Re: Regulating coil: here's a big one!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Scott Hanson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <huil888-at-surfside-dot-net>
> 
> In an attempt to help put an end to the "variable capacitance" debacle, I
> thought I'd add a bit to the "regulating coil" thread.
> 
> Saturday I picked up the largest variable inductor I've ever seen at the TRW
> electronics swap meet in So Cal. Unlike the commonly seen ex-military roller
> inductors, this one is "really big", and utilizes utilizes a pair of solid
> copper "shoes" on a slider to make contact with the windings. The
windings are
> rectangular section solid copper, .185" thick X .325" wide. The coil is
6" long
> X 6.5" in diameter, with 14 turns total. As measured using a Sencore Z-meter
> II, the range of inductance adjustment is from .1uH to 20uH. Weight is just
> under 20 lbs.

	Any idea who sold it? That looks suspiciously like a coil I used at
Northrop several years ago.  Belonged to Dick Norton.
 
> As soon as I can rig up a gear motor to allow this to be adjusted from a
> distance, I'll add it to my 6" X 32", 120ma NST-powered coil and see if
dynamic
> tuning is worth the effort.
> 
> Scott

	I tried tuning the primary of a small coil here with a small rotary
inductor from a "command series" transmitter.  I simplified the rotation
by putting a pulley on the shaft and turning it with a long nylon cord. 
Not as neat as a motor, but a LOT less work.  I'd think that something
similar (but with a big pulley) would work in your case.

	Qualitatively, I could notice some difference in streamer length when
running the coil at very low power, but not much.  I have the actual
measurements buried somewhere in my "archives".  Conclusion was that,
for a primary with as many turns(~15) as I was using it wasn't worth the
bother, but probably would be for a primary with only a very few turns. 
Of course, the external inductor alters the coupling as well as the
resonant frequency, so things get a little complicated.

	I can't remember anyone posting the results (such as streamer length vs
primary inductance) of "fine tuning" a big TC.  Is there anything like
that in the archives, and if so how would one find it?  For the little
coils I've built and the couplings I've used, a 5% change in primary
inductance wasn't particularly significant.

Ed