Jay,
Thanks for that comment and your experience. I do not have a WM-2, but
I do have a homebrew version of its predecessor, A Simple and Accurate
QRP Wattmeter by Roy Lewallen W7EL. It originally appeared in QST
Februrary 1990.
The calibration method using a DMM to measure the voltages is
theoretically correct, but assume that everything in the directional
coupler and the diodes is perfect and without variation or loss. I
believe it is accurate to within 20% for all cases, but to get closer
than that figure, one must include the variations and loss in the
coupler in addition. Doing final adjustments by accurately measuring
the power output is a good way to include all elements in the calibration.
Both Roy's wattmeter and the WM-2 do have a great advantage in that they
will be just as accurate at low power levels as it is at higher levels,
and that is where many wattmeters fall flat on their face. It is a very
good wattmeter for QRP levels.
73,
Don W3FPR
Jay wrote:
Randy,
Don said:
The WM2 must be calibrated and verified before it can be trusted, and
similarly the wattmeter section of the KAT2, KAT100 and KPA100 must be
calibrated before they can be trusted. After any of these instruments
have been calibrated, the dependency on the precise 50 ohm dummy load
goes away. but some instruments will indicate forward power and reverse
power, and the two must be subtracted for the net power, while others
will do that calculation automatically. As a user, you must know the
characteristics of your measuring instrument before you can correctly
interpret its readings.
Don is very correct in this.
I have a WM-2 and found it to be "way off" when adjusted according to the
instructions and working into a 50 ohm load. I finally hung a scope on the
input to
the dummy load and set my K2 to 40m and 5 watts. I adjusted the WM-2 to read
the
calculated input power at the dummy load based on the pk-pk signal. The K2 and
WM-2
were then "close enough" with each other on 40m. Since then, I have relied on
the K2
power output.
Vy 73
Jay
AJ4AY
Mobile, AL
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