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
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