It's possible that those indications are accurate and related, but not
necessarily so. Most wattmeters show total forward (including reflected)
power, so if the reflected power goes up as the incident power goes
down, the total remains the same. If your external wattmeter shows
actual delivered power (forward minus reflected), then you would expect
the displayed power to drop as SWR increases, although at 1.5:1 the
effect would be minimal.
It is more likely that the SWR increase you are seeing on your MFJ is
real, and the lower power reading is just the KPA-100 delivering less
power on 10m into the higher SWR.
BTW, I have been doing a lot of work lately, along with a friend, to
determine just how accurately power can be measured. We have determined
that manufacturers' claims about absolute power measurement accuracy are
extremely optimistic. We have access to tens of thousands of dollars of
precision loads and measurement equipment, and find it terribly
difficult to measure absolute power within .2dB accurately (~5%), and
sticking a Bird 43 in line with typical ham dummy loads is definitely
not the way. The Bird is a good example of misleading ratings. It is
rated at 5% of full-scale with a high quality dummy load. In other
words, if you had a dummy load with >40 dB return loss, the Bird would
only be within 5% at full scale. At half-scale it would be 10% at best.
And even if your load is 50 ohms exactly, the makeup of that 50 ohms
plays a part... if part of the 50 ohm Z is reactance, the accuracy
typically suffers as well.
To make a long story short, I think most hams are lucky to have 10%
accuracy, which is OK if you realize it.
Larry N8LP
Gottlieb, Jonathan wrote:
I have just completed the Power Calibration (R26 andf R27) on my KPA100. The
SWR shows 1.0-1 on 40 and 80M, but then goes up from there. It is 1.5-1 on 10m
and power output as indicated on my external power meter shows power declining
as the SWR is increasing. I am using an MJF 264 dummy load which is not the
best. Any thoughts? Could I just be seeing a poor load?
Jonathan Gottlieb
WA3WDK
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