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Need help with antennas and ground plane

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Sep 11, 2011
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#1
I have a 2005 Freightliner Coronado and just purchased 2 Predator 10K antennas. I was told by the store to use just a single antenna on the driverside. I ran new coax to the driverside antenna since the old set-up was for 2 or co-phased. Hooked the antenna to a mirror mount with a Fire-stick stud . Thinking I'm on my way, I checked my SWR's and it was over 3. So I started looking for connection problems. When I couldn't find any thing, I just happened to open the door and saw the SWR drop to 1 with it open about 2". I was confused about this and started to research and found that the problem might be a no ground plane issue since the cab is all fiberglass. But why am I getting great SWR's with the door open? is this a no ground plane problem? Would it help to go back to a co-phased with both antennas? Any help would be GREATLY appreciated. Thanks
 
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#2
Not necessarily a ground plane issue (though that would be part of the problem with antenna performance), but rather I am imagining the mount may not be electrically grounded properly. And when you open the door, some metal makes contact with other metal and improves the electrical ground.
 
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#3
The first question is did you tune that antenna? They all require it you know. By opning the door you changed the electrical characteristics (reactance) present -around- that antenna. That will certainly make the SWR change, among other things.
- 'Doc
 
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#4
I drove a Freightliner Columbia and i can tell you that the factory coax that goes to both antennas are trash. The swr's with the stock antennas are not even close to where they should be. They must just install them on there and there you go. (Yours could be different but i doubt it). I used a Firestick with an adjustable tip and 18 feet of coax that i bought. I have my own swr meter and know how to tune it so i done that myself. Im thinking i got it down to a 1.5 on Ch19. I have had to run a ground wire from the mirror mounts to a ground somewhere, but not always. I slipseated alot and drove alot of trucks and it seems every truck i drove was different.

Yes when you open the door the swr's will change no matter how you got the antennas tuned good or bad, so you have to tune it with the door closed. Dont tune the antenna next to another truck or a building or it will change when you get away from it. That sounds silly but i know its true.

So i would say run about a 10 gauge ground wire to a good ground somewhere and try that, its not gonna hurt it any, just try to keep it away from your coax.
 
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#5
First, thanks for the responses, I really appreciate all the info i can get. This is all new to me and I'm determined to get it right. I did install RX8 and what a difference from the 58. I did have the door closed but seen the high SWR and was told if was above 3 it must be a connection problem. I did trim a little off the whip maybe 5" in 1" cuts. When I didn't see much of a difference I was baffled. So, I will be going back to work tomorrow and will try the ground wire. Thanks again.
 
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#7
I never did like trimming and cutting on the whips or any other antenna. You have to be just right when you cut it. If you cut it too short, well you cant add more to it.
A 4' Firestick 2 5/8 wave i believe with adjustable tip is what i use. Everyone you talk to will recommend a different antenna. Dont remember what kind of coax i use but I know i bought it at the Pilot along with the Firestick 10 years ago i guess.
 
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#8
Well, today was a great success. I ran a 12 gauge solid wire and grounded the mirror and the antenna mount. And presto the SWR's came down to 1.1 on channels 1 and 20 then 1.2 on 40. I was lucky to run the ground wire through the mirror frame for a factory look. So thanks again for all the replies it was very helpful.
 
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