http://elecraft.85.s1.nabble.com/OT-Need-Antenna-Tuning-Help-tp7327188p7333646.html
and have been dealing with the tuning issues. A quarter-wave-ish wire over
an FCP and an isolation transformer is a complex, dual tuned impedance. It
sometimes does NOT prune simple like a dipole. We don't have a lot of
based on Jim's experience. We will post on the W0UCE web page when we have
> 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. :-)
> >>
> >>
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