I think you have to identify which of the two resonances is due to the antenna and not due to the antenna + feedline. Elevated radials that exhibit resonances of their own can also be confounding.
The multiple resonance may be due to a fortunate length of coax that presents a 50 ohm at the shack end. You can identify which resonance is due to the antenna by attaching a short length of coax at the antenna feed and find the resonance with your antenna analyzer. Then you will know if you have to add or subtract wire. The fact that SWR "goes up" when you keep adding wires (@ complex part =0) only means that the impedance at resonance is changing. At resonance (complex part=0) different antennas have different values of Z Inverted L's and verticals have Z's of 30 to 35 ohms at resonance with a really good ground system. That isn't an SWR of 1:1. A yagi may have a Z of 15 ohms-- hence a big SWR. Once you have the antenna resonant where you want, then you have to deal with the mismatch. A simple L network at the base of the antenna will take care of the mismatch. 500 watts doesn't require monster components. 73 de Brian/K3KO On 2/28/2012 7:20 PM, Don Wilhelm wrote: > Jim, > > I did a little chatting with Guy K2AV when he was starting to work on > that project (W0UCE lives only a few miles from me), and I recall that > changing the height above ground changed the behavior. > So if you do not have the lowest wire 8 feet above the ground, it will > not be tuned properly. > Maybe Guy will comment, he frequently posts on this reflector. > > 73, > Don W3FPR > > On 2/28/2012 5:22 PM, Jim Bennett wrote: > >> 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 t r >> > an > >> 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. :-) >> >> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > > ----- > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1424 / Virus Database: 2113/4837 - Release Date: 02/28/12 > > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
