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Author Topic: swr temp changes for the better after I smit  (Read 867 times)
VE4AOD
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Posts: 23




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« on: April 22, 2010, 08:09:36 AM »

Hi.I have a KLM 3 band beam. 15 and 10m swr are ok. 20M shows best swr (1.8:1) at 14.330 on my ant analyzer UNTIL I go back to using the radio and xmit. SWR then drops to where it should be lower on the band and when the analyzer is reconnected an swr graph is great. After about 3 minutes swr goes back up on the analyzer until I repeat the above steps. Same thing agn after a few minutes. Any help/ideas would be greatly appreciated. Why would the swr temp change under power then revert back? 73...Jack
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N3OX
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« Reply #1 on: April 22, 2010, 08:16:39 AM »

Sounds like an oxidized connection of some kind to me. Some power gives enough voltage to break through an insulating barrier but maybe it reforms...

This happens with relay contacts a lot.  You got a remote antenna switch in there somewhere?

Actually... I guess a MINOR resonance shift would probably point to an oxidized contact in reflector or director more than it would DE or somewhere in the feedline...
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WX7G
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« Reply #2 on: April 22, 2010, 09:09:55 AM »

It sounds like something is heating up. The 3 minutes it takes for the SWR to revert tells us the part heating has some thermal mass. So it's not a relay. The change in resonant frequency tells us the L or C is changing. It sounds like a trap heating. That it does not happen on 10 and 15 meters (or does it?) tells us that it is a 20 meter heating when it is operating as a loading coil (that's what it does on 20 meters). A loose wire in the trap? Water in the trap? You might transmit for a long time - perhaps late at night when 20 meters is dead - to see if heating whatever it is dries it out and fixes the problem. By a long time I mean an hour.
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WB2WIK
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« Reply #3 on: April 22, 2010, 03:45:26 PM »

You didn't say what model beam it is, but the KLM tribanders I've had didn't have any traps.

What model beam is it?
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KE3WD
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« Reply #4 on: April 22, 2010, 04:35:33 PM »

I'd be suspecting moiskture intrusion or possibly oxidized connections somewhere. 

Aluminum, rivets, etc. 

Might even be the feedline, I'd check that also, moisture intrusion, connections, etc. 

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W8JI
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« Reply #5 on: April 22, 2010, 05:36:55 PM »

Sounds very much like oxidation on a connection, probably in the reflector if it is actually in the antenna. It COULD be elsewhere, but it is common in the antenna.

Since it is a minor change in frequency upwards and minor SWR increase, it is almost certainly in the reflector.  If a director opens or partially opens, the antenna normally will shift lower in frequency. If a reflector opens or partially opens, SWR goes up and so does resonant frequency.

I would check all my coax connections first though, because they can cause an unpredictable shift if they have a bad connection.
« Last Edit: April 22, 2010, 05:38:34 PM by Tom Rauch » Logged
WA3SKN
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Posts: 5457




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« Reply #6 on: April 23, 2010, 05:17:49 AM »

There is a resistance change.
Can be caused by water, corrosion, bad connections, spider nests, etc... lots of possibilities.
First replace the antenna with a dummy load and verify good connections and no water affecting the feedline.
Then check, inspect, and clean all the antenna's connections, insulators, etc for corrosion and oxidation.  You will find something.
73s.

-Mike.
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