Others have mentioned arcing as a possibility. We see more of than than we 
would like. There are many poor UHF connectors and (especially) adapters 
around. This is especially true with right angle connectors.

There is another possibility that could be happening. RF loves any wire it can 
find for return currents. The outer braid of the 40 feet of coax could be doing 
a great job of picking up RF and conducting it back to the KPA500. We usually 
see this with vertical antennas which are very happy to use the feedline for 
return current, but this also could be happening with your setup. The idea is 
that at HF, current flows on the edge of the braid. In this case there are two 
edges, one inside the cable (closest to the center conductor, with the second 
on the other (outer) side. The current you expect is flowing on the inside, and 
the outer can pick up anything stray, including the signal you are trying to 
radiate. This is conducted back to the amplifier where it really messes up the 
sample signal. I would also expect you to see RFI with this setup.

A third possibility comes from where you now live. As I learned long ago as a 
young ham in Miami, the South Florida heat is very harmful to coax cable. Over 
(an amazingly short) time the coax inner conductor can migrate through the 
insulation and come into contact with the shield. This is very common, occurs 
within just a few years (or months in some cases) and requires coax to be 
replaced every few years. It almost always occurs at bends in the coax. This, 
of course, leads directly to the arcing issue described above (and by others). 
Foam dielectric coax is probably the worst with this issue. The solution is new 
coax. And do your best not to bend the cable!

The solution to the second issue is a good balun choke. K9YC would advise 
running the coax through a large toroid several times to help resolve the 
problem. An alternate is to use a large number of coax-size toroids fed onto 
the coax, which will also choke off the unwanted currents. You might want to 
prefer the latter because of the same migration issue described above. In this 
case the turns on the toroid _will_ fail within a short time.

See K9YC’s excellent web page for more information. He has a very good paper on 
baluns, especially choke baluns.

Best o flick solving this issue. That South Florida sun and heat can play havoc 
with ham stations. Much different problems than those seen in 1-land!

73,

Jack, W6FB, ex-WA4FIB


> On Feb 23, 2018, at 9:42 PM, Chris Hallinan <challi...@gmail.com> wrote:
> 
> Greetings from southwest Florida.
> 
> My KPA500 is kicking out with REFL HI warning (high SWR).  But here is the
> strangeness. My setup is K3->KPA500->40' coax->MFJ 600W remote tuner -> 4:1
> balun -> open wire feedline to 80-meter doublet.  In words, I have a remote
> autotuner feeding an 80-meter doublet fed with 600-ohm open wire line.  A
> Balun Designs 4:1 balun provides the interface from the open wire line to
> the tuner.
> 
> With the autotuner tuned at low power, the SWR shows 1.5:1 on my KPA500 and
> external SWR meter.  As I run the power up to 300, 400, 500 watts,
> somewhere in there all of a sudden the SWR instantly jumps off the scale,
> into the red LEDs on KPA500, and it faults.  It's as if the SWR were
> dependent on power.
> 
> Even stranger, when I back the power off to say 250 Watts, holding key down
> the SWR is stable in the green (<1.5:1), but when I key it at CW rates, the
> SWR bounces into the red on every dit and eventually faults, when at steady
> state key down it never leaves the green zone!
> 
> I fear it is the tuner falling over, but it should be able to handle the
> power.  I see this on all bands from 80-20.
> 
> Operating into my 20-meter resonant vertical, with no tuner, everything
> works fine.
> 
> Any ideas or hints on how to correct this?
> 
> Thanks,
> 
> Chris
> 73 de K1AY
> 
> 
> -- 
> Life is like Linux - it never stands still.
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