Posted by
Vic Rosenthal on
Sep 04, 2020; 6:26am
URL: http://elecraft.85.s1.nabble.com/KPA-1500-SWR-fault-above-700-800-watts-on-160m-tp7664618p7664690.html
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
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.htmlMessage delivered to
[hidden email]