Question: Recently I’ve detected an annoying humming noise coming from my AM/FM stereo radio. It doesn’t seem to happen when we are at anchor, only when we are in our slip and plugged into shore power. Any idea how I can figure out what might be causing this noise?
Answer: You are experiencing what is known as RFI or radio frequency interference. One of the clues you have provided here is that your problem only occurs at the dock with the boat plugged in. If you are also running some lighting inside the boat that you don’t normally use when you are anchor, I’m betting the problem is coming from a fluorescent light.
Here’s how to figure it out for sure. You will need a multi-band radio like the one shown in the photo here. You might also want to have a handheld VHF radio available as well. I use these two radios all the time to trace RFI because it is a frequency-sensitive problem. Interference will only occur if the emitter of the “noise” is operating on a frequency very close to the frequency the receiver is running at—in this case your AM/FM stereo. It will make a difference whether your radio is making the noise on the AM or FM band, and to some degree what station on a given band you are listening to when you hear the noise.
Tracing the source of the noise is a process of elimination. Try and figure out which band (AM or FM) and which station on your built-in radio is most susceptible to the humming noise and tune your AM/FM portable radio to the same channel. Now move around the cabin. It’s also helpful to have someone turn lights (and any other shore power-supplied equipment) on and off. Listen for the noise as you home in on the various pieces of equipment. If the humming gets louder as you move your portable radio closer to a piece of gear, then you have found the source.
To eliminate the noise, you might have to replace the offending piece of equipment. Aside from replacement, noise filters (capacitors) can be purchased at marine electronics equipment outlets and are sometimes effective. These outlets can tell you how to install the filters.
Sometimes relocating equipment to get it away from the antenna on the receiving radio will do the trick. Distances will vary slightly, but usually a distance of at least three feet between the noisy device and the antenna does the trick.