K2 power meter question, again

[Chris Kantarjiev K6DBG]

A similar topic came up a few months back, but I can't find an answer
to my question in the archives.

I have a QRP K2 with KAT2, running to a Hustler vertical (which
is not purely resistive - typically has C=6 R=40, more or less).
I have noticed recently that the LED power bargraph shows more
than 5 (lights the next segment) when I have the power set to 5.0.
Using the ATU's Pout mode, it tells me about 5.8 - 6.1 F ...

I'm wondering why the K2 isn't controlling the power better? Or
is this as close as it gets?

I notice that if I turn on TUNE and adjust POWER to bring the output
down a bit (back to about 5.0, it's pretty jumpy), then the power out
seems to get back in sync, for a while - but not for long.

I'm not particularly concerned with how accurate the Pout is - last time
I checked, I had calibrated it reasonably well. I'm concerned about the
difference between what I dial in and what the K2 itself measures.

Does this sound at all familiar? It almost feels like a firmware issue.
I have 2.03d 1.07.

73 de chris K6DBG
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[Don Wilhelm-3]

Chris,

It is likely not a firmware issue, but a problem with calibration - the
adjustment of the trimmer cap is critical.  I can give you a 'non-Elecraft
manual' method to calibrate the KAT2 wattmeter.

If you want to be assurred of a proper trimmer cap setting, make up a 100
ohm and a 25 ohm dummy load - they do not need to be high wattage if you are
careful with the power output while using them - a couple watts is
sufficient.  Then alternately connect the two dummy loads (KAT2 set for
CALS) and observe the reported SWR for each - if the reported SWR is not the
same for both of these loads, the trimmer is not balanced - tweak it a bit
and repeat (repeat as many times as necessary until the SWR readings are the
same).  After getting both loads to report the same SWR, then set the
Reflected pot to indicate a 2.0 SWR with both loads (OK, 1.9 to 2.1 will
likely be OK, but shoot for the 2.0 value) - you may have to touch up the
trimmer during this process.

After you have completed the trimmer adjustment and reflected pot adjustment
as above, then set the forward pot to indicate the proper power output with
a good 50 ohm dummy load (KAT2 set for CALP).

If you have two 50 ohm dummy loads, you can create a 25 ohm load easily by
connecting them in parallel (easy with a Coax Tee coupling), and the 100 ohm
load can be created by connecting the 2 loads in series.  Keep the lead
lengths to a minimum becaus a substantial length of coax will have
transmission line transormation effects - yes, even a couple feet will make
a difference at 7 MHz.

Once you have calibrated the power output as above, you still may see some
fluctuation of power in TUNE - the firmware decreases the amount of power
level control so you can tune an external tuner without the power drifting
all over the place.  The keyed output will be more steady (and accurate)
than TUNE with the KAT100 in Pout state.

73,
Don W3FPR

> -----Original Message-----
> A similar topic came up a few months back, but I can't find an answer
> to my question in the archives.
>
> I have a QRP K2 with KAT2, running to a Hustler vertical (which
> is not purely resistive - typically has C=6 R=40, more or less).
> I have noticed recently that the LED power bargraph shows more
> than 5 (lights the next segment) when I have the power set to 5.0.
> Using the ATU's Pout mode, it tells me about 5.8 - 6.1 F ...
>
> I'm wondering why the K2 isn't controlling the power better? Or
> is this as close as it gets?
>
> I notice that if I turn on TUNE and adjust POWER to bring the output
> down a bit (back to about 5.0, it's pretty jumpy), then the power out
> seems to get back in sync, for a while - but not for long.
>
> I'm not particularly concerned with how accurate the Pout is - last time
> I checked, I had calibrated it reasonably well. I'm concerned about the
> difference between what I dial in and what the K2 itself measures.
>
> Does this sound at all familiar? It almost feels like a firmware issue.
> I have 2.03d 1.07.
>
> 73 de chris K6DBG
>

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