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?
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?