AM Transmitter Hum Problems

Hum can be an annoying problem for all low power AM transmitters, but by following the guidelines below, you should be able to locate and eliminate the problem.

Possible sources of hum and recommended fixes.

EVERYTHING that follows assumes you have connected the transmitter to GROUND!


  • Increase the audio source signal level (volume)


Hum can be introduced on the audio cables by a ground loop. A ground loop will occur when the audio source device is connected to a ground that is different from the transmitter ground.
  • You can check for a ground loop by unplugging your audio source from the transmitter.
  • If the hum drops a lot or goes away, you may have a ground loop.
  • Try disconnecting the transmitter earth (earthed via audio cable)
  • Fit a “ground loop isolator” in the audio cable path (earthed via transmitter)


The most common source of hum with indoor installations is AC hum modulation induced on the RF signal by the building wiring.

  • You can check for this type of hum by disconnecting the audio cables from the transmitter.
  • If the hum persists, you have AC wiring induced hum.
  • AC wiring can induce hum on the part of the signal that it radiates. Often you will note that a radio placed close to the transmitter will not have any hum, but a radio in a distant room will have hum.
  • This problem can arise when the transmitted signal is picked up by the house mains cables and then re-radiated with modulation from the mains.
  • Connecting the transmitter ground to a grounded metal point such as copper water pipe can bypass much of the signal that normally couples into the wiring.
  • Try to eliminate the source of the AC wiring hum. Any device that has a switching-type power supply can cause hum. For example low energy lights, lamp dimmers, mobile phone chargers, Wi-Fi router, PC’s etc. Also, turn off all lamp dimmers completely.
  • If your radio has an internal directional antenna, try rotating the radio. Often you can find a position where the signal is still strong, but the hum is markedly reduced.
  • Changing the operating frequency can have a big effect on AC wiring induced hum. (The higher the frequency the better..!)