Ken Scharf wrote:

The biggest problem with a home brew rotor inductor
is going to be a low resistance contact with no
dead spots as the coil is rotated.

For an antenna tuner, we can make do with a tapped
coil and a small variable inductance in series with
the tapped coil to "fill in" the gaps between the taps.
To do this I would use a 'blast from the past', a variometer.
This consists of two coils, one fixed and one rotating inside
the fixed coil. If both are the same inductance (so the outer
one is would with wider spacing than the inner) when they are
in "series aiding" the total inductance will be the sum, plus
the mutal inductance. When they are series opposing, the total
inductance will approace zero. (over simplifcation).
Variometers have NO moving contacts so they are easier to build
than rotary coils. They can't be made as large in inductance
without avoiding other problems (large distributed capacitance),
but a small variometer in series with a multitapped coil is a
good compromise.


"Leon Heller" wrote in message
...

"Tim Wescott" wrote in message
...

Leon Heller wrote:

"Gregg" wrote in message
news:t4eld.149312$df2.42254@edtnps89...


I stumbled across a few at surplus outlets, I'll see if I can dig up
URL's. $50 to $150 USD, depending on KW rating.

You can build your own fairly simple though, for a lot less :-)



An old-timer I used to know made his own from plastic drain pipe.
He used a lathe for cutting a spiral grove in the pipe for the
wire, and machined most of the other parts himself.

73, Leon


I'm not sure I'd qualify anything requiring a machine lathe as being
"simple".


It is if one has a lathe. 8-)

Strictly speaking, he wasn't making roller inductors, anyway, as the
contact was moving across the turns giving one turn resolution, like
a rheostat. He only used QRP. Making a proper roller inductor for
high power like those made by Henry would be quite difficult.

73, Leon




Difficult indeed!

I worked at Barker & Williamson during the early 60s when they were
still making these things. First of all, you have to go out and
located silver plated bare wire for the coils. Then you're going to
have to machine yourself a precision roller that also needs to be
silver plated, and a silver plated rod to carry the roller with as
little lateral friction as possible.

The balance is comparatively simple, if you have a source for G7 or
G10 material to fabricate the frame, and are comfortable in machining
fiberglass-epoxy laminates to a reasonable degree of precision. (B&W
of course used custom molded and glazed ceramics, not laminated
fiberblass, but that is simply a nit!

I personally wouldn't want to even attempt it. Far cheaper to buy a
surplus variable inductor.

Harry C.

If your antenna tuner has two caps then you can often just use the
tapped inductor, with no rotary at all (says Tim, who's antenna tuner
has a rotary inductor...).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com