KPA500 and VSWR

[Randy Farmer-2]

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
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[ANDY DURBIN]

"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."

I have published actual test data that shows the very strong influence of PA load on PA dissipation for my KPA500.  SWR is not a good indicator of the load mismatch.  You must consider the complex load impedance to see the real picture.

For those who missed it the first time, one set of my test data is documented here - https://tinyurl.com/y93m99sw

The KPA500 monitor that I created is shown here - https://tinyurl.com/yalv5o6f


Andy, k3wyc


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[Randy Farmer-2]

Good point, Andy. I never thought to use the complex impedance
measurement capability of the LP-100A to see what the exact load
impedance was with the old configuration.  If I have occasion to do any
modifications on my 20-15-10 dipole, I'll try to remember to document
the before and after load impedances (if it changes anything).

73...
Randy, W8FN

On 7/15/2020 12:25 PM, Andy Durbin wrote:
> I have published actual test data that shows the very strong influence of PA load on PA dissipation for my KPA500.  SWR is not a good indicator of the load mismatch.  You must consider the complex load impedance to see the real picture.

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