FM Transmitter 60Hz Hum

Thread Starter

Chris65536

Joined Nov 11, 2019
160
After reading some recent posts about the problem of a 60Hz (or harmonic) hum on audio signals, I remembered an odd one that I encountered. I have a Ramsey FM25b transmitter that I use to distribute music in my house. I have the output power turned down, so the signal doesn't even reach the street, but that had nothing to do with this problem. It had a persistent 60Hz hum, and turning up the RF power actually made it worse. I tried all sorts of filtering on the power supply, but still there was the hum. I even tried powering it from a 12v gel cell, using my cell phone as the audio source, but it still hummed!

This problem was eventually solved by removing the the PCB mounted whip antenna, and connecting a piece of coax running to an old VHF dipole 8 feet above. So my question is: How could only the antenna's proximity to the transmitter PCB, cause a 60Hz hum?
 

Thread Starter

Chris65536

Joined Nov 11, 2019
160
It seemed like the location of the transmitter and antenna didn't matter too much, while the proximity of the antenna to the transmitter made a huge difference. I could hold the dipole close and the hum would come back. If I turned up the RF power, the hum got louder. Hummmmm? It's a mystery.
 

vu2nan

Joined Sep 11, 2014
103
I suppose it's an RFI problem - antenna proximity resulting in RF getting into the power supply leading to regulator malfunction.

Regards,

Nandu.
 

Thread Starter

Chris65536

Joined Nov 11, 2019
160
Thank you. Isn't it odd though, that it was modulated at 60Hz? I guess the power line frequency is everywhere inside the house, and it was somehow affecting the RF feedback. It was an odd problem, which was unavoidable using the PCB mounted 1/4 wave antenna.
 

DickCappels

Joined Aug 21, 2008
6,101
Looking at Audioguru Again's thoughts in post #2 brought about the idea that if the problem is in some way 60 Hz hum entering the transmitter via the antenna, a high-pass filter might help a lot by keeping the hum out.

1578714218307.png

In the test in which you powered the transmitter with a gell cell and fed audio from your cell phone, was any part of the transmitter including the "ground" connected to anything plugged into the AC line? If so what was that?
 

vu2nan

Joined Sep 11, 2014
103
Looking at Audioguru Again's thoughts in post #2 brought about the idea that if the problem is in some way 60 Hz hum entering the transmitter via the antenna, a high-pass filter might help a lot by keeping the hum out.

View attachment 196636

In the test in which you powered the transmitter with a gell cell and fed audio from your cell phone, was any part of the transmitter including the "ground" connected to anything plugged into the AC line? If so what was that?
Hi Dick,

Here's a video which demonstrates the problem and its solution!


It appears to be a case of induced RF/mains hum mixing at some rectifying junction in the shack wiring and getting reradiated.

Regards,

Nandu.
 
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