K7ITM wrote:

In at least some editions of "Reference Data for Radio Engineers,"
there's a graph of how much the inductance of a solenoid coil is
lowered by being placed inside a shield. If the shield isn't too
close to the coil, the percentage decrease in inductance is rather low
(the coil isn't affected very much). The graph doesn't tell you how
much the Q is lowered, but I did some comparisons of Q estimates from
a couple different solenoid coil programs I have and Q estimates for
helical resonators, and came to the conclusion that you can account
for most of the lowering of Q by assuming the RF resistance remains
constant and the inductance is lowered, per the R.D.R.E. graph
(attributed to RCA). That assumes a highly-conducting shield, I'm
sure. The observation helped dispel the "magic" aura of helical
resonators: their Q is actually lower than the Qu of the coil they
contain, if that coil is in free air. (A big advantage, of course, is
that they are fully shielded.)

This gets to be a jello-y area if you follow the clues far enough. The
coil's Q, or better said, the circuit's Q has a lot to do with how much
spacing is required. Old timey Rule Of Thumb suggests one diameter
spacing away from enclosures but thats typically relative to lower Q
coils which are common rote. I make that comparison against high-Q
coils in dx crystal sets where 12-18 inch separation isn't uncommon.

The textbook Q formula doesn't address this...nor is it a necessarily a
biggie because most of the coils that we encounter in the situation
presented in the original post aren't of high enough Q so as to present
any problem.

But, it needs to be reminded that coil Q is definitely subject to its
surroundings. Re-tweaking the inductance value is easy enough but you
cannot recoup the loss of Q. It always depends on the specific application.

-Bill