Topband: To short or not short unused coil turns

[Guy Olinger K2AV]

On rotary coils providing an adjustable inductance, the usual way to
reduce the inductance is for a moving shorting contact to short to one
end.  Whether that is the top or not depends on it's use in the
circuit.  The arc problem becomes critical when the tuning of the
moment requires a small fraction of the entire coil, as on a higher
frequency.  At that point the voltage would be extremely high on the
unused end of the coil if the moving contact were NOT shorted to the
end.

The use of a rotary coil in this fashion goes back to the beginnings
of radio and is without problems as a matter of design.

If you want to retain the unused part of the coil and its connection
to the rest of the coil, just short the unused portion, and you will
have it intact for use in some later project.

73, Guy.

On Sat, Oct 2, 2010 at 9:56 PM,  <WD8DSB at aol.com> wrote:
> Typically I see comments that unused coil turns should be shorted, but
> logic tells me that for maximum coil efficiency unused coil turns should not be
>  shorted unless the application of unused turns results in the generation
> of  excess voltage which then creates an arc or spark.
>
> I am building a remotely controlled base loading coil box for my 53 foot
> 160 meter vertical, and during prototype testing I have not experienced any
> problems running with the unused coil turns not shorted, but now that I am
> hard  wiring the final version of my box I thought I would like some feedback
> from  others regarding this topic.  The coil is also used to run the
> antenna on  80 meters and have not had any difficulty with arcing when not
> shorting the  unused turns.
>
> Please comment on my belief that unused turns should not be shorted unless
> arcing occurs.
>
> Thanks,
> Don Kirk (wd8dsb)
>
>
>
>
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