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Author Topic: mag mount grounding  (Read 651 times)
AE7GL
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Posts: 62




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« on: March 01, 2005, 04:27:35 PM »

How does a mag mount antenna get its connection to ground? I mean since many mag mount antennas have a plastic coating on the base to protect the vehicle surface how do they achieve a ground connection? To form the ground side of the ground plane as it were? Or do they?

Maybe this is a simple question but short of taking my mag mount apart I haven't figured it out yet.

Thanks,
Charlie
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KB1LKR
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Posts: 1898




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« Reply #1 on: March 01, 2005, 04:48:18 PM »

Capacitance. At DC it isn't grounded at its base (though the coax shield may be at DC ground at the other end) but at VHF/UHF the capacitance to the metal roof provides a moderately low impedance path to ground.
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AE7GL
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Posts: 62




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« Reply #2 on: March 01, 2005, 05:03:50 PM »

OK, that makes sense. So then at HF freqs would the capacitance be such that the grounding would not be as efficient?
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K0BG
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« Reply #3 on: March 02, 2005, 07:04:22 AM »

Far too many amateurs confuse RF and DC grounds, which leads to misunderstandings about the differences in DC ground and groundplane.

At low frequencies, a vehicle represents a capacitance to ground, not a groundplane. The losses involved are referred to as ground losses.

As the frequency is raised, the vehicle starts acting more like a groundplane. At VHF frequencies, it can act easily as a groundplane if the mounting scheme is sufficient.

A mag mount may indeed be DC grounded which doesn't mean a thing with respect to RF ground. A mag mount capacitively couples to the groundplane (in this case the vehicle under it), just like a vehicle capacitively couples to the ground at HF frequencies and adds losses.

The worse aspect to mag mounts has nothing to do with their additional ground losses however. It's the safety concerns should you have a crash. There has already been one reported case of a death caused by one, and most vehicle insurance plans do not cover such a loss.

Alan, KØBG
www.k0bg.com
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AE7GL
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Posts: 62




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« Reply #4 on: March 02, 2005, 02:09:58 PM »

Alan,

I fully understand the safety part of it. One could easily come off in an accident. That's why I have an NMO on the roof. I just use mine when parked and I want a better antenna for my HT. Rather than pulling the mobile radio outside I can just set the mag-mount on the vehicle and run the coax to the HT sitting on a table or somewhere.

My question was because I just didn't understand how the coupling worked to get the ground effect. And, by the way. You've got a great website. I've learned a lot from it.

Thanks,
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AA4PB
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« Reply #5 on: March 02, 2005, 03:26:23 PM »

You are correct. The lower the frequency, the less efficient the capacitive coupling between the mag mount and the vehicle. The efficiency is not too bad on 2M but on 75M it is very poor, even if you use one of the big mounts with three magnets on it.
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KF8RX
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« Reply #6 on: March 03, 2005, 10:03:07 PM »

I have a Little Tarheel on a three magnet mag mount. It works great on 40 mtrs, it's lowest freq. In fact it works better than the trunk lip mount I tried.

Eric
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K4KAL
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Posts: 13




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« Reply #7 on: March 08, 2005, 05:18:24 PM »

I have a High Sierra Sidekick mounted on a 3 Magnet Mount, using a Icom 706MKIIG.  SWR is 1.1 on 10 thru 80 meters.  Interesting discussion on the grounding, but I do know that the "T" Type 3-Magnet Mounts don't work as well as the one High Sierra sells which has a triangular metal plate instead.
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N3ZKP
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Posts: 2008




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« Reply #8 on: March 08, 2005, 06:30:14 PM »

<< I have a Little Tarheel on a three magnet mag mount. It works great on 40 mtrs, it's lowest freq. In fact it works better than the trunk lip mount I tried. >>   
 
<<I have a High Sierra Sidekick mounted on a 3 Magnet Mount, using a Icom 706MKIIG. SWR is 1.1 on 10 thru 80 meters. Interesting discussion on the grounding, but I do know that the "T" Type 3-Magnet Mounts don't work as well as the one High Sierra sells which has a triangular metal plate instead.   >>

With all due respect, I hope you two have lots of money in the bank. No insurance company will pay off on damage or injury caused by flying mag mounts.

ANY mag mount antenna is a projectile waiting to be launched in a 60 mph panic stop. An antenna as big and heavy as the little Tarheels and Sidekicks will not only come off in such a panic stop they will probably go right through the window of the vehicle in front of them. There is already been one confirmed death cause by a mag-mount coming loose and several other injuries, as well, and those were much smaller antennas.

No matter if the antenna makers sell them, they are outright dangerous when used on a vehicle in motion at highway speeds.

Lon

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