Don and all,
This is exactly the method I have been using on all the Elecraft
wattmeters that I have worked on for several years now. I believe you
mentioned it first, so I do have to give you all the credit for this
great idea.
It does result in a more accurate setting of the trimmer capacitor than
just observing the null, but you have to just nudge the trimmer setting
a bit at a time. As you stated, the 'manual' adjustment done first
will get things close, and these loads can be used to refine the adjustment.
With the KAT2 (and KAT100), I connect each load to the two antenna jacks
and use the ANT 1/2 button to switch between the loads - with the KPA100
it is harder because I have to connect and disconnect the loads for each
comparison. I could devise a switch, but that would introduce an
unknown SWR element.
I have a set of 25 and 100 ohm loads constructed from 30 watt thick film
power resistors ( 50 ohm 1% resistors), so they will handle up to 60
watts continuous, and I like to do a final check at higher power with
the KPA100 and KAT100. If you choose this route, be certain to keep the
connections to them VERY short - connecting to these (non 50 ohm) loads
with even a short piece of 50 ohm coax will alter the SWR due to
transmission line effects. I use male to male connectors.
73,
Don W3FPR
[EMAIL PROTECTED] wrote:
Hi
I have a method that works well for me to adjust the power/SWR bridge.
You will need to make up two additional dummy loads besides your 50 ohm.
These need to be 25 ohms and 100 ohms. I used 2 50 ohm 2 watt MOS resistors
mounted inside a PL239 connector wired in series for the 100 ohm and in
parallel for the 25 ohm. For the kAT2 you will also need UHF to BNC
adapters.
After you do the procedure below from the manual set the forward power pot
per the manual. Set the reverse power pot to about the same position as the
forward pot. Now using about 2-3 watts with the 25 ohm load note the SWR
reading. Now install the 100 ohm load and adjust the null in very small
amounts until you get the same reading with both the 25 and 100 load. It
does not matter what the reading is yet, just so it is the same. Once that
is done adjust the reverse pot so the K2 displays 2.0:1 with either load.
Now go back and check the power and SWR with the 50 ohm load. The SWR should
be 1.0:1. If the power is off adjust the forward power pot to correct it and
recheck the reverse again with the 25 and 100 ohm loads to make sure you
still get 2.0:1. All of this will interact a little so you may need to go
through it several times.
Hope this helps
Thanks
Don Brown
KD5NDB
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