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  Radio Antenna Affects Compass

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Author Topic:   Radio Antenna Affects Compass
dave14 posted 07-25-2012 10:33 PM ET (US)   Profile for dave14   Send Email to dave14  
My marine radio antenna definitely had impact on the compass unless it was mounted on the top of the rail on the opposite side of the console.
Jefecinco posted 07-26-2012 09:19 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Dave,

I assume your VHF radio antenna only had impact on your compass when the radio was transmitting.

I believe that in receive mode the antenna is passive. I believe VHF radio antennas are constructed of non-ferrous materials and that the mounts, too, are usually non-ferrous so they should not affect compass performance.

I cannot remember the last time I used the transmitter on my VHF radio except to make a radio check call once or twice a year. For that reason I would not hesitate to mount a VHF radio antenna near a compass if it was the most convenient place to mount it.

Butch

jimh posted 07-26-2012 08:40 PM ET (US)     Profile for jimh  Send Email to jimh     
I can't imagine the typical VHF Marine Band radio antenna would affect a compass. Most of these antennas are made of non-ferrous metals and fiberglass. I don't know if radio frequency currents from the antenna (as would occur when transmitting) would affect the magnetic compass, but, if they did, one could just avoid taking compass readings while transmitting. Radio frequency current is an electro-magnetic wave, but it is an alternating current, and one would expect that an alternating current might not deflect the compass, as long as it was a symmetrical wave. I would be more suspect of the metal in an antenna mounting base. Perhaps the base was made of ferrous metal and only plated with chrome, insted of being a non-magnetic material like stainless steel.
davej14 posted 07-28-2012 10:15 PM ET (US)     Profile for davej14  Send Email to davej14     
The antenna is a GAM-2. I cannot recall the details because it was installed severa years ago. My antenna is mounted to a 1 foot stainless extension and fixed to the side of the grab rail by a Shakesphere stainless ratcheting clamp. No ferrous materials are used. I installed the compass after the antenna was in place with the same mount on the port side. Moving the mount to the starboard side fixed the problem for me.
Jefecinco posted 07-29-2012 10:33 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Dave,

Perhaps some of the materials used were not as stainless as advertised. I would be suspicious of the extension. If you used mounting bolts other than those supplied by Shakespeare they could be of lesser quality stainless steel.

Due to the difficulty in finding high quality marine grade hardware in my local area I often use "stainless" steel hardware from the usual big box stores. So far the big box stuff seems to be serving well.

Butch

knothead posted 08-04-2012 06:35 PM ET (US)     Profile for knothead  Send Email to knothead     
Here's a funny story about compasses and electricity that occured when I was solo moving a J-30 racing sailboat from Beaufort,NC back to our home port at Wrightsville Beach,NC after a Memorial Day weekend regatta. Being that it was 1986 the boat had no navigation equiptment other than two compasses each mounted on the starboard and port bulkheads respectively and a knotmeter with a knot log. The VHF was somewhat nonfunctional.


The wind was light leaving the seabouy at Beaufort Inlet so I decided to motorsail back. The rhumbline from the Beaufort Inlet seabouy to the Masonboro Inlet(Wrightsville Beach)seabouy is approximately 240 degrees, distance is about 65 miles most of which is out of the sight of land. My boatspeed was 6.5 knots so I figured the trip should take about 10-11 hours.

About an hour into the trip, being somewhat bored I pulled one of the stereo speakers out of the cabin and set it on the cockpit bench seat and spent the rest of the day fishing and listening to tunes while casually sailing along.

Along about 6:00 pm the wind began to freshen and the boat began to heel so I decided to take speaker below. While moving it I noticed the compass take a big spin. Hmmm, Lord knows what course I'd been sailing the last seven hours while out of the sight of land. One nice feature about the Atlantic Ocean, if you want to hit land just head due west, which I did, eventually making port about 6 hours later.

It was, as they say, a very good lesson.

regards---knothead

dfmcintyre posted 08-04-2012 06:56 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Another good lesson:

Years ago, my folks were on their annual cruise. Dad had the brand new autopilot set, and was in a relaxed but alert state. Nice calm water running up Lake Huron. All of a sudden, and for no reason, the autopilot started acting up. Disengaged and went to manual.

This behavior manifested itself throughout the trip. More often in the morning then afternoon run. Sometimes it would swerve to port....other times to starboard. This went on for a week.

He thought maybe temp related? Then noticed it would almost always happen when my mom went below.

To retrieve a jacket because of the morning chill.....

Huh?

He went below, with her at the helm and rummaged through the closet where the jackets were hung.

On steel hangers....

Right below (like two inches) where the electronic tech had installed the remote compass sender unit to the autopilot.

They had words.


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