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Re: [TowerTalk] Antenna problem?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna problem?
From: Ian White G/GM3SEK <gm3sek@ifwtech.co.uk>
Reply-to: Ian White GM3SEK <g3sek@ifwtech.co.uk>
Date: Thu, 6 Oct 2005 17:48:30 +0100
List-post: <mailto:towertalk@contesting.com>
K7LXC wrote:
>Howdy, TowerTalkians --
>
>    I've got an antenna problem and I'm stumped. An M2 15M monobander
>developed some high swr - 3:1 or more where it used to be 1.1:1.
>
>    I took a crane and man-basket up to the feedpoint and measured the swr
>with my MFJ at the balun; it was 1.2:1. I next measured at the jumper to the
>tower - same thing. Then I measured the end of the feedline in the 
>shack - same
>thing. As far as I was concerned, the antenna system was fine.
>
>    Plug it into the FT-990 and the swr on the rig was 3:1. Antenna tuner was
>off. Add different lengths of RG-8, the swr varied from 2.5:1 to 5:1.  It
>doesn't seem to be wx related.
>
>    Like I said, I'm stumped. Any help out there? Solutions appreciated!

Something very similar happened here a few weeks ago, and it turned out 
to be a bad PL259. The center pin had become intermittently open-circuit 
when plugged into the jack at the end of the feedline, but it was making 
contact when plugged into the different jack on the MFJ.

I had left a 50R load on the shack end of the cable, so from out at the 
antenna I was able to test the SWR of both the antenna and the cable. 
Both looked OK.

What I didn't do (but should have) was to pull and twist the plug and 
the jack while each one was connected to the MFJ.

Shake head, screw it all back together, go back to the shack... high SWR 
again. But now I knew it had to be the connector itself, and when I 
unscrewed it again, the plug had failed completely.

That was an intermittent center conductor, but in this case the 
variations of SWR with cable length suggest an intermittent shield.


In another case a few years ago, a defective double-jack adaptor was 
failing to make contact with a perfectly good PL259, while the same plug 
worked fine in the jack of the MFJ.



-- 
73 from Ian G/GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
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