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Author Topic: Mobile grounding question  (Read 256 times)
W1FRR
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Posts: 11




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« on: February 18, 2008, 05:11:54 PM »

With a Diamond K9000 motorized mount the advice is to ground to the car body. My question is how it should be accomplished. Where to take off the ground strap from the mount?
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W5FYI
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« Reply #1 on: February 18, 2008, 06:34:52 PM »

   The instructions probably would be clearer if they explained what type of ground they wanted. Chances are, they mean electrical ground--a conductor that leads to the negative terminal of the battery. It's much like the headlight and taillight bulbs on many cars, a single wire goes from the light switch to the bulb socket's center terminal, but the barrel of the socket uses the common ground, for example the car's steel chassis, to which the battery's negative terminal is also attached. Your antenna mount needs this electrical ground to have its motor work properly. For a non-metal car, you can simply run another wire from the mount back to the battery to accomplish the same thing.

   The radio-frequency ground is a bit more complicated. It doesn't have to connect to the battery at all, but it should tie all the metal in the car into one massive "radial" system or counterpoise. That is, the chassis, hood, trunk lid, doors, side panels, bumpers and motor should all be grounded to each other, as well as to the coax braid near the antenna feed point. While this ground doesn't need to connect to the battery, it doesn't matter if it does.

   Hope this helps. Stew, W5FYI
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K0BG
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« Reply #2 on: February 19, 2008, 06:44:45 AM »

Doesn't make any difference if you RF ground it at all. The antenna is mounted well above its image plane, and as a result the losses are high. While it may be trick, from an operational standpoint, it sucks.

The reason they tell you to ground it is for a battery return for the motor.

Alan, KØBG
www.k0bg.com
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WA7NCL
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Posts: 625




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« Reply #3 on: February 19, 2008, 03:51:22 PM »

Seems like the motor power should have two wires in the control cable.

I also seem to remember some of the commercial mounts don't provide an RF ground and the instructions mention using a "ground independent" antenna.

Since the mount you are talking about is probably for VHF or UHF, you might get away with marginal grounding if the antenna is derived from some sort of half wave antenna type.  Then it might qualify as "ground independent".

K0BG is right about image plane but since you're not drilling holes in your roof, I assume you are willing to make compromises.  I would try to connect a ground through the shortest path to the car body I could, then check the SWR.  If you can't get it matched up, you might not have a good enough ground.

Sometimes I think if the coax runs along the car body for a ways, you can get some ground coupling that way as well.

All of this stuff is not the best system, but you might get it to work well enough to live with.  If it doesn't work, you can try more heroic measures like drilling holes, giving up on the motor mount, etc
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