Question about Cobra 29 LTD CB Radio

1 Answer

Cant get SWR below 3.5

Hey,

I have a Uniden PC76XLW, which I have read is pretty much the same as the Cobra 29, that's why I posted here. I have it hooked up in my pickup truck. The antenna is a wilson 5' fiberglass, mounted on a plastic tool box, as far back on it from the cab as possible. I have a ground wire running from the negative shield of the coax where it connects to the antenna to the chassis of my truck. I have checked for continuity, and get good continuity from the positive lead on the PL-259 connecter at the radio to the tip of the antenna. There is no short along the coax or any connecters, and there is also good continuity from the negative lead on the PL-259 to the chassis. I think this is how it's supposed to be hooked up. I cannot get any transmission or reception on the radio, although I can hear faint static. When I try to tune the antenna to the radio with its built in SWR meter, the needle doesn't move no matter if I adjust the tip up or down. It reads well over 3. I'm guessing because there is no short in the antenna system or anything and I can't adjust any SWR that it's something internal.

When I got the radio, it had a blown protection diode, which I replaced. I was using a friend's magnet mount antenna which would get the SWR almost to 2, just to try it out. Now with a new antenna, it worked briefly while I was testing it very quickly, with high SWR (I hadn't had a chance to ground the antenna yet. Did that mess it up maybe), but then once I hooked up the ground, it hasn't worked.

Any suggestions? Thanks

Posted by on

  • halebp May 31, 2009

    Doesn't the chassis serve as the ground plane?

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  • Master
  • 437 Answers

A ground wire will not replace the lack of ground plane. Try a different antenna altogether (possibly one with a magnet mount) and mount it on some metal surface like even the front hood and retest.

I am sure the problem you will find is with the lack of a proper ground plane.

Posted on May 31, 2009

  • Thomas Neubauer
    Thomas Neubauer May 31, 2009

    Think of ground plane as a physical reflector of surface area. You could try bolting the antenna down temporarily to just about anything but the plastic tool box. You need metal under that antenna to act a ground plane. There are antennas that do not require the ground plane. These are special use antennas for things like fiberglass boats etc.

    Because these radios are pretty tough you can even have a friend move the antenna around while you are attempting to take the SWR reading (dead key) and watch for it to fall. Twin antennas (like on big trucks) are easily tuned this way.

    I hope this helps.


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