Electrically short CB antenna (1 Viewer)

[Hayes]

Hey folks.
Wondering if anyone has some wisdom to share:

I just finished installing a CB radio system in my LX470. I've got a Cobra 75WXST and a 3' Wilson FGT-3 fiberglass antenna mounted on the rear ds corner via a LabRak style mount bolted into the lift gate bumper, and connected via an 18' run of Wilson coax with PL-259 and FME connectors. The antenna sits on a ss spring and a quick-connect.

Ground at the mount is good.
No shorts that I can find.

Using the little Astatic PDC-1 I'm getting an SWR near 1 on ch40 and ~3 on ch1. That's with the tip tuned all the way out.
Electrically short.
If I connect the extra ground that is on the Wilson antenna the SWRs drop across the board.

Should I try another antenna?
Should I just use the extra ground wire as intended? (Although this defeats the quick connect)
Thoughts?

Thanks for your time.

 

[1911]

My experience is that quick-connects and springs almost always complicate tuning the antenna and makes SWR worse. Try it without those and see what you get for SWR. If you can tune it acceptably that way, then add the quick-connect and the spring back in one at a time and test again; then you will know which is the problem.

The other possibility is insufficient ground plane, if not enough of the antenna reaches above the roof line. The Wilson web site does not say which wavelength that antenna is made to, but you may need a 1/2-wave antenna back there as they are much more forgiving of ground plane than a 5/8 or 1/4-wave antenna.

 

[ChaseTruck]

...
Should I just use the extra ground wire as intended? (Although this defeats the quick connect)
...

How about installing a disconnect plug in that extra ground wire?

About that radio - make sure you have a good place to hang the handset. My wife has that radio in the (open...) FJ40, it's usually laying on the passenger bench seat. And, inevitably, it ends up speaker-side down...

 

[Kofoed]

There should not be that amount of SWR difference across the cb band. You have added the QD and spring to the system, which has changed the load, as somebody has pointed out. That extra length should make the SWR higher on ch 40 than 1. Close the doors and hatch, etc., and try it again it a parking lot.

So the whip is like one foot away from the body for its full height? Not good placement.

 

[Hayes]

Thanks for the input.
Yeah, I tried with just the antenna and no springs or quick connects and got the same result.

 

[1911]

Thanks for the input.
Yeah, I tried with just the antenna and no springs or quick connects and got the same result.

Then try a 1/2-wave antenna.

 

[Hayes]

I tried a 4' Firestik II on the same spring and quick connect. Some quick tip tuning got me to SWR 1.2 on both ch 1 and ch 40, with ch 19 at essentially 1.0. Success.

Both this antenna and the 3' Wilson are 5/8 wave antennas. Not sure if the solution was the Firestik itself or the 4' length getting more antenna above the truck, or both, but I'll take it.

Thanks for the input everyone.

 

[Kofoed]

Maybe the antenna element of the first whip was no-good. I don't know why firestik sells the 5/8 wave spiral wound around the stik. You get ZERO benefit of the directivity of the 5/8 wave vertical antenna. They should just make a 1/2 wave and forget it