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" <[email protected]>
To: [email protected]
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
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