There's a question as whether the Q function on an LCR meter measures Q - or
rather, which Q it measures......
The usual LCR or bridge (which measures 1/D as well) measures at some fi
xed, and generally low frequency, so the resistive portion measured is the
LF
resistance. The classical Q meter, on the other hand, allows Q to be measured
at the frequency of operation, because it operates on the basis of
magnification factor at resonance. Even so, the Q meter will measure apparent
Q,
rather than actual Q, although for all practical purposes, the difference can
be
neglected. Apparent Q can be very different, though, for a coil with a high
self capacity, such as a pot core without the use of a sectionalised former.
My main experience has been with the Marconi instruments TF329 ( which is
pre WW2!) and the later TF1245. Unfortunately, the diode used for the voltmeter
in the TF1245 is now made of 'unobtainium' - or over $50, which is much the
same thing! I did a lot of work on high Q coils some 30 odd years ago. This
involved end cheeks of synthetic resin bonded fibre, with a hole in the
middle,
six rods of fibreglass in hexagonal formation round the hole, and 180
strands of 38SWG enamelled wire as a bunched conductor. At 2 MHz, I could get
a Q
of over 500: the idea was to se if we could drop the losses when feeding a 16
foot whip at 2MHz. In the end, we just used B & W type coilstock - it had a
Q of about 150 from memory. It got warm at 400 watts into a 10 ohm, 250pF in
series load, and even more so when matching to 5 ohms in series with 50pF.
These days, on the few ocasions I want to know Q, I use a resonance method
and determine the 3dB points: that, at least measures real Q, although you have
to be careful how much you load the circuit. It is of course, the way to
measure a cavity or a transmission line resonator. I suppose for a cavity, you
could get there from measuring logarithmic decrement, but that's a bit messy.
An exercise I would not recommend if you want peace of mind is to measure
the same high Q coil at teh sam frequency on 3 different Q meters.......the
difference can be frighteningly large!
73
Peter G3RZP
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|