Dear Peter, LF Group,
This is one of the well-known techniques for measuring Q; as Rik pointed
out, the important thing is to minimise coupling between generator and
detector and the tuned circuit under test, to ensure the Q is not being
decreased by loading by the test gear. The way to check is to reduce the
coupling so the signal amplitude is reduced, say by about half, and measure
the Q again - if it is significantly different, there is significant
loading. You need to do this with both generator and detector, since either
one can cause loading. With very high Q coils, the coupling must be very
small; the equivalent parallel resistance of a high Q LF loading coil can
easily exceed a megohm, so almost anything actually connected to, or even
anywhere near a high potential point in the circuit will really clobber the Q.
What you really want to know is the equivalent series resistance of the
coil. I usually do this as follows: Connect generator to meter and measure
ampliude V1. Then connect a series resonant circuit using the coil to be
tested in series with a suitable resonating capacitor across the generator
terminals, and tune for a null in meter reading, ie. series resonance, and
measure voltage V2. At resonance, the reactance of L and C cancel, and the
remaining Rseries forms a potential divider with the paralleled source
resistance of the generator (Rs) and load resistance (RL) of the meter. If
you know what Rs and RL are, you can calculate Rseries:
Rseries = (RsRL/[Rs+RL])*1/([V1/V2]-1)
Typically, The meter needs to measure a reduction in voltage of 10 - 30dB,
which should not be a problem for a level meter, or a scope. The main
problem with this method is that harmonics of the generator signal will not
be nulled out, and will produce an increase in apparent Rseries if that
resistance is very low. However, this is not a problem if the generator
output is clean (harmonics < 1%), or if a selective level meter is used to
measure the voltages. It works well for series resistances less than the
generator source impedances, which they should normally be. There is no
other connection to the junction of L and C, which is the sensitive, high
potential point in the circuit, and provided you know what it is, the
source and generator impedance does not cause errors. A selective voltmeter
with tracking generator is the ideal tool for this job.
Once you have Rseries, Q = XL/Rseries, = 2pi*f*L/Rseries
The resonant frequency of the coil for the large coils we are using will
depend on stray C between windings and connecting leads, so the apparent L
will wary with frequency, as will Rseries. So again I agree with Rik and
Andy that it is important to measure at close to the desired operating
frequency. There will always be variations in stray capacitance between
measurement of Q and connection to antenna, so the effective L will be
somewhat variable. Also, with large diameter coils, nearby conducting
objects will absorb energy from the coil, and again affect Rseries and L.
So keep coil as far as possible from ground, metallic objects, walls etc.
both when measuring and in use.
My 136k Loading coil has 80 odd turns of Decca litz wire on a sectional
manhole former. L is about 4mH, and Rseries about 5ohms, making Q around
700. For 73k, another sectional manhole is stacked on top, wound with about
120 turns of 19/0.25 Teflon insulated stranded wire, which gives a total L
of about 15mH, and a Q of around 300. The coils are wound in sections, with
the total turns divided fairly evenly between the 14 slots on the former
The required number of turns wound in to each slot before moving to the
next, with the aim of minimising inter-winding C and maximising breakdown
voltage, rather like the old-fashioned RF chokes. So G3LDO's Qs of less
than 200 suggest either poor inductor performance or Q measurement errors.
Having said that, with most combinations of antenna and loading coil, even
reducing loading coil losses to zero would only lead to modest 10-20%
increases in antenna current, because losses are dominated by the antenna
itself. Other effects, like how wet the weather is, will produce similar
variations. My main reason for winding big loading coils was to stop the
things melting!
Q measuring seems to have gone out of fashion in the last few decades - all
the major test gear companies have stopped making Q meters, which is a pity
because the impedance meters which have replaced them do not cope well with
measurements on high-Q circuits. Older textbooks, like Scroggie's "Wireless
Laboratory Handbook", discuss Q measurement at some length.
Cheers, Jim Moritz
73 de M0BMU
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