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vhf antenna mounting question

Old 02-26-2012, 06:43 PM
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Default vhf antenna mounting question

Iam adding a second vhf and to get distance from the two antennas I will need to put the new one real close to my gps antenna. Will this cause any interference to either unit. Thanks for any help. Don,t won,t to drill and then need to move one
Old 02-26-2012, 06:46 PM
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Generally not recommended. Most manufacturers will recommend at least three feet between active antennas.
Old 02-26-2012, 06:56 PM
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That's what I was afraid of. I,ll look around at how some of the other guys have there's set up. I have room front to back but it will be hard to keep wires out of site and keep a clean look
Old 02-26-2012, 07:35 PM
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I'm not sure there will be much of an issue. The VHF is only transmitting when you key the mic. Otherwise it's receive only. All GPS antennas are receive only, too. The newer ones are powered and output data via NMEA 0183 or NMEA2000. Older antennas were simply an antenna. The only conceivable interference would be interference when you key the mic. Since they are on VERY different frequencies, I'd be surprised if you would have any issue unless you literally had them touching. I've had them mounted a foot apart and never had an issue.
Old 02-27-2012, 05:23 AM
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I'm with Buoy Scout on this one. On my last boat I had the following on my top.

VHF Antenna, AM/FM Antenna, Garmin GPS Antenna, Garmin Radar, XM Weather! I had a decent size top but since it was a soft top I was limited to the aluminum structure for mounting locations. Not to mention the fact that I had outriggers up there as well. I had this setup for over two years without any problems. The only suggestion I would have is to keep the two VHF antennas as far apart as you can and don't worry about the others.
Old 02-27-2012, 06:33 AM
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Thanks guys I think I,ll put it near my gps witch will be away from the other vhf and hope for the best
Old 02-28-2012, 04:51 AM
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I too had the same question. I called Shakespeare technical support and they strongly recommended that the VHF antenna not be within 3 feet of any other type antenna as it could and likely would interfere with output or reception signals.

I do see where many are not meeting these criteria in distance and folks seem to not have an issue, or they think that they don't, but how do they know until they are 50 miles offshore and can't reach anyone? This is a science so far no one has been able to answer...when the device does not put out as much as before some level of interference was created.

I suppose, mount them and then see. Fortunately, both my VHF's are exactly 3' from each other and on the back two corners of the hardtop. I have been debating to install a AM/FM in the foward middle, but will look to mount maybe more forward to keep it 3' away if possible. If I find an issue, I can always lower the other antenna's to see if it is a problem if I ever need to transmit to the furthest extent possible....kinda like when you need a tow.....

So, it is good to hear folks say it is not an issue. I have moved forward with that, but always in the back of my mind is the issue could be there and is not easy to detect. Sorry, but that bugs me. But that is just me...
Old 02-28-2012, 06:33 AM
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The 3' "rule" applies to antenna's that have an appreciable length as compared to the length of the VHF antenna. This interaction of two antenna's of similar length can cause a de-tuning effect. The greater the ratio of antenna length's, the less interaction. GPS antenna's are extremely short in comparison to the VHF antenna and would cause little to no interaction. In fact, most GPS antenna's use a horizontal patch antenna element inside the antenna housing that has virtually no vertical height at all. Keep your VHF antenna's at least 3' apart but you can get much closer to a GPS antenna without problems.

Eric

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