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truthiness 03-31-2013 11:35 PM

Wacky SWR readings
I'm running a Midland 75-822 handheld with the Cooltech Gatemount setup which includes 18' coax with molded Firering and 4' tunable Firestik firefly antenna. Initial install gave me SWR hitting about 3; wound up sanding paint off of my ground point under the rear tie down loop and dialed in my SWR to a respectable 1.2 on channels 1 & 40. At channel 19, needle didn't move, a solid 1, I was psyched.

Hopped in my car tonight and noticed my SWR was 1.2 on 1, but had creeped up to 1.5 on 40 and I wanted to tweak it. There was a slight drizzle and after I removed the cap on my Firestik and adjusted my tune, I went back into the Jeep and my SWR was 3+ and I couldn't get it to move no matter what I did with the antenna or ground.

Any suggestions?

Jerry Bransford 04-01-2013 01:06 AM

Reinstalling the vinyl tip-cap adds electrical capacitance which will dramatically change the antenna's SWR. You have to tune the antenna to one side of its point of minimum SWR so reinstalling the vinyl tip cap will bring it back down to where you want it. It's a trial and error thing but it's not so hard. You'll quickly find which side of minimum SWR the antenna needs to be tuned to so reinstalling the vinyl tip cap lowers the SWR back down to its minimum (good) point.

truthiness 04-01-2013 01:12 AM

It was tuned to 'in use' conditions with the vinyl cap down to 1.2.

With a light drizzle SWR went to 1.5 on channel 40, when I took cap off to tweak (from my previous trial and error I knew it was probably less than a half turn on my Firestik) I came back into the cab and everything was above 3 and tuning did not respond.

Any ideas? Was it because it was wet out?

I guess what I'm trying to say is that I couldn't even get it back to previous 1.2/1.5, it just blew everything to 3 with just me removing the antenna cap and fiddling with the tune. I thought maybe something got wet somewhere because it was sprinkling out. Probably not a smart move, I know.

Jerry Bransford 04-02-2013 07:43 PM

Anything touching the antenna can change its SWR. So while I've never measured the SWR with a dry vs. a wet antenna, knowing the many factors that can affect an antenna's resonance point there is no doubt that rain could affect it.

truthiness 04-02-2013 07:51 PM

SWR is still stuck at 3+ no matter what I do even after doublechecking my ground contact. I'll have to check my connections this weekend when it warms up.

Jerry Bransford 04-02-2013 08:39 PM

Perhaps the nylon insulating shoulder washer isn't properly seated & allowing the antenna to touch the steel mount which shorts it to ground. Or the mount's lower part that is supposed to touch bare metal on the under side of the mount isn't tight or not enough powder coating was removed to make the RF ground's electrical connection reliable.

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