Reg Edwards wrote:
There is a coil.
It is 1" in diameter.
It is 2" long.
It has 20 turns.

How accurately can coil Q be determined at 30 MHz?

(1) Using an Autec antenna analyser?
(2) Using the best commercially available instrument.

Those are good questions, although not particularly germane to the
current discussion. A toroid is much easier to measure than an air core
inductor with fair accuracy, since the field is largely confined. What
makes solenoidal coils relatively difficult is the problem of avoiding
coupling to the measuring device and nearby objects.

I'll mention again that ferrites are most commonly used at RF for
wideband transformers, baluns, and EMI suppression. In those
applications, Q is typically very low (1 or less) and generally immaterial.

To answer the question, though, I first note that your program predicts
a Q of about 500 for this coil, with a Z of about 960 ohms and an ESR of
about 1.4 ohms at 30 MHz. If it's correct, an antenna analyzer would be
poor choice for measuring it for several reasons -- poor accuracy at
that high an impedance, poor resolution of the ESR, and residual
resistance in the measuring device. So probably +/- 50% would be wishful
thinking. On the other hand, a good Q meter might make 20% if you could
get the coil far enough away from the fixture, and I could probably do
around 30% with my GR impedance bridge.

But what's the point you're trying to make?

Roy Lewallen, W7EL