Guess FCP as a concept has hit the big time if it's being bandied on Elecraft reflector. :>)
We do not have much feedback from 80 meter use at all. It's been 99.9% about 160 meters where the huge space crunch hurts the worst. You DO need to do all the measuring at the output of the isolation transformer. Otherwise the coax can completely screw up the clues you need to prune the thing. I'll wait to hear what you measure at the IsoT. The only way you can correctly measure at the shack end of the coax is if you are using an AIM 4170 and have run a remote calibration to the end of the coax. There are some other analyzing devices that can do the remote measure thing besides the AIM. The program that runs with the analyzer keeps the calibration data for that specific piece of coax and then uses a mathematical transformation to adjust out the effects of the coax. Cannot do that with the MFJ analyzers. On another issue, it was not clear from your post, but understand that a +/- 33 foot FCP is a 160 meter device only and the isolation transformer for that is 20 bifilar turns, and you will need something in the order of magnitude of 125 feet in the "L" to resonate. For 80 meters ONLY, the FCP is +/- 16 feet, and the isolation transformer is 15 bifilar turns to work with something in the order of 60-65 feet. You can use the 20 turn, but electrically it has a lot more leftover inductance, and most likely runs with shorter L's for the "simple" configuration. If you were doing that, you would be the first "guinea pig" on how to use the 160 IsoT on 80 meters. Nobody has figured out how to do a dual band FCP, nor can you just put them in parallel. You might get an impedance dip, but the field cancelling effect that makes an FCP work is simply absent off-band, and the ground loss is back. The "simple" configuration is the FCP, the isolation transformer and something approximately a quarterwave L, whose length gets adjusted to bring the zero reactance crossing point where you want it. If you want to get into the theory, there are more complex applications, like how to do a 4 square with FCPs, while not using any radials. We are talking here about the "simple" configuration. The actual value of the resistance at zero reactance a given person gets depends on the specifics of the "L" that you have up. If you get the zero reactance point right, significantly off-50 R values can be dealt with using a series matching transformer of 50 ohm then 75 ohm, then 50 ohm coax rest of the way, calculated by a little free program called SMC.EXE. Most often the basic shape of the L is dictated by available trees, staying hidden, or..., very specific to the individual. So we don't publish "ideal" dimensions for the L. The double dip is possible, but explaining why without a slide show of EZNEC graphs ain't possible for us normal mortals. Or it could just be an effect of your specifics and the length of your coax. I would like to know the specific dimensions of your L. If you are getting 50 +j1 at 3654, then the current wisdom is you need to add length to force it down. But we don't have 80 meter information on how far down you can force it before the FCP starts to "argue with you" and won't move any more. Moving it from 3654 to 3535 is adding something like 4 and a half feet. My current 160 meter antenna, up 90 out 105 over a 160 FCP, has a "sort of half-way decent" spot down around 1700, but it's nowhere close to adjustable to 50 +j0. It's also lossy, as the FCP has flipped out of its cancellation mode. This leads me to believe the upper dip is the one you want to manage. For the extended audience here, the concept is on W0UCE's web page at http://www.w0uce.net/K2AVantennas.html Sorry about the OT. We don't have an "FCP reflector". Reflector discussion has been entirely on TopBand. 73, Guy K2AV. On Tue, Feb 28, 2012 at 6:51 PM, Jim Bennett <w6...@mac.com> wrote: > Well, crud - my fat fingers messed up the original post. That reading at > 3.654 should have been R=50, X=1, SWR=1.0. I'll take the MFJ out to the > antenna base and see what I get. > > On Tuesday, Feb 28, 2012, at Tuesday, 3:43 PM, Fred Jensen wrote: > > > On 2/28/2012 2:22 PM, Jim Bennett wrote: > > > >> > >> 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. > > > > You have something seriously wrong, 0+j1 ohms is a short circuit, or > > extremely close to one. I'm sure the SWR reading at that point is > > bogus, the MFJ was not designed to operate into a short circuit. In > > fact, the apparent "short circuit" could be bogus, I've gotten some very > > improbable results a few times with my MFJ. > > > > I'd suggest: > > > > 1. Take your MFJ259 out to the feedpoint of the antenna and see what > > you get. > > > > 2. Put a dummy load at the antenna end of your coax run and measure the > > impedance at the shack end. It should be 50+j0 ohms everywhere [within > > reason] > > > > 3. Check the SWR with your K3 with the tuner in bypass at 3501 and up > > the band a few places and draw a graph of the readings. Keep the power > > down, you can't really hurt the K3, it'll quit making RF if it gets > > really annoyed. If the null at 3461 is really the resonant point of > > your antenna, you should see the SWR climbing rapidly as you go up the > band. > > > > When really weird things happen in conflict with known physics, it's > > almost always a simple error in the system or measurement configuration. > > I'd look carefully at the wiring. Let us all know what you find, I'd > > like to write it into my notebook for future ref. > > > > 73, > > > > Fred K6DGW > > - Northern California Contest Club > > - CU in the 2012 Cal QSO Party 6-7 Oct 2012 > > - www.cqp.org > > > > > > ______________________________________________________________ > > 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 > > ______________________________________________________________ > 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 > ______________________________________________________________ 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
