OT: Need Antenna Tuning Help

Posted by Jim Bennett on Feb 28, 2012; 10:22pm
URL: http://elecraft.85.s1.nabble.com/OT-Need-Antenna-Tuning-Help-tp7327188.html

I've got a K3 and a three month old KPA500. Trying to get 5BWAS; 80 meters is my biggest hurdle. I put up an 80 meter Inverted L several months ago - prior to getting the KPA500. The vertical part of the antenna is 40 feet in length. I had been using two radials, about 14 inches above ground - small lot, CC&R's, HOA, etc. The antenna worked fairly well with my K3 and the internal tuner. I had done some pruning of the radials to get it resonant on 3.550. The K3 tuner took care of the rest. Well, along came the KPA500 and as yet, no KAT500. So I started looking at the antenna to see what I could do. I opted to replace the two single radial wires with a K2AV Folded CounterPoise (FCP). Not sure if this was a mistake or not. Everything I've read said that the FCP approach on 80 / 160 meter Inverted L antennas would result in a greatly improved signal. What has happened is that I cannot for the life of me get this thing resonant around 3.520. And yes, I did build the isolation tran
 sformer, per the instructions.

Here is what is happening and I am certainly perplexed. I'm using an MFJ-259B meter, at the shack end, to take my readings. I get TWO SWR nulls. Yes, two. One is at 3.461 and has R=35, X-0, and SWR of 1.3. The second null is at 3.654, R=0, X=1, SWR=1.0. In an attempt to get the antenna resonant at 3.520 or so, I started adjusting the length of the antenna. Adding wire to it makes the two "nulls" go down in frequency, as I would expect. If I instead shorten the antenna wire, the "resonant frequencies" both go up. However, there comes a point with both approaches where the change of frequency stops, but the SWR goes up.

As you can guess, I am NOT an antenna expert. Why do I get two different resonant frequencies, separated by only 200 KHz?

I've spent the better part of two days futzing around with this thing and am about at my wit's end. I don't necessarily want to toss the entire FCP assembly in the trash, and go back to the two radials, but this is not going as I'd hoped! If someone on this list who has a good handle on wire antenna tuning can give me some pointers and/or hints, off the list, I'd certainly appreciate it. :-)

Regards, Jim / W6JHB
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