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KC8AXJ
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Posts: 303




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« on: December 13, 2003, 12:50:05 PM »

I have a coil :

http://www.godshand.com/ebay_stuff.htm

I am going to put a 102" whip on it,with a big spring.

Question is this.

The BEST place (ground plane wise) for the antenna is over the roof of my Dodge Durango.I have thought of making a mount to go on the roof rack and guy it as well.

As there is always a trade off, the problem here is that baby will be really up there and could wack things.

Second choice is making a mount to my rear hitch and having it (mast) come up enough so the whip portion would be at about roof line therefore having a lower height (better than one foot) to the deal.

The second choice would also give me a stronger mounting system overall too, and protect the coil.

As the second choice would make my radiation pattern SOMEWHAT directional, as the ground plane is all on one side basically of the antenna's "view", is it THAT big a performance loss ??


73
Steve
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AA4PB
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Posts: 12800




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« Reply #1 on: December 13, 2003, 03:42:41 PM »

On the HF bands the size of the vehicle is small enough in terms of wavelength that it really doesn't act as a ground plane but rather capacitive coupling to the earth. I don't think you'll notice a big difference in radiation pattern in either location. I have my whip mounted on the rear corner of the pick up bed in a stake pocket (thus there is no metal for 270 degrees around the antenna) and I haven't noticed any directivity. It is more important to keep the whip itself away from metal in order to minimize capacitive coupling to the vehicle. Low on the bumber is bad from the perspective that a good percentage of the whip runs close to the metal. Up on the edge of the bed and at the rear corner keeps it well away from surrounding metal.
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K0BG
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Posts: 9867


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« Reply #2 on: December 13, 2003, 10:25:48 PM »

Bob is giving you the correct scoop. While there is some directivity to side, back, front or side mounting, in the real world the differences are minimal. There are more important things to worry about. Like proper bonding, proper wiring, noise abatement, mounting the rig (or faceplate) is a safe and convenient matter, and permanently mounting any device to minimize dangers in the event of an accident.

Do a search here on eham.net for articles to help you on your way.

Alan, KØBG
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