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Author Topic: Can DC ground cause RF in the Car?  (Read 572 times)
K6JCR
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« on: February 20, 2010, 06:45:35 PM »

Hi Folks,

I'm always interested in safety and a thought occurred to me the other day regarding mobile antenna grounding.  I understand there is a difference between RF ground and DC ground.  I have a 2m/70cm whip mounted on a roof rack.  The mounting bracket has piece of rubber between itself and the rack.  Since the mounting bracket is essentially insulated from the roof, I'm puzzled then how the roof can act as a ground plane without a direct connection like radials have on vertical antennas.  I had taken comfort that the roof acts as an RF shield.  But is it possible that the DC ground wire from the rig to the battery is acting as the second element of a dipole and radiating inside, blasting me with up to 50W of UHF?

I don't have an RFI problems that I know of, I assume everything is working as it should.  I'm just puzzled by this since I'm still doing a lot of learning.

Thanks,

John
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K0BG
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« Reply #1 on: February 21, 2010, 07:40:41 AM »

When there is an inadequate ground plane, and/or excessive ground plane losses, there will be common mode current flowing on the outer surface of the coax shield. If it is enough to cause RFI problems, you definitely want to correct the problem.

This thread would go on for weeks if you get into the moot scenario about mag mounts vs. clip mounts vs. permanent mounts. The truth is, the best way to mount any vehicle antenna, is to place as much metal mass directly under the antenna as possible. Mag and clip mounts cause more problems (ground loops?) than just ground losses, and should be avoided.

As for roof racks... people use them to avoid drilling holes (another moot scenario). While antennas so placed appear to work well, most amateurs don't have the equipment to measure the field strength difference, which can be significant. One question you should ask yourself is this: Why don't commercial two way radio shops use mag and clip mounts?

Let me add something here. There are dozens of flawed experiments posted on the internet about the dangers of RF exposure. The truth is, unless two criteria are met, the chances of damage to human tissue is zero. Those two conditions are: The power AND the frequency have to be high enough to emit ionizing energy. That's almost an impossibility at legal amateur power levels below about 1 GHz.
« Last Edit: February 21, 2010, 07:46:05 AM by Alan Applegate » Logged

K6JCR
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« Reply #2 on: February 21, 2010, 12:38:23 PM »

Oh cool, that was good info.  I searched on common mode current and finally got to my next level of understanding how it all works.

Thanks,

John
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AA4PB
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« Reply #3 on: February 21, 2010, 12:41:01 PM »

The capacitance between the bracket and the carrier may some path (although poor) for the RF ground currents. The performance would probably be improved by providing a good, short connection between the mount and the roof but that may not be practical.
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N3MQM
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« Reply #4 on: February 23, 2010, 04:53:38 PM »

I agree with Allan on this, but the rack is BOLTED to the roof, so it is not insulated from the body. That being said, it is a bad connection.
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K0BG
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« Reply #5 on: February 24, 2010, 06:23:34 AM »

Think about this scenario for a moment.

You install an HF mobile mount on a frame extension bracket, with a 4 foot post welded to it. You mount the antenna atop the post. The post is well DC grounded, and so is the base of the antenna. However, the question which needs to be ask is, is the base of the antenna RF grounded? Or, what does the ground plane look like? The truth is, there isn't an adequate ground plane directly under the antenna. Thus, ground losses increase, and in this case rather drastically. What's more, the RF has to return to the source, so it flows down the coax cable and/or the motor control leads as common mode current. In this case, the CM is high enough, that you almost can't choke it off, so the cables act like part of the antenna, causing all sorts of RFI problems.

With respect to a mag mount and VHF antenna; If you have the equipment to measure the field strength, between an antenna mounted via a mag mount, and that exact same antenna mounted via an NMO mount, you just might be surprised at how much difference there is, especially at angles below 20°. And, I suspect you could measure a similar change between the NMO and the roof rack, and it doesn't make any difference if the roof rack is DC grounded or not. Again, it isn't DC ground we have to worry about, it is an RF ground, which is actually the ground plane, and the missing half of our antenna.

Putting this into more common terms; How good of a radial field is the roof rack, compared to the radial field offered by the roof of the vehicle? To some, these are moot questions, when in reality, they are not!
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