Good news Ray, I swear by Ian's choke design. It has worked miracles here. As I mentioned in an earlier post I went straight to the 3 section GM3SEK choke, in fact two of them, one at each end of the coax section. Works a treat. I can't speak highly enough of the KPA500/KAT500 combination.
73 Stephen G4SJP On 15 January 2014 15:59, Ray Coles <raycole...@gmail.com> wrote: > I am pleased to relate that I seem to have finally nailed this problem, and > I think many of those who gave me helpful advice and the Elecraft design > team will be interested in learning what the final diagnosis and cure > turned > out to be. A quick recap: About 2 months ago, operating at about 75W into > my > trusty OCFD, the KAT500, KPA500 and K3 SWR readings shot up into the red > and > all DX fun ended abruptly. The problem remained, so I started looking for > the culprit: I pruned tree branches that were touching the antenna after a > recent storm. No joy. I replaced my 300W 4:1 balun at the end of my RG58 > buried coax feeder with a 1KW unit. No Joy. I replaced the buried RG58 > feeder with UR8 which took a new route, avoiding burial. No Joy. I dropped > the antenna and replaced the 300ohm twin feeder from the balun to the T > piece, the T piece itself and the antenna wire and dog bones. No joy. At > this point I consulted the Reflector Oracle, and the most common response > was common mode current on the feeder caused by OCFD unbalance. I did > research and decided to build the renowned GM3SEK choke balun. I chose the > mid range version which covers broadly 5-14MHz and installed it just south > of my 4:1 balun at the North end of the feeder. No Joy. Further ideas from > reflector pundits suggested that it might still be common mode, but coupled > from the antenna into the feeder where my GM3SEK choke wouldn't help. > > Fortunately I had bought enough Fair Rite oval cores to build all three > (Lo/Mid/Hi) balun designs, so in a final roll of the dice I built the two > core Hi Band balun and installed it in the Shack next to my rig. The hi > band > version has a fairly flat impedance curve and reaches down to 7MHz and up > to > 30MHz. IT WORKED! I have run 100W on all the bands from 40m to 10m and I > have seen no run away SWR readings. My 25W tune value stays rock solid. As > a > final push-my luck step I turned the KPA500 to operate and ran 400W SSB. No > Sweat! All of these things were no-nos yesterday. > > The final diagnosis is as follows: Some simple fault perhaps a connector or > a crimp termination caused the first occurrence. I then compounded the > problem by replacing my buried RG58 with an unburied and, I now see, a more > parallel routed feeder which locked the problem in via antenna coupled > common-mode currents. This also explains the fact that I could keep > operating provided I kept power below 40W. Some respondents felt there was > a > KAT problem which compounded the problem, I always felt that the problem > lay > outside the shack but that maybe, just maybe, the KAT might be a bit > sensitive to the problem. The only thing I would ask the Elecraft design > team to investigate is whether the SWR circuitry has to respond to the > common mode currents as it does. Probably not a firmware issue though! > Thanks to the many people who made so many helpful suggestions. E Pluribus > Unum! > > > > Ray Coles, C.Eng. M0XDL > > 10 Littlemoor Road, > > Weymouth DT3 6AA > > Tel: +44 (0) 1305 833699 > > Mob: 07831 516517 > > > > > > > > ______________________________________________________________ > 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
