Topband: Inverted L SWR Jumps ???

[Jack Henry, OA4TT]

Hi Ashton,

I had some arcing into a coral tree at the 20 ft level.  The vertical part of the inverted L was  supported by a 40 ft bamboo pole that was nested close to the coral tree.  The wire was #14 with varnish that probably had long departed.  As the wind blew, the wire would come in contact with a branch and burn into it.  I was running an old TL-922 which doesn't have any fancy protection so it kept on doing it's job.  This was at about 1 kW.  I didn't notice any observations in the shack until my wife came screaming into the house that the tree was on fire.  I burnt halfway through a 3 inch  diameter branch. So there is a possibility this might be the problem depending on how your wire is routed.

73  Jack



--- On Tue, 11/27/12, Ashton Lee <Ashton.R.Lee at hotmail.com> wrote:

From: Ashton Lee <Ashton.R.Lee at hotmail.com>
Subject: Topband: Inverted L SWR Jumps ???
To: topband at contesting.com
Date: Tuesday, November 27, 2012, 9:49 PM

So I am trying to get set up better on 160 meters. I now have two antennas up (pretty well separated). One is an Alpha Delta DX A sloper hung in a tree with a grounding wire led to a ground rod and small radial field. The other is an inverted L on a good radial system of about 2000 feet in various lengths of about 50 feet each as fit the yard. Both are resonant at about 1.830 .

The sloper loads fine all the way up to 1500 watts. The inverted L loads just fine to about 700 watts and then causes the Alpha amp to fault out. I think I am getting a sudden change in antenna impedance. The antenna is fed through a 5 KW rated choke balun. The feed line exits the base between radials. I've tried various feed line lengths, I've replaced every component in the system except for the antenna wire. The antenna does climb along the branches of a tall pine before L-ing outward at about 55 feet. I think the problem is worse at night time when things are cold (and perhaps more humid).

What I see on the amp is output power suddenly seem to surge to 2500 watts, and reflected power jump from a few watts to over what the amp can read… then in a flash the amp faults out. This all happens with only about 20 watts of drive, so the amp can't actually be putting out 2500 watts unless something very strange has happened. As I noted, using the other antenna all is good.

I need to get the inverted L working since it seems to have substantial receive gain vs the sloper, so I assume it will be equally better on transmit.

All advice is welcome. Am I likely to be "arc-ing" to the tree branches? Could the wire be the problem? Do inverted L's have trouble with full power? The same wire worked fine for the last few years, but fed against a much lesser radial field and run through a less dense, lower tree.

I'll be trying everything I can think of tomorrow afternoon, starting by trying to minimize contact with the tree branches. All suggestions welcome.

73
KQ0C
Ash
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