The flux at the center of the coil is highly concentrated. This flux then
leaves one end of the coil and returns to the other end via an external
path, just as it is supposed to do, but at a much smaller value of flux
*density* in the external path. This external flux can induce "eddy"
currents in a shield that can decrease coil inductance and Q, usually only
a small amount if the shield is not too "close".
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The above sparks-off my following question:

If an antenna matching unit (some prefer calling it a tuner) has air
/ceramic/polystyrene wound inductors ,to which extent will a metal cabinet
affect the Q of the said inductors ?
I have read somewhere that to maintain the best possible Q , the distance
between the inductor(s) and other metal parts should be not less than the
diameter of the inductor. Does this make sense ?

TIA for any response

Frank GM0CSZ / KN6WH