Circuit Q = omega*L/R. Reducing L has little or no effect on Q because,
after winding L, you will find R has decreased in about the same proportion.
The fewer the number of turns, the shorter the length of wire, and the lower
the resistance.
The ratio of L to C also has little effect on circuit Q because the
intrinsic Q of capacitors is usually an order of magnitude or more greater
than Q of L. L and C values of a tuned circuit are usually selected by the
reactances required of them at resonance for reasons independent of circuit
Q. Eg., the reactance of L and C may be required to be 300 ohms at
resonance because other components will have to be connected to them.
Usually it is the value of C which controls the value of L. C may have to
be trimmer. If it is a fixed value it will have to conform to a preferred
series of values and tolerances. If it is too small it will get lost in
stray and other circuit capacitances.
IMPORTANT - Intrinsic Q of a solenoid is directly proportional only to its
physical size. Double all dimensions, including wire diameter, and Q is
doubled. Its the the amount of space you have which decides the value of Q.
And there's a similar relationship even for magnetic cored components. If
you havn't got the room then you will have to put up with a low coil Q. And
it's always lower than what you think it is. Its impossible to measure in
situ. Spice is of no help.
Don't forget that a tuned circuit is never used in isolation. If it is used
as a filter in transistor collector circuit then it forms only part of the
transistor load. And whatever else is connected will reduce the effective
circuit Q. It could be that it doesn't matter what the intrinsic Q of the
coil may be provided it is not ridiculously low. Which I suspect to be true
in your case. You may be doing your nut about nothing.
To put it crudely, it is seldom that coil Q matters. Nearly always whatever
you've got is good enough. If in a particular application you might think it
does then you are barking up the wrong tree.
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Reg, G4FGQ
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