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Author Topic:   Interference to SONAR from Radio Transmission
highanddry posted 09-03-2006 11:24 PM ET (US)   Profile for highanddry   Send Email to highanddry  
Odd thing that since panel/flush mounting my 987c and installing the new radio I have found an odd problem. On high power when I key the microphone the 987 goes beep and the screen flickers. It does not do this with the VHF on low. I think I will proably have to move the antenna which is mounted now on the console grab rail. The Lowrance did not do that but then--well--things happen. I have both wired to the auxilary fuse buss and grounded to the common ground bus that were both provided on my Nantucket from the factory. I think that the antenna being so close is producing a strong enough field to interfere with the 987c with the VHF at high power. I guess I need a new antenna anyways. I had it cabled to a handheld but this panel mount has more power. I suppose mounting the antenna to the bow rail and letting the coax run under my Dri-Dek may work. I hate drilling holes and the console is so convienient--darn that! Oh, the antenna is a 50 dollar Shakespere 5241R. Seems to work fine other than the glitching with the 987c. Both a question and a heads up.

Anyone have similar occurences?

Here is a picture and no, the antenna was not flipped down during the problem interference. Little changes cause big problem. I also have the antenna coax coiled in the console for the first time in an 8 to 12 inch coil, hmmm.

Casco Bay Outrage posted 09-04-2006 02:05 PM ET (US)     Profile for Casco Bay Outrage  Send Email to Casco Bay Outrage     
Don't think it is the antenna.

I have read here that if you have to coil the extra wire, make the bundle in long loops, then twist it to make a figure 8. This helps cancel out the interference. Locate the extra cable as far away from the radio and GPS as possible. [The value of such a configuration is doubtful. See below. --jimh]

highanddry posted 09-04-2006 04:48 PM ET (US)     Profile for highanddry  Send Email to highanddry     
I think I cured it. I had cable tied the coax as it came out of the console to the hand rail. Removing the tie and pushing the coax away from the hand rail eliminated the glitching. I do have to relocate that antenna at some point or get an extension or whatever but it is working now. Go figure.
jimh posted 09-04-2006 10:40 PM ET (US)     Profile for jimh  Send Email to jimh     
Can you tell me where you read about twisting the transmission line to the antenna into a particular shape. I want to find that bad advice an remove it from the website.



highanddry posted 09-05-2006 02:05 AM ET (US)     Profile for highanddry  Send Email to highanddry     
The instructions for the antenna did say to coil it into a loop of at least eight inch diameter. It mentioned nothing about figure eights or pretzals but he is just trying to help out and often strange things do help.
jimh posted 09-05-2006 11:31 AM ET (US)     Profile for jimh  Send Email to jimh     
Forming a coaxial cable transmission line into a solenoid coil can help to suppress the flow of antenna currents on the transmission line by creating some inductive reactance which will oppose them, but if such currents are a problem it is really a symptom of other problems in the installation that ought to be addressed and remedied.
Casco Bay Outrage posted 09-05-2006 12:43 PM ET (US)     Profile for Casco Bay Outrage  Send Email to Casco Bay Outrage     
Jim -

Sorry for the incorrect advice. Did a search and found it (love your engine!)

Guess I forgot the other good information in the thread.

Sorry guys :(


davej14 posted 09-05-2006 02:50 PM ET (US)     Profile for davej14  Send Email to davej14     
Here is another option for your antenna mount. Higher is better.

I also have the 5241R and I elevated the mount with t 2 foot Shakesphere stainless antenna mast extension.

highanddry posted 09-06-2006 03:15 AM ET (US)     Profile for highanddry  Send Email to highanddry     
Nice boat in the picture and that is exactly what I am considering doing, an extension. I notice that the coax is cable tied to the hand rail and that alone seems to be causing the glitching onmy installation. If the cable touches the hand rail--glitches--- if I push it away so that it does not touch I get no glitches on the 987c sonar when I key the mike on the VHF. The glitches are such that the 987c will beep, the screen will flicker and once it powered off. This does not occur at all now that I removed the coax from touching the hand rail.

I think your extension idea looks good. I will look more into that.

jimh posted 09-06-2006 07:23 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the pointer to the previous discussion. The advice contained in that prior discussion on CONTINUOUSWAVE to form the excess cable into a figure-eight does not suggest that there is any electrical improvement to that configuration. The suggestion that such a configuration will provide an electrical benefit seems to be entirely the invention of this current discussion.

Forming the excess transmission line into a figure-eight is a good way to mechanically lay the cable into a bundle because cable will not be twisted. This technique is used to lay up large diameter electrical cables so that they can be stored or carried without introducing any twists in them which would make later unwinding very awkward. There is nothing wrong with such advice, it is just that such a configuration is unlikely to also confer any special benefit to the cable if it is a coaxial transmission line cable.

There is a benefit if a coaxial transmission line cable is formed into a cylindrical coil. Such a winding will create an inductive reactance which will tend to oppose the flow of any antenna currents on the outer skin of the coaxial transmission line.

There is a great deal of misinformation contained in boating websites about radio transmission lines, and my goal is to not contribute further to that problem by publishing more of it here.

jimh posted 09-06-2006 07:34 AM ET (US)     Profile for jimh  Send Email to jimh     
The interference between the radio transmissions and other electronic devices located in close proximity to the base of the antenna which you describe is one of the precise reasons why I changed my antenna installation to use an elevated antenna. See

VHF Marine Band Antennas For Small Boats

davej14 posted 09-06-2006 09:01 PM ET (US)     Profile for davej14  Send Email to davej14     
I wonder if bonding your hand rail to your battery negative bus would help with the interference problem? Might be worth a try.

Next year I plan to replace the Shakespeare 5241R with the GAM antenna recommended by Jimh. I like Jim's installation but I found that the fiberglass extension was about twice the cost of stainless (which doesn't make sense). I stayed with a 2 foot extension because that was the maximum overall height that I could fold down inside my bimini. Since you have a larger boat you can probably utilize a longer extension.

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