[Milsurplus] Silly Inaccurate Wattmeters and a Tool to Fix Them.

[David Stinson]

I recently was graced and blessed by a dear soul
with a modern, calibrated scope which has made it
possible to begin calibrating my test gear, most
of which has been shockingly "out to lunch."
There is a web site with calculators which are
invaluable for this.

For instance:
If you set your calibrated scope for 50V per
vertical division (5 V if using a calibrated
x10 probe), connect a *non-reactive*
50-ohm load to your transmitter, key a carrier
and scope the output, you can read the peak-to-
peak waveform.  Say it's 4 divisions, which is
200V Peak-to-Peak.  Easy way to tell your true
power into that load: Divide the reading by 2,
which gives you "Peak Voltage," then multiply
by 0.707, which will give you RMS Voltage
(70.7 V in this example).

Take that figure to this page:
https://www.pasternack.com/t-calculator-power-conv.aspx
Select Input Type as "Volts," output type as "Watts,"
enter the RMS voltage and hit "Calculate."
The result: You are delivering a true 100W to the load.
If you modulate your carrier and read the vertical scale,
you can calculate peak power with modulation.

These folks have a bunch of useful calculators at
https://www.pasternack.com/t-rf-microwave-calculators-and-conversions.aspx

The point of all this is to talk about our typical ham Wattmeters.
I have Drake W-4, the Drake 7-line meter, other makes and
a couple in antenna tuners.  NONE of these are even close
to accurate, nor can you validly calibrate them.  They
are ridiculously non-linear.  If you pump a scope-confirmed
20 watts through them and set the adjustment for 20 Watts,
then drop the power to 10 or raise it to 40 the meter
will read way off.  All of them.  I have a Bird at
work but no HF slugs- maybe that one would do better.

I conclude that "ham grade" Watt meters are good for
relative power indications and tuning-up, but any
Power reading they give you is likely nonsense.

What has been your experience?

GL OM ES 73 DE Dave AB5S

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