[Elecraft] KPA-1500 SWR fault above 700-800 watts on 160m

[Victor Rosenthal 4X6GP]

I had an interesting experience trying to use a balun in a very 
demanding situation. I'll describe it, and at the end I have a question.

Antenna was a dipole with total length of 10m fed with approximately 480 
ohm open-wire line. Line length was 10m. I was trying to use this 
antenna on 40m with a power of 1200w.

The first attempt was to use a 5kw DX Engineering 1:4 balun in the shack 
between the open line and coax, feeding the coax with an unbalanced 
T-network tuner. There was a lot of RFI with computers, etc. in the 
shack. The antenna seemed to pick up a lot of local noise. During a 
period of contest operating, the balun overheated, causing soaring SWR 
and ultimately internal arcing.

Next I added two capacitors in series with the line to tune out the 
reactance (the 1/4 wavelength line inverted the capacitive reactance at 
the feedpoint of the antenna making it inductive at the shack end). This 
eliminated the overheating (it ran a little warm), but the RFI problem 
remained.

Then I rewired the balun to a 1:1 configuration. This seemed to help a 
little with the RFI. It made tuning easier. But the antenna was still noisy.

At this point I had some really good luck and was able to purchase an 
old Johnson KW Matchbox, which is a link-coupled balanced tuner. I 
replaced the T-network tuner, the balun, and the series capacitors with 
this unit. The antenna is now MUCH quieter and RFI problems have been 
eliminated.

The lesson I learned is that baluns don't work very well with high SWR, 
especially when the impedance is very reactive.

And now for my question: did I permanently damage my balun when it 
overheated? If so, what is the mechanism of damage? I took it apart and 
didn't see any obvious signs.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
.
On 03/09/2020 19:05, Jim Brown wrote:
> <snip> A choke inserted into a poorly
> matched or un-matched line is much more likely to overheat and fail.
> Dissipation (heating) is addressed in the 2018 Cookbook.
> 
> 73, Jim K9YC