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Author Topic:   SONAR Causes VHF Marine Band Radio Interference
Lohff posted 01-29-2008 12:35 PM ET (US)   Profile for Lohff   Send Email to Lohff  
I have had [interference] on the FM radio while the Bottomline Fish Finder is on. I've tried an additional battery and different wiring, but to no avail. Can only listen to CD's or the wife singing. HELP!!
David Pendleton posted 01-29-2008 06:46 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I have an older Raymarine Fishfinder that just spews RF. I don't have an audio system, but it does interfere with my VHF.

In my case, after working with Raymarine, I just had to live with it.

You might try a few ferrites (google: ferrite, or toroid ferrite, or clip on ferrite) on the transducer and power cables for the fishfinder.

You could also try them on the power and antenna cables for the stereo also.

Or, you may just have to live with it.

jimh posted 01-29-2008 08:42 PM ET (US)     Profile for jimh  Send Email to jimh     
Based on your description, it sounds like the SONAR is emitting radio frequency interference. The interference is not being conducted by common-mode power wiring. It appears to be radiated and picked up by the receiver antenna of the VHF Marine Band radio.

There is no cure for this except to suppress the radiation from the SONAR. You could try to isolate the cable between the SONAR control head unit and its transducer, keeping the cable away from the radio, its transmission line, and the antenna.

Use of ferrite beads might help. Try obtaining some large torroidal cores. Wind a few turns of the SONAR line on them, and see if there is any effect. Locate them as close to the control head unit as possible.

Chuck Tribolet posted 01-30-2008 08:57 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Ferrites on the antenna cables is probably a bad idea.
The ferrites suck up the higher frequencies, which is exactly
what you want to pump down the antenna cable.

The ferrite on the transducer cable should be as close as
reasonably possible to the fishfinder. Fortunately, in this
case the right thing is also the easy thing.

Jim: he said FM, not VHF, though that doesn't affect the


jimh posted 01-30-2008 08:36 PM ET (US)     Profile for jimh  Send Email to jimh     
Chuck--In an antenna transmission line no transmission line current runs on the outside of the outer conductor. There is absolutely no problem in using ferrite beads on a transmission line, although that is not I suggested. Ferrite beads are very commonly used on radio frequency transmission lines in order to suppress flow of antenna currents on the transmission line.

I suggested using ferrite inductors on the SONAR line, and placing them close to the control head unit. The purpose of that inductance is to suppress flow of radio frequency current on the SONAR line.

handn posted 02-03-2008 09:27 AM ET (US)     Profile for handn  Send Email to handn     
Excuse me if this is slightly off topic.
On my boat, my radar transmissions cause much interference with the sonar reading of my Interphase Twin Scope Sonar. However, the radar transmission cause no interference with my Furuno Sonar. I have no clue why this is so.
My vhf radio performance is not effected by either the radar or the sonar.

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