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
