At the risk of becoming a bore, I think it might be time
to repeat some well-worn procedures on measurement techniques for the
benefit of newer list members. I was prompted by Bob's description
of measuring his coil.
> I took the secondary to my work bench and rigged up a 555 Timer
> circuit and played with caps and resistors until I found resonance
> of the secondary. Then I took the timer signal and shot it into
> the primary, put the secondary back in and hooked a scope to the
> secondary output. I was mis-coupled only by 1/16" so I had made a
> lucky guess in the beginning. I did notice on the scope that as
> I walked by (6 feet from the coil), not only did the frequency
> change on the scope but the signal amplitude as well!
(a) Frequency : You NEED a low impedance signal source (the lower
the better - zero ohms ideally), a frequency counter, and an
Method : Place the secondary (propped up at the height you wish to use
it) at least 6 feet clear of any objects including your measuring
equipment. Suspend the scope probe about level with the top of the
secondary and AT LEAST 3 feet away (further the better). Connect your
sig gen directly to the bottom of the coil using the shortest
possible lead and keep as much of the lead as far away from the coil
as possible. Connect the frequency counter directly across the signal
generator. Tune the generator and monitor the scope for peak reading,
the frequency of which the freq counter will give you.
NB - there is absolutely no way you are going to come even close if
your scope probe is hooked to the coil. A 10x probe adds 10pF or so
of additional capacitance to the coil and 1x is far, far worse.
(b) Q : This is even more critical than frequency, so maximum
isolation for the coil is the key ingredient. Connect/arrange the
equipment as for (a), then tune the generator either side of resonance
noting the frequencies at which the waveform drops to 70% of its
peak amplitude at the resonant frequency. Q = Fr/(Fhi-Flo)
Measuring the Primary
Both Fr and Q of the primary are fundamentally the same as the
secondary, but the equipment is hooked up differently. Both of
these measurements are done with the spark gap shorted (which means
the Q you obtain is indicative of component quality and the degree
to which the primary couples into the ground). Also, although you
want to disconnect the primary chokes/transformer etc from the primary
to get these measurements, you may be interested to try this with
these components connected at some stage to see what effect they have.
Connect the generator in series with a resistor of not less than
10kohms across the primary cap, and the oscilloscope directly across
the cap. The relatively high value of Cp makes the scope capacitance
insignificant for this measurement. Tune the generator and measure as
for the secondary.
Leave the equipment hooked to the primary, mount the secondary where
it would normally go and ground it well (or at least as well as you
would in operation). Try and maintain as much isolation for the
system as possible. Tune the generator from well below Fr to well
above. You will see two peak responses on the scope with a null
between them (if prim and sec are tuned to around the same frequency).
The coupling constant for the arrangement you have is approximately
k = (Fhi-Flo)/Fr where Fr is the exact null between
Fhi and Flo.