To make these kinds of measurements with amateur equipment and accuracy, I
suggest acquiring some directional couplers and/or high power attenuators. A
spare K3 or a decent SDR receiver then makes an excellent "power meter" with
significant dynamic range. Gain measurements are really ratio measurements and
seldom, if ever, require NIST traceability. The ancillary hardware can be
characterized by substitution.
One further point: Although it's possible that two 10% accuracy power meters
will have 20% uncertainty, it's highly improbable. In industry, all
uncertainties: mismatch errors due to reflection coefficients of sources and
power sensors, linearity errors, noise, temperature effects, etc are combined
into one uncertainty by RSSing the individual errors. You never add all the
worse cases to get the answer.
Wes N7WS
On 6/19/2018 3:42 PM, Don Wilhelm wrote:
John,
You are exactly right for bring that factor to attention. Typically wattmeters
use diodes in their detectors, and the response will vary by frequency and by
the power level.
I would trust only something that has been calibrated to NIST traceable
standards. The Telepost LP-100 is one example (and those are used on many
Elecraft test benches).
While 10% is an OK deviation for amateur purposes, two wattmeters each with
10% accuracy can lead to a 20% error in the final measurements.
It is too easy to jump to conclusions by not considering the potential errors
in measurement accuracy. If you want 5% accuracy in your conclusions, your
measurement tools should be accurate to 0.5% - a far stretch for wattmeters as
we know them.
73,
Don W3FPR
On 6/19/2018 6:25 PM, John Oppenheimer wrote:
Hi Scott,
I wonder how the linearity of the two power meters was evaluated?
Some years ago I checked the K3 and KX3 power meter against two other
meters. They were just within 10%
http://www.kn5l.net/Elecraft/Power.html
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