VHF Echoes

Warren Updike of Towson, Maryland, asks:

“My Simrad RS8300 VHF has two full-function handsets. The VHF is a black-box type with a DSC class-C transceiver that is about 15 years old; it’s mounted behind a panel in the main saloon. The receiver is very sensitive and very easy to use. The transceiver interfaces with my Raymarine ST-50 GPS, which is mounted nearby. My AM/FM stereo receiver is also located near the GPS and VHF transceiver, and the only FM antenna is a short wire that leads to a nearby fixed port.

I’ve been told that my transmissions frequently include background music and, occasionally, voices; at times the music is so loud that my voice transmission can’t be understood. The problem is intermittent, but it does seem to occur at particular locations on Chesapeake Bay, where I sail. The U.S. Naval Academy has large transmission towers, and an Air Force facility is not far away. Have you ever heard of this phenomenon? Is there a RF filter I can install? Would an FM antenna splitter let the VHF and FM share the signal?”

Gordon West replies:

First, your FM antenna wire has no effect on the music and voice problem. What I think is happening is that the strong radio signals ashore are being induced onto your VHF remote head cables. When you are on VHF receive, you may hear a faint signal coming out of the VHF speaker. When you transmit, those shoreside signals are getting into your mike cords and remote heads and are mixing with your own transmissions.

Putting a ferrite snap-on filter choke at both ends of each of your two remote-control head cables will minimize or eliminate these induced signals; now the cables are acting like a dipole antenna. You can get a pack of four ferrite filters at your local marine radio store for about $25. When the filters are installed at both ends of the two cables, your problem should disappear.

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