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Author Topic: Dual Band uhf vhf antenna issues  (Read 527 times)
GRADY
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Posts: 14




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« on: April 06, 2011, 07:48:00 PM »

I think I am about to pull my hair out trying to figure this one. FT8800R and a Comet CA-2x4SR antenna. The SWR on 2 meter is about double as advertised, and on 70cm it ranges from 1.3 to 2 which I have no issues with. On the 2m side, I get 3.8 from 147.000 and up, but below that is is 4.0 on the nose. Any suggestions? I have triple checked all connections. It is on an NMO mount from Larsen, and the vehicle is a Jeep Wrangler. The mounting location is on the rear tailgate, with about 1/2 of the antenna rising above the soft top. The atenna mount ground is pulled via a 14 gauge stranded wire back to the body, with the paint sanded away at the body and where the ground wire mounts to the antenna bracket I fabricated. The paint is sanded away as well for the 2 feet of the NMO mount to make metal to metal contact on the bracket. I would think that if it were a ground issue that it would show up on both bands. I have not yet checked reflected power or anything other than SWR. I cant find anything on the net about tuning this thing either, even at the comet website. All help appreciated.
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KF5KZX just the new guy on the block!
KC7YRA
Member

Posts: 256




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« Reply #1 on: April 06, 2011, 07:52:38 PM »

There is a significant difference between an electrical ground and an RF ground.  Jeep Wranglers are notoriously difficult to get a proper groundplane on.  And running a wire does not constitute a groundplane. 

Dollars to donuts, this is your problem.

Brad
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GRADY
Member

Posts: 14




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« Reply #2 on: April 06, 2011, 08:44:26 PM »

Would it be possible to effect one band but not the other? Maybe trying a ferrite core with a couple of wraps close to the antenna?
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KF5KZX just the new guy on the block!
WB6BYU
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Posts: 12973




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« Reply #3 on: April 07, 2011, 09:02:09 AM »

Grounding is most critical close to the base of the antenna - within inches, not feet.
A 6" wire connected to the vehicle is no ground at all on 440, and poor on 2m.

As a test, take some copper wire and make quarter wave ground radials connected
right at the NMO mount - use 4 wires each 19" for 2m and about 6" for 440.  You
don't need to be able to drive with the wires in place, just do it as a test.  If that
changes the SWR, you know that you don't have a good enough ground.

And, yes, the problem can show up on one band but not another.  For example, if
you had a pair of 12" wires running from the antenna base to the chassis, that
would be 1/2 wave on 440, and they would present a low impedance for RF.  But
on 2m they would be somewhat less than 1/4 wave and shorted at the far end,
which will be rather reactive and not a good ground.
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