Full power tuning

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Original poster: "S & J Young by way of Terry Fritz <teslalist-at-qwest-dot-net>" <youngs-at-konnections-dot-net>


I am seeking engineering opinions again:

I am about to experiment with adding a variable inductor in series with my
primary circuit so I can tune for max streamer length while the coil is
running.  My external inductor is two small flat spiral coils of 5 turns
each (look like miniature primaries) connected in series.  By moving the
coils closer together or farther apart, the mutual inductance of the two
coils changes, causing the overall inductance of the external coil assembly
to be variable.  (I will move one of them in relation to the other one with
a "linear actuator" made from a cheap electric screwdriver driving a
threaded rod.)

There are two ways of series connecting the two pancake coils.
Configuration A results in the mutual inductance adding to the inductance of
the two coils (2L+M).  Configuration B results in the mutual inductance
reducing the inductance of the two coils (2L-M).

In my situation, my twin primary tank circuit is tapped at turn 6 for each
coil and resonates at 180 kHz.  Now I add in the external variable inductor
coils.  With Configuration B (series-opposing) the primary taps need to be
at 5.6 turns each.  With Configuration A (series-aiding), the primary taps
need to be at 4.6 turns each.  Either case, the primary resonant frequency
can be varied by about plus or minus 5 kHz from the nominal 180 kHz simply
by moving the external coils closer or farther apart.

So which configuration results in the least losses or the best efficiency
and why?  I assume the loss in the two external spiral coils is the same for
either configuration - true?  So I am thinking Configuration B that
preserves the highest primary inductance is probably a slightly better
choice.

Your comments will be appreciated.

--Steve Young