It appears that the higher voltages at QRO are enough on return to fault the 
amp.  That could be an arc or the match isn’t really 1:1.   Using an antenna 
analyzer at the balun would be useful. 

If you’re on 80M, without the tuner, you’re likely to see ~3:1 untuned at the 
balun, within the range that should almost work, BUT it also depends on the 
length of the open wire feed.  (Using a 75 ohm feed would be wiser if used for 
only 80M). 

Try modeling it on EZ-NEC, then adjust the feed (in the model) by some feet to 
see what that does.  If you’re using it for multiband, you’ll have to run it 
for each band.  Getting it to work well on all bands is a serious challenge, 
near impossible; something has to give. 

It’s likely too that the model won’t show coupling or losses exactly to the 
real world (metal, trees, walls), so if it suggests you make changes, you’ll 
still have to adjust to your real antenna.  In short, go play. 

My 340’ 80M EDZ was textbook perfect (2 opposing 5/8 wave, center fed, NOT 
resonant) but didn’t load that way, until I added almost 30’ more antenna AND 
shortened the feed by 50’ in the real world.  After more than 200 countries 
later (on all bands 160-10), I’d say it worked pretty well.   Not easy, but I 
was patient.  Having a K3 made the difference, often the DX did too. 

Good luck, it’s one of the fun things to do in ham radio. 


Smell Czech correction happen

> On Feb 23, 2018, at 9:42 PM, Chris Hallinan <> 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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