Recently I described some experiments with multilayer coils based on a Decca
coil.
The resultant coil is now similar to that shown in the LF book, page 13 (See
the second coil back) except my coil has two layers, double wound. I had
been advised by some of you that the Q of a multi-layer coil is low compared
with a single layer solenoid. The question is just how much lower is the Q
and how significant is it in practice.
The coil is at this moment being tested with a full kW on a T/A test and I
get just as good results as I did with the single solenoid although the
single solenoid was a much smaller coil wound with thinner wire.
To progress I need to measure Q.
It is quite surprising how little there is in general Ham literature on
measuring
Q. I found something in the 'ARRL Electronics Data Book' and Wes Hayward's
'Radio Frequency Design', although the latter is related to filters and
measuring loaded Q.
I made a test rig up using a signal generator (with frequency counter) and
the Pegelmesser D2155 level meter set on wide band.
I connected the coil under test with a parallel air-spaced capacitor to the
signal generator via a variable capacitor. I coupled the coil to the level
meter with with just a clip on the insulation of the coil. I reduced the
coupling to the signal generator with the variable capacitor to minimum
while maintaining a good signal on the level meter.
The parallel capacitor caused the coil to resonate at 182kHz . Using the
3dB B/W points (down from resonance) on the meter I measured the Q as 185.
With the variometer in the coil set to minimum the coil resonated at 209kHz
and the measured Q was 167.
Is this the right way of going about it? If I use a larger capacitor across
the coil won't this give a sharper selectivity and improved apparent Q?
Regards,
Peter, G3LDO
e-mail <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
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