[Elecraft] KPA500 and VSWR

[Randy Farmer]

I just finished the 1300Z session of the CWops CWT and thought I'd pass 
on an observation on the KPA500. When I'm operating in contest mode, one 
of the things I like to keep an eye on is the KPA500 PA temperature, as 
monitored via WIN4K3. (WIN4K3 manages the multiple connections for CAT 
data needed to run my heavily automated SO2R station.) For the past few 
weeks, my regular strategy has been to run continuously in the early CWT 
sessions, usually on 40 meters. Late last week I made a tweak to my 
80-40 inverted V antenna that resulted in raising it around 15 feet from 
its previous height of around 40 feet. As a result of this change, the 
40 meter VSWR at the transmitter produced by my KAT500 dropped from 
about 1.4:1 indicated to 1.04 indicated on a well-calibrated LPA-100A. 
At the duty cycle of constant run operation at around 34 WPM, the PA 
temperature was previously running in the very high 50s, sometimes 
getting to 60° or even 61°. I'm actually slightly exceeding the amp's 
500W output rating, since I set the drive to produce 500W as measured at 
the output side of my switched bank of Low Band Systems bandpass 
filters. With the lower VSWR, I never saw the PA temperature exceed 50°. 
I suppose this shouldn't come as a surprise, but it does show the PA 
heating effects of even moderate load mismatches on a solid state 
amplifier designed for a 50 Ohm load. Just an anecdote that some may 
find interesting.

73...
Randy, W8FN