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Author Topic:   Recommended Separation Distance Between VHF Transmitting Antenna and GNSS Receiving Antenna
Don SSDD posted 09-09-2015 09:33 PM ET (US)   Profile for Don SSDD  
I have a VHF with a GPS mounted on top of it, somebody told me the VHF antenna could cause [problems in GPS receiver reception or cause] damage [to] the GPS [receiver]. Anyone heard of that? Thanks--Don
jimh posted 09-10-2015 01:44 AM ET (US)     Profile for jimh  Send Email to jimh     
As a general rule I would not locate any sort of second radio antenna or radio receiver in the very near field of a VHF transmitting antenna.

The sensitivity of a particular GNSS receiver to being damaged by exposure to strong levels of radio-frequency energy around 156-MHz radiating from a transmitting antenna is not some sort of universal constant. The ability of a particular GNSS receiver to continue to operate properly while very close to a 156-MHz transmitting antenna would be a property of the individual device, not some general property of all devices of that kind.

It is best for your VHF Marine Band radio transmitter's antenna that it is not very close to anything else on the boat, including a GNSS receiver.

In the particular case being asked about here, where the GNSS receiver is part of the radio, the transmitting antenna needs to be at least three-feet away, not just to keep the GNSS receiver working but to keep the radio working, too. Radio transmitters are generally not designed to be exposed to strong radiation from their own transmitting antennas.

Don SSDD posted 09-10-2015 10:34 AM ET (US)     Profile for Don SSDD    
Thanks Jim, my VHF antenna is maybe 18 inches from the radio and the Raymarine GPS, it is all on the starboard side of the console.

Simple solution would be to move the antenna and leave the VHF and GPS as is.


jimh posted 09-10-2015 12:31 PM ET (US)     Profile for jimh  Send Email to jimh     
Maybe you could elevate the antenna a few feet with a short extension mast. Vertical separation is really more effective than horizontal separation.

If the antenna happens to be one of those not-too-expensive 8-foot-long fiberglass antennas, the actual antenna portion of the assembly is probably contained in the top three-feet of the fiberglass tube. If the antenna is an expensive 8-foot-long antenna, the actual antenna probably begins about two feet up from the base.

Don SSDD posted 09-10-2015 04:03 PM ET (US)     Profile for Don SSDD    
The antenna is a stubby one, maybe 18 inches. I guess I should get a cheap long one?


jimh posted 09-10-2015 08:22 PM ET (US)     Profile for jimh  Send Email to jimh     
Get a GAM ELECTRONICS SS-2 with an ADAP-II mount. Install it atop a Shakespeare 498 four-foot extension mast. See

Don SSDD posted 09-11-2015 05:38 AM ET (US)     Profile for Don SSDD    
Excellent Jim, thanks for the link and advice.


jimh posted 09-11-2015 11:01 AM ET (US)     Profile for jimh  Send Email to jimh     
An antenna for 156-MHz (the VHF Marine Band frequencies) that is only 18-inches long is a quarter-wavelength antenna, and would only be effective if it were operated above a metal ground plane, such as the roof of an automobile. I don't know of any antennas designed for marine use on the VHF Marine Band that are only 18-inches long. I think you will find a big improvement in performance when switching to a longer antenna that is designed to work at the VHF Marine Band on a boat, where a large metal ground plane is typically not available.
jimh posted 09-12-2015 09:46 AM ET (US)     Profile for jimh  Send Email to jimh     
It just happens that this morning I was looking at the specifications for a rather good GNSS receiver designed for marine use, the GARMIN 19x. The mounting instructions suggest at least a 3-foot separation, and to keep the GNSS receiver out of the main beam of the transmitting antenna. I think that is typical of most manufacturer's advice regarding minimum separation of a GNSS receiver from a transmitting antenna.

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