Update I took the Balun designs 4:1 out of line and put a Dx Engineering maxi core 5kw 4:1 in its place. It tunes a little better on 160 but the KPA1500 finally faults out and says cannot find match and shows 9:1.... better than 99:1
So I put the KPA1500 straight to a 2.5kw bird dummy load and it tunes instantly 1:1 in 160M So this fault is with the antenna for sure The only other Balun I have to try is a DX Engineering 6:1 which I will try in the morning. Failing that I will order a 1:1 feom DX Engineering But at least I know it’s the antenna that is causing this problem I’m going to also borrow a antenna analyzer tomorrow and see what gives. Thanks for all the suggestions! We will whip this yet Ronnie w5sum Sent from my iPhone > On Sep 2, 2020, at 6:05 PM, Alan - G4GNX <g4...@g4gnx.com> wrote: > > Ken, sorry this is rather long, but here goes............... > > I had a similar problem in the UK, with a OSCFD antenna, purchased from a UK > company. I didn't question the theory and just assembled it according to > instructions. > > As you would expect the antenna is made from two differing lengths of > ruggedised, plastic coated copper wire, which is fed by a 4:1 Guanella balun, > rated at 400W. The balun is potted in a weatherproof box and is fed via the > customary PL259/SO239 connector with Mil Spec RG58 coaxial cable. About 6' > down from the feed point is a common mode choke (sometimes called a sleeve > balun) which consists of 8 ferrite inline cores, around the co-ax and held in > place with heat shrink sleeving and cable ties. > > The rig was just the K3S/100W with internal tuner and all was well for many > months, mostly using 40 metres SSB. A while later, I added the KPA500/KAT500 > combo which I ran for a couple of very short periods at 400W to test. For a > weekly net, I used only 200W and all seemed OK for a while, until suddenly > the VSWR started to increase over the period of an 'over'. Over a few weeks > this got more frequent and I would have to re-tune part way through an > 'over'. At that time, I wan't sure whether it was an antenna issue, or the > KPA500/KAT500, although I suspected that the KPA500 was getting too hot, so I > posted a question on a forum and was contacted by Jack Brindle of Elecraft, > who kindly looked at the KPA500 fault reports and sent me a comprehensive > explanation of his findings based on the figures and his experience, which > concluded that the antenna was almost certainly the culprit. > > I ordered a new 1KW Guanella balun and some much bigger ferrites, with the > intention of replacing the RG58 with Westflex 103. In the meantime I > soldiered on with the original setup until one day all hell let loose, with > the KPA500 and KAT500 fault lights flashing and even the K3S got 'ticked > off'. At this point I switched to my HF vertical, to keep me on the air. > > When I dismantled the wire antenna, I expected a balun fault, but it was > actually OK. On further checking, after disconnecting the co-ax from the > balun, the open ended co-ax showed a DC short on a test meter! > > I used the Rig Expert 600 to determine exactly where the short was and it > turned out to be exactly where the sleeve ferrites had been, BUT there was no > discoloration of the outer covering, or signs of melting, so I knew it was > unlikely that the ferrite cores had got heated. I cut the section out of the > co-ax and started to take it apart. With the outer covering all removed, > there was still no sign of discoloration of the screen braid, but as I tried > to remove the braid, it was obvious that there had been considerable heating > as the center section of the braid has melted into the center conductor > insulation, and somewhere in there, it has made it right through to the > center conductor. > > Unfortunately I haven't yet found the exact point of the short, because I > left the partly dismantled piece of inner co-ax on a table and one of my cats > seems to have run off with it! :-D > When I eventually find it, I'll continue investigating. > > In the meantime, one theory is that the common mode choke presented a high > impedance to RF coming back down the co-ax and this in turn produced a very > high voltage causing an arc between inner and outer and as the > characteristics changed, in turn a high current node was created, which > caused the heating effect. Another theory is that it is just a crappy piece > of cable and it just failed. > > Unfortunately this is not the sort of fault that can be detected in its > development stage by a low power antenna analyser, it needs a fair amount of > power. > > This long tale may just give you some clues to your possible issue on 160m > although on a different power scale.. > > 73, > > Alan. G4GNX > > > ------ Original Message ------ > From: "Ken Ramirez de Arellano" <kp3mm...@gmail.com> > To: elecraft@mailman.qth.net > Sent: 02/09/2020 22:41:30 > Subject: [Elecraft] KPA-1500 SWR fault above 700-800 watts on 160m > >> Every bit of antenna hardware ( coax, balun, insulators, etc. have been >> changed out or proven good. Connections are all solid. My KPA1500 will >> transmit above 800 watts for about 15-30 seconds and then the SWR jumps up >> and it faults out for high VSWR. I can run 1500 watts on all the other >> “Legacy” bands (10,15,20,40,80) with no problems. I’ve forced an STU >> relearn and still the same issue. The antenna B amp is a Clipperton L and >> that transmits at a KW out and I see no change in VSWR in the external >> wattmeter using all of the same cabling. It’s starting to look like >> something heating up in the KPA1500. Is anyone aware of a known issue for >> this with the KPA1500 or a known fix? I would rather do a field repair >> instead of sending to Elecraft from KP4. TIA Ken, KP3MM >> -- >> Ken Ramirez >> KP3MM >> > > ______________________________________________________________ > 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 > Message delivered to w5...@comcast.net ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
