Reg Edwards wrote:
John, thanks for describing your detailed and recent interesting
experience and your thoughts on the subject.

However, once again the question is raised - how does one calibrate a
Q meter? Rhetorically, is calibration traceable to National or
International Standards?

Q is the ratio of two values (and several different pairs of values
can be used to arrive at the same end result). For a coil, one ratio
that produces a value of Q is the peak energy stored during an AC
cycle to the total energy dissipated during a cycle. If you can
measure those two quantities, separately, you can calculate Q. But it
is often much easier to work with a resonant tank and use a capacitor
that is known to have a much higher Q than the coil being tested, so
that you can assume that all the losses are in the coil. Then the
tank circuit Q is the coil Q. This is the bases of the Boonton 160A
and also the signal generator, voltmeter method I have been using.

So much depends on the Q quality of the meter itself.

The meter quality has to be higher than the Q of the device being
measured, of a compensation has to be made for the meter losses. For
instance, with the signal generator voltmeter method, I have
calculated the losses in the two 10X probes, to prove to myself that
the errors they cause are not significant up to the highest Q values I
measured, this way. I would have had to measure a Q near 1000 before
they would have altered a significant digit of the measurement. But I
did convince myself that energy absorption outside the coil in
surrounding objects is significant, since shifting my position in my
chair did change the measurement.

Meter
manufacturers are unable to state degrees of accuracy at various
frequencies and actual values of Q. Nobody knows what the actual
value actually is! Least of all the user!

Boonton originally sold Q standard coils (inductors with known Q) to
be used to check the accuracy of the Q meters. I don't know how those
coils calibration got back to basic measurements traced to the Bureau
of standards.


Fortunately, the exact value of Q of a coil is never required. It is
used only to provide coarse estimates of other quantities. And there
are usually other means of finding the other quantities. They can be
estimated by calculating from values which CAN be measured or
estimated.

I agree. Usually, proving that a given device has at least a certain
Q is enough, or tests made on various devices by the same method can
show which ones have higher Q than others. This is what I am doing
with the measurements.

So Q meters provide support and back up for experimenters who have
other means of finding the answers they are looking for. By itself a
measured value of Q is inaccurate and of no use. What matters is what
can be derived or guessed from it.

Q is a way to measure losses. If losses are important to the
application, Q is one way to get information that is useful. An
infrared thermal imager may be another.

It is merely an intermediate variable in a chain of deductions or
calculations. Above Q equal to a few hundreds it is anybody's guess.

In some ways it is similar to an SWR measurement on a line which isn't
there.
----
Reg.