power cord at the entry to the box. With this arrangement I don't have hum regardless of where I place the transmitter. My transmitter has a whip antenna attached to the circuit board so I drilled a hole in the top of the metal case, installed a grommet, and the antenna exits the box through this hole.
Here's a PHOTO of the setup.
Neil