Ever since I received my KPA1500 (SN 00178), I've observed that the ATU
sometimes won't achieve a match better than about 1.4:1, even though the
antenna SWR is well within range. (ATU STOP TUN SWR = 1.0:1, tune power
40 W) Now, 1.4:1 doesn't sound that high, but the amplifier will often
not operate at full output at that SWR, either due to excessive current
or drive power. This is particularly troublesome when tuning across a
band. Clearly, the KPA1500 is designed to deliver 1500 W to a 50 Ohm
load, with little margin for a mismatch. More margin would have
increased cost, size, and weight, of course.
Just to be sure there isn't something unique wrong with my KPA1500, have
others observed similar behavior?
A little ice on the antenna this morning provided an opportunity for a
test. On 40 meters, the antenna SWR was 2.2:1. The best match the
KPA1500 ATU could attain automatically was 1.4:1, in multiple attempts.
I was able to manually get it down to 1.1:1. (SWR as indicated by the
KPA1500 display.) It was a little tricky, requiring one value to be
increased beyond the SWR minimum before the other was adjusted, as often
happens with manual tuners. With the KPA1500 ATU bypassed, the K3 ATU
automatically got it down to 1:1 on the first try.
My conclusion is that there's something about the KPA1500 ATU algorithm
that prevents it from achieving a better match, in some cases. (I'm
assuming that, aside from their power ratings, the components in the
KPA1500 ATU and the K3 ATU are similar.) I'm wondering if the KPA1500
ATU algorithm could be improved to perform more like that of the K3 ATU.
That would allow operating the KPA1500 closer to full power across a
band without having to ride the power control.
Further details of ATU component values, etc. are available.
73,
Scott K9MA
--
Scott K9MA
k...@sdellington.us
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