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Old 05-18-2006, 05:47 PM   #1 (permalink)
sgtrogerp3
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high SWR - CB antenna ground

i mounted an antenna on the driver's side fender. The center wire in the antenna cable goes to the stud, and the antenna wire outer braid (ground?) was grounded to one of the fender bolts. i still can't get the swr's below 2, and i've adjusted the tunable antenna tip. do you think i'll have better results if i ground it right to the antenna mount itself, or does it matter as long as it's grounded? the wilson fiberglass antenna i'm using is about 7 years old, so i ordered another (in case the antenna is shot).
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Old 05-18-2006, 06:30 PM   #2 (permalink)
88mag
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I'm not sure exactly how your doing that, but normally the only connection is the coax connector itself, with the outer ring being connected to the coax shield. The actual ground connection to the vehicle is thru the stud. The stud has an insulated connection thru the mount, to the whip antenna....If you are splitting out the coax rather than having the coax terminated on a proper connector, you can disrupt the impedance of the transmission line (coax)...This can certainly throw out your SWR...

Mike
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Old 05-18-2006, 07:08 PM   #3 (permalink)
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What Mike said...and also make sure your radio itself is well grounded.

Another note: some radios adjust SWR internally. I've seen these radios' SWR hover around 2 no matter what else you do. An SWR of 2 is not catastrophic by any means. It's nice to get it below 2 obviously, but as a practical matter, you're not going to see a big performance gain going from 2 to say...1.3. Nor will an SWR of 2 damage your radio. Just my 2 cents.
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Old 05-18-2006, 07:26 PM   #4 (permalink)
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I've been out of CB and ham for a while, but I always aimed for 1.5:1 or better. Just to echo, grounds are the place to start.

If that doesn't work try repositioning the antenna.

I was always obsessive with keeping swrs down - I spent hours on a 40 meter long wire, dropping and trimming an inch at a time. Final result was 1.04:1
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Old 05-18-2006, 09:26 PM   #5 (permalink)
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Another thought....If the radiator, the whip itself is very close to the fender, that forms a capacitor, which would alter normal characteristics...

If you do a search, you will see that I made a homemade bracket...Although I made sure to ground it properly, the whip is very close to ground (the fender) about an inch from the end...

To reinforce the poster above, he is correct about the performance gain issue...If we are going to mount our antennas based on asthestics, there will be a price to pay....A little less efficiency...but I'm happy with mine..

Also, keep in mind that the SWR will vary wih the channel used to test it....I set mine up using channel 20....I didn't get below 1.5....the best mount is in the very center of the vehicle...


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Old 05-18-2006, 09:55 PM   #6 (permalink)
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the wilson fiberglass antennas have a grounding wire built into them. depending on the application it may work better without it. I was running dual 5 footers on my old truck and they had better SWRs without the ground wire attached. but if you do use it you have to make sure its a clean conection or it wont work, so no paint no grease no dirt.
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Old 05-20-2006, 04:13 AM   #7 (permalink)
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Quote:
Originally Posted by VB Dually View Post
What Mike said...and also make sure your radio itself is well grounded.

Another note: some radios adjust SWR internally. I've seen these radios' SWR hover around 2 no matter what else you do. An SWR of 2 is not catastrophic by any means. It's nice to get it below 2 obviously, but as a practical matter, you're not going to see a big performance gain going from 2 to say...1.3. Nor will an SWR of 2 damage your radio. Just my 2 cents.
SWR can only be adjusted at the antenna. Anything else is just fooling your meter similiar to what an antenna tuner does. The center core of your coax goes to the stud or antenna only, the ground does not, that's why its insulated. Make sure you have a good coax 18 feet long to start with for any wilson antenna. The only purpose of the ground it to create a "Ground Plane" to develop the radiation pattern of the antenna system. Also, if you're using a cheap SWR meter or the one in a radio don't worry too much about what it says just get it as low as possible. The ohmage of the system is more important than the reflected power. A good Meter will cost at least $300 such as the MFJ 259B. Hope this helps.
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Old 05-22-2006, 09:44 AM   #8 (permalink)
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I used the antenna mount near the fender well also, and I started out with a 4' firestik antenna, and could not the SWRs below 2:1. I tried a 3' firestik, and believe it or not I was able to tune the better than 1.2:1 in the band center and better than 1:5:1 across the whole band. I talked with firestik, and they said it was most likely to be because of the lack of ground plane at that location.
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Old 05-22-2006, 03:36 PM   #9 (permalink)
zr2yz125
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I could not get my SWR down while using a shorter coax cable. I bought a 16 or 18' (I dont remember which) and I can pull SWR around 1 on ch 20
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Old 05-22-2006, 10:21 PM   #10 (permalink)
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I'm going to start an argument here, but I will say this anyway:


IF your mounting bracket is truly grounded, then the number of feet of coax should be irrelevent.

Just do what I did, and take a grinder and grind down to bare metal, then screw down your mount....Then, spray that lithium grease all over it....for corrosion protectin.
t
I don't claim to be the know it all, but something tells me you don't have the proper mount...If you are a welder, run a bead from the mounting stud to the fender...And you might run a ground strap from the fender to the firewall..

Also, TRUE SWR can only be adjusted at the antenna itself, like SB says..


Good luck..


Mike
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