posted 06-21-2001 09:59 AM ET (US)         

There are two ways that this interference could be making it's way from your fishfinder to your VHF.

1) Noise that is generated from the fisfinder and then reftlected back down the power leads. The filter that Mark suggested may help with this type of problem. Just make sure that the filter is designed to pass as much current as your VHF draws at full power. A pie (greek symbol) filter would be best, but the sales clerk at Radio Shack would probably glaze over if asked.

2) Radiated noise either from the unit or one of the cables. The only way to correct this is to physically seperate the units and cables. Most wave propagation decreases in intensity logarithmically; radiated "interference" does so as well. So moving the units away for each other will improve the situation.

How, does one determine which type of interference one has? Try these in your garage:

Method one. If your radio is easily removable, such as on a gimbal mount, make a longer power cable and move the VHF temporarily to the bow or the stern of the boat. Don't move the mount just make a longer power cable and lay the radio in the stern, bow, or even on the floor of the garage. Use a short piece steel clothes hanger to act as an antenna, it should fit into the antenna jack but don't force it. DO NOT TRANSMIT!!!, not only would it be illegal, but you risk blowing the transmitter section without a proper antenna load. Turn on the VHF and the fishfinder. Is the interferrence gone or diminished? If yes you probably have radiated interference. If not then you probably have noise in the power line.

Method two. Disconnect the power and the gound lead from the VHF, yes both of them. Also disconnect the antenna, use a small piece of clothes hanger again as an antenna. Using a small 12 volt battery or 2 6 volt lantern batteries in series; connect the VHF up to the temporary power source. DO NOT TRANSMIT!!! Turn on the VHF and the fishfinder. Is the noise gone? If it is then you pobably have interference that is being reflected back down the power line. If not you probably have radiated noise.

Power line noise may be helpd with a filter.

Radiated noise is a different problem. The noise could be leaking from the fishfinder or one of the cables, including the power cables. My bet would be on the fishfinder or the sonar transducer cable. The only solution is to find the source and isolate it from entry point on the VHF. You might try disconnecting the sonar transducer cable and see if the noise stops. Is sonar transducer cable and the antenna cable close together, especially coiled up together? If so try seperating them. Are the power cables coiled up together or physically parrallel? If so try re-routing one of them.

Hope this helps.