VHF Radio: Effect on Compass - Moderated Discussion Areas

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  VHF Radio: Effect on Compass

search | FAQ | profile | register | author help

Author Topic:   VHF Radio: Effect on Compass

Royboy posted 06-11-2005 04:56 PM ET (US)
I'm mounting a Icom 302 radio in my 13 Sport and the instructions caution to mount the transiever no closer than one meter from the boat's compass. I'm not too sure how to achieve this in a 13, so: what's the reason for this requirement?
Thanks,
Roy
fourdfish posted 06-11-2005 07:44 PM ET (US)
My radio is only 16 inches from the radio and I hooked it up and tested it for variation before I mounted it. (Moved it around while on and while broadcasting) it does not affect it's performance. Still right on according to the GPS. However the antenna is further away.
Sal DiMercurio posted 06-11-2005 07:45 PM ET (US)
The magnet in the speaker of the radio can pull the compass off by as much as 180 degrees.
Sal
fourdfish posted 06-11-2005 10:06 PM ET (US)
Only if it's close enough!
Sal DiMercurio posted 06-11-2005 10:24 PM ET (US)
Thats why they say 3' or 3 meters.
Sal
Royboy posted 06-11-2005 11:05 PM ET (US)
I hooked everything up and testee it out before I mounted the transceiver. The compass didn't move during broatcast, so I mounted it as far as I reasonably could, which is about 24". Everything seems to be working normally, but I'll have to check it all out (GPS, Compass, and Radio) on the water. Thanks everybody for the responses, as always you were very helpful.
Roy
fourdfish posted 06-11-2005 11:06 PM ET (US)
Sal--3 meters is 9 feet and after 60 yrs and five boats I have never had my radio close enough to cause any variation in my compass. The speaker is small and any effect can be checked easily. In a 13 Sport practical application is necessary. It is better to mount the compass in a central location and the radio as far as possible away and still be at the disposal of the pilot. 1 meter(3 ft) is not always practical nor needed.
Knot at Work posted 06-12-2005 08:27 AM ET (US)
Explains why I get lost.
and can't tell them on the radio where I am.
jimh posted 06-12-2005 08:55 AM ET (US)
The magnetism in the loudspeaker in the VHF Marine transceiver is a permanent magnet, and the flux of its magnetic field is not affected by the electrical conditions of the radio, that is, whether the radio is on, off, receiving or transmitting, the magnetic field from the loudspeaker magnet is more or less constant.
When direct current flows in a wire it generates a small magnetic field. If a wire was close to a compass and a very high current flowed through it, it could affect the compass. However, in most direct current wiring there are two wires in the circuit, with current flowing in opposite directions, and thus, if the wires are in close proximity arranged symmetrically to each other, the magnetic field generated from the current tends to cancel. In some cases to enhance this self-cancellation of any residual magnetic field, the (insuated) wires are twisted together in a very uniform fashion.
Most VHF Marine radios only draw a modest current on transmit, and their power leads are usually paired and molded together. I do not think you will find it likely that current flowing to the radio during transmission will affect the compass, unless the wiring for the radio was running through the middle of the compass and had very poor self-cancellation characteristics.
It is commonly seen that the wiring to the internal electric lamp which illuminates the compass is twisted to enhance self-cancellation. Of course, the lamp is in very close proximity to the compass, so even its tiny current needs to be considered as a possible influence.
fourdfish posted 06-12-2005 11:27 AM ET (US)
Jim--The speakers have a permanent and an electromagnet which work together to move the cone to produce the sound. They both
produce fields which can affect the compass. If you were testing the effect relative to distance you would want to test it both on and off.(By the way E-TEC fuel injectors are based on this) I have tested it and have found an effect up close next to it but not 15 or 16 inches away. Most VHF radio speakers are small needing a very small magnet and need to be close to cause any signicant variation. At 7 inches away I found almost no effect with my Standard Horizon. However, up close when I turned it on the needle moved 2 degrees. Without noticeing, I once left my knife which is magnetized next to the compass and we were off all day.
jimh posted 06-12-2005 07:55 PM ET (US)
The current which is applied to the VOICE COIL of the permanent magnet loudspeaker in order to produce an aural output from the loudspeaker is principally an ALTERNATING CURRENT. In general alternating currents do not produce a steady magnetic field because they alternate polarity. Although the excitation voltage applied to the loudspeaker to modulate its magnetic field is not precisely symmetrical, it is generally a symmetrical alternating current, so its effect would probably not be great in terms of creating a new magnetic field.
In very old radios there were speakers which were not made with permanent magnets. A DC Current was applied to the loudspeaker to magnetize it. This type of loudspeaker is virtually unknown these days.
An extraordinarily sensitive detector of magnetic fields is the color registration in a three-color CRT--a color television. If you take a powerful loudspeaker and hold it near the CRT, you will observe the effect on the display by the distortion of the colors. Now, apply some audio frequencies to the loudspeaker and see if they distortion changes. I bet it does not. I believe that the magnetic field emitted by the loudspeaker's permanent magnet is stronger than any influence from the exciting voltage. Or at least it will be until the loudspeaker is OVER DRIVEN. The aural distortion you hear when a speaker is over driven is a result of the exciting voltage on the loudspeaker being greater in force than the fixed flux of the permanent magnet.
It is probably feasible, but I don't think too likely, that the loudspeaker's effect on the compass could be modulated by the exciting voltage, but I would suspect you would need to have the volume turned up very high to see any variation. Because the radio sits with the audio squelched about 99-percent of the time, I don't think this is a serious problem.
If the compass needle jumps when the radio is just turned on, it is probably due to magnetizing current in the power leads. Check that the leads are not too close to the compass, and, if possible, twist the leads or otherwise make them as symmetrical as possible so their fields cancel.
fourdfish posted 06-12-2005 10:39 PM ET (US)
Sorry Jim, I think we are off the point of this thread. I don't really have any problems with my situation. Just relating my experiences.

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: VHF Radio: Effect on Compass
Royboy	posted 06-11-2005 04:56 PM ET (US) I'm mounting a Icom 302 radio in my 13 Sport and the instructions caution to mount the transiever no closer than one meter from the boat's compass. I'm not too sure how to achieve this in a 13, so: what's the reason for this requirement? Thanks, Roy
fourdfish	posted 06-11-2005 07:44 PM ET (US) My radio is only 16 inches from the radio and I hooked it up and tested it for variation before I mounted it. (Moved it around while on and while broadcasting) it does not affect it's performance. Still right on according to the GPS. However the antenna is further away.
Sal DiMercurio	posted 06-11-2005 07:45 PM ET (US) The magnet in the speaker of the radio can pull the compass off by as much as 180 degrees. Sal
fourdfish	posted 06-11-2005 10:06 PM ET (US) Only if it's close enough!
Sal DiMercurio	posted 06-11-2005 10:24 PM ET (US) Thats why they say 3' or 3 meters. Sal
Royboy	posted 06-11-2005 11:05 PM ET (US) I hooked everything up and testee it out before I mounted the transceiver. The compass didn't move during broatcast, so I mounted it as far as I reasonably could, which is about 24". Everything seems to be working normally, but I'll have to check it all out (GPS, Compass, and Radio) on the water. Thanks everybody for the responses, as always you were very helpful. Roy
fourdfish	posted 06-11-2005 11:06 PM ET (US) Sal--3 meters is 9 feet and after 60 yrs and five boats I have never had my radio close enough to cause any variation in my compass. The speaker is small and any effect can be checked easily. In a 13 Sport practical application is necessary. It is better to mount the compass in a central location and the radio as far as possible away and still be at the disposal of the pilot. 1 meter(3 ft) is not always practical nor needed.
Knot at Work	posted 06-12-2005 08:27 AM ET (US) Explains why I get lost. and can't tell them on the radio where I am.
jimh	posted 06-12-2005 08:55 AM ET (US) The magnetism in the loudspeaker in the VHF Marine transceiver is a permanent magnet, and the flux of its magnetic field is not affected by the electrical conditions of the radio, that is, whether the radio is on, off, receiving or transmitting, the magnetic field from the loudspeaker magnet is more or less constant. When direct current flows in a wire it generates a small magnetic field. If a wire was close to a compass and a very high current flowed through it, it could affect the compass. However, in most direct current wiring there are two wires in the circuit, with current flowing in opposite directions, and thus, if the wires are in close proximity arranged symmetrically to each other, the magnetic field generated from the current tends to cancel. In some cases to enhance this self-cancellation of any residual magnetic field, the (insuated) wires are twisted together in a very uniform fashion. Most VHF Marine radios only draw a modest current on transmit, and their power leads are usually paired and molded together. I do not think you will find it likely that current flowing to the radio during transmission will affect the compass, unless the wiring for the radio was running through the middle of the compass and had very poor self-cancellation characteristics. It is commonly seen that the wiring to the internal electric lamp which illuminates the compass is twisted to enhance self-cancellation. Of course, the lamp is in very close proximity to the compass, so even its tiny current needs to be considered as a possible influence.
fourdfish	posted 06-12-2005 11:27 AM ET (US) Jim--The speakers have a permanent and an electromagnet which work together to move the cone to produce the sound. They both produce fields which can affect the compass. If you were testing the effect relative to distance you would want to test it both on and off.(By the way E-TEC fuel injectors are based on this) I have tested it and have found an effect up close next to it but not 15 or 16 inches away. Most VHF radio speakers are small needing a very small magnet and need to be close to cause any signicant variation. At 7 inches away I found almost no effect with my Standard Horizon. However, up close when I turned it on the needle moved 2 degrees. Without noticeing, I once left my knife which is magnetized next to the compass and we were off all day.
jimh	posted 06-12-2005 07:55 PM ET (US) The current which is applied to the VOICE COIL of the permanent magnet loudspeaker in order to produce an aural output from the loudspeaker is principally an ALTERNATING CURRENT. In general alternating currents do not produce a steady magnetic field because they alternate polarity. Although the excitation voltage applied to the loudspeaker to modulate its magnetic field is not precisely symmetrical, it is generally a symmetrical alternating current, so its effect would probably not be great in terms of creating a new magnetic field. In very old radios there were speakers which were not made with permanent magnets. A DC Current was applied to the loudspeaker to magnetize it. This type of loudspeaker is virtually unknown these days. An extraordinarily sensitive detector of magnetic fields is the color registration in a three-color CRT--a color television. If you take a powerful loudspeaker and hold it near the CRT, you will observe the effect on the display by the distortion of the colors. Now, apply some audio frequencies to the loudspeaker and see if they distortion changes. I bet it does not. I believe that the magnetic field emitted by the loudspeaker's permanent magnet is stronger than any influence from the exciting voltage. Or at least it will be until the loudspeaker is OVER DRIVEN. The aural distortion you hear when a speaker is over driven is a result of the exciting voltage on the loudspeaker being greater in force than the fixed flux of the permanent magnet. It is probably feasible, but I don't think too likely, that the loudspeaker's effect on the compass could be modulated by the exciting voltage, but I would suspect you would need to have the volume turned up very high to see any variation. Because the radio sits with the audio squelched about 99-percent of the time, I don't think this is a serious problem. If the compass needle jumps when the radio is just turned on, it is probably due to magnetizing current in the power leads. Check that the leads are not too close to the compass, and, if possible, twist the leads or otherwise make them as symmetrical as possible so their fields cancel.
fourdfish	posted 06-12-2005 10:39 PM ET (US) Sorry Jim, I think we are off the point of this thread. I don't really have any problems with my situation. Just relating my experiences.