"Pipex News Server" wrote in message
...
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

I prefer the term "tuner" because the unit very seldom actually "matches"
impedances. Instead, it "transforms" the impedance at the sending end of
the transmission line, coax or whatever, to the 50 ohms resistance that the
transmitter is usually (these days) designed for. The term "tuner" is common
usage and it's OK.

The coil in a CLC tee-type tuner can have a Q as high as 400, and stray
coupling to the metal cabinet or ground plane can easily cut the Q in half.
What effect that has depends on the load impedance of the antenna
feedpoint. If the load is highly reactive (high X, low R) the coil can get
quite hot.

I believe it is important that the open ends of the coil should be at least
one coil diameter away from any metal surface. The sides of the coil are
less critical, but the mechanical design should do a pretty reasonable job
of reducing that stray coupling to a low value also.

Stray capacitance from coil to ground or elsewhere can be a problem
sometimes.

I have been thinking about my previous inputs to this thread, and I am not
entirely satisfied with them. I will try to improve them sometime today.

Bill W0IYH