In article , "Jan-Martin Noeding,
LA8AK" writes:

A rule of thumb says that the screen around an oscillator coils could
be 3 times the coil diameter, and Radiotron Designer's handbook, 3rd
edition, 1941 says that the screen should go at least one coil radius
above the coil. But how far from the chassis should the coil winding
start?

About the same distance. :-) The old Radiotron book (I have a CD
for one of the editions as a keepsake) has charts showing the
inductance reduction and Q reduction for various relative diameters.
The closer the enclosure, the more reduction of both values. That
sould be accounted for in any resonant applications, especially those
for tracking as in superhet or TRF front-ends.

The conducting enclosure or shield contains the magnetic field on the
coil and, ideally, should have the same clearange from both top and
bottom of the winding. It will still work with an assymetric clearance
but L and Q are spoiled by the closer spacing.

Toroidal cores are much less affected by conducting enclosures due to
the magnetic field contained largely IN the torus' core material.

Len Anderson
retired (from regular hours) electronic engineer person