Some years ago I found that leaving the unused turns open-circuit caused
very serious interactions with the remaining part of the coil at certain
frequencies, and actually made the tuner untunable at those frequencies. I
was sure that the open turns and the capacitances involved with those turns
was resonating, causing "suckouts". If the unused turns were shorted, the
spurious resonances in the coil disappeared, a well as I could determine.

Assume the shorting bar is perfect (lossless). Then:

The equivalent circuit just for the coil alone is the used part with its
resistance, magnetically coupled to the shorted part with its resistance.
The coupling from the shorted part to the used part adds an inductive
reactance plus some resistance in series with the used part. The shorted
turns have nearly the same ratio of inductive reactance per inch of coil
length to resistance per inch of coil length as the rest of the coil, so the
Q should not be degraded by this coupling. In other words the net loss is
the same as that of the entire coil operating alone. The perfect short does
not add power loss into the coil.

Assume the shorting bar is not perfect (not lossless). Then:

Some additional power loss is added to the shorting bar and the Q should
decrease.

Bill W0IYH

"Peter Orban" wrote in message
...


William E. Sabin wrote:
...

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.

...
Hi Bill,

In practical tuner applications the unused part of the coil is usually
shorted. The reasoning is that it prevents the generation of high RF
voltages. Shorting part of the coil should ruin the Q quite a bit.
Any comment on this practice?

Thanks, Peter