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Re: Continuously variable primary

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Original poster: "S&JY" <youngsters@xxxxxxxxxxxxxxx>

There is another way that could be easier to construct.  Just  use a normal
primary, with the inductance set 10% or so lower than what is needed.  Then
put in series an off-axis variable inductor that is not magnetically coupled
to the primary.  One might think the off-axis inductance would noticibly
decrease overall coil efficiency, but it doesn't.  Just keep it away from
large metal things, the same as you would for a primary.

The variable inductor can be a beefy roller coil, such as some hams use in
antenna tuners, for example.  Or it can be two small opposing spirals that
you can move closer or farther from each other with an insulating rod, or a
cheap electric screwdriver turning a threaded rod (poor man's linear motor).
I experimented with the latter, but decided it's not worth the hassle.

--Steve Y."

I tried the roller-coil approach with a small coil here. Maximum inductance was maybe 20% of the resonant primary. I tuned it with a pulley wrapped with nylong cord so I could stand back a bit. Tuning was noticeably better when the power was too low to cause breakout but once streamers appeared it didn't appear to change things enough to be worth the trouble.