Calibrating wattmeters and HF voltmeters. (Was KAT2 power calibration)

[Ken Rice]

Hi Don and everyone.

It does seem that calibrating a wattmeter is a hard thing to do if you don't
have an office at NIST! When I care about accuracy, I measure RF voltage
with the Elecraft DL1 and hope for the best.

Suppose you have a good DC standard, such as Don's or Joe Geller's designs.
Is there anyway to bootstrap accurate RF voltage measurements from accurate
DC measurements?

It's true that measuring power at +/- 20% is plenty good if what we care
about is signal strength on the other end of a QSO.  Still, it would be
useful and satisfying to be able to measure RF voltages more accurately when
doing experiments.

Thanks and 73,
Ken
K3VV
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[Don Wilhelm-3]

Ken (and others interested),

The DL1 has a formula in its manual that considers the diode forward
voltage.  You can use that to achieve accuracy if you are using a DL1.  You
can include similar terms in your measurements and calculations if you are
using an alternative.  Apply Ohm's law for power in whatever form your
measurement technique finds more suitable.

Short tutorial on measurement accuracy follows:

If you draw out the measurement circuit then figure out all the voltage
drops or current loops, that will provide you with an accurate picture of
how to do the math to figure out the total solution.  Then one only needs to
consider the accuracy of the measurement equipment in recording the values
obtained during measurement - i.e. your DMM scale may be accurate to .03%
+/- 2 counts in the low order digit, and that must be considered when
calculating the accuracy of your measurements and results.  It is often
informative to calculate the range of error in the result considering the
limits of your instrumentation - the specs for your measurement device will
tell you what its limits are.  Just because you can see a number in a
digital display does not mean it is telling you the whole story.  Knowing
the limits of your measurement equipment goes a long way toward achieving
accuracy.  Any piece of measurement equipment is not to be trusted until it
has been verified.

In power measurements, an item that is frequently ignored beyond the diode
voltage is the accuracy of the load resistor at the frequency of
measurement.  A good antenna analyzer can tell you what the real impedance
is at the frequency of interest.  If the load has some reactance, you can
include that in the calculations too - it is a simple application of Ohm's
law for power, but if there is any reactive component, one must use it in
the calculations too, and including the complex values (R+jx) in the
computations can become messy (but it will be accurate).  All the efforts to
have a non-reactive dummy load is a quest to simplify the calculations (and
keep the math in the real plane).

There are components readily available today that are useful in calibrating
equipment - such as precision voltage references and precision resistors (I
have found thick film 1% 50 ohm power resistors by Caddock to be good at RF
(values other than 50 ohm have proven to be capacitive).  If you want a
sanity check on items purchased for use as standards, buy 3 or more of them
then meausre and compare them before use - cast out any that do not agree
with the rest of the batch (that does not increase the tolerance unless your
measurement equipment is sufficiently accurate and calibrated).  Care for
these 'standards' well - mark them as standards and put them in a safe place
and use them only for calibration.  Using this technique, even homebrew
equipment can be calibrated and that calibration tracable back to your set
of standards.  Knowing the limits of your measurement equipment is key to
achieving measurement accuracy - and yes, you will have to 'do the math'.

73,
Don W3FPR

> -----Original Message-----
>
> Suppose you have a good DC standard, such as Don's or Joe
> Geller's designs.
> Is there anyway to bootstrap accurate RF voltage measurements
> from accurate
> DC measurements?
>
> Thanks and 73,
> Ken
> K3VV

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