[Elecraft] KX1 says "PO.4" -- is this 0.4 W or 4 W?

[W3FPR - Don Wilhelm]

Andrew,

That elusive 'known accurate wattmeter' may be a challenge to find -
especially at the power levels you are attempting to measure.  Many (most)
wattmeters spec the accuracy as a percentage of full scale - so if you use
one good for 10% on a 50 watt scale, the error can be as much as 5 watts -
and that is greater than the power that you are trying to measure - not a
good situation.  So even if you do find a wattmeter that is 'known
accurate', it is always prudent to ask "accurate at what power level?".  If
one is dealing with 100 Watts, 10% may be a tolerable accuracy, but at low
power levels, things become more critical on this issue.

May I instead suggest that you use a known 50 ohm resistive dummy load -
they are easy to check with an antenna analyzer (ideally X will equal 0),
along with either an RF Probe or a 'scope.  The RF Probe will read Vrms,
while the 'scope (properly calibrated) will read Vp-p.

The formulas are (Vp-p^2)/8R or (Vrms^2)/R where R is the resistance of your
dummy load.  It is easier than the 'telling of it', and will provide better
accuracy than most wattmeters.

You can call me a 'test equipment skeptic' if you want, but I learned long
ago that trusting test equipment without question will lead you into many
deep traps.

73,
Don W3FPR

> -----Original Message-----
>
> > 0.4W ...
>
> I was afraid of that -- I recalibrated the tuner with a 50 ohm dummy
> load and now it's showing 1.9 W when running on batteries, which sounds
> correct.  I will further calibrate when I have access to a known
> accurate wattmeter.
>
> Thank you for clarifying.
>
> --Andrew
>