[Lowfer] 8 kHz VLF loading coil idea

[W2MXW]

Hi all,
I have an idea and a question I want to run by you good folks. I'm in the
process of constructing a VLF beacon to operate at 8.7 kHz, with a vertical
antenna (either a 'Tee' or umbrella, not finalized yet). It will operate
24/7 in CW (normal) & QRSS modes, at about 50 watts TPO. Not much for VLF,
but the problems associated with getting up to higher power levels (*really*
high antenna voltage, for one) are for the time being discouraging me from
attempting same. Besides, as with Lowfering, the fun is in the challenge of
trying to hear/see the tiny signal! :-)
I think that the loading coil required at VLF is the most difficult part of
the whole system, especially making the coil easily tunable. My idea is to
make a tunable loading inductor by using a high voltage transformer, such as
a MOT (microwave oven transformer) or plate transformer, having a bit more
than the required inductance, and applying a variable DC bias to it to
basically create a saturable-core reactor. By varying the DC bias it
*should* in theory vary (by reduction) the inductance in a controllable
manner thus allowing fine tuning of the antenna system to resonance. I don't
think it's an entirely new idea, I seem to recall reading about something
like that used (on a HUGE scale) at one of the Navy VLFers, but I could be
wrong.
Here's my plan: take the MOT or PT, immerse it in an oil tank to get higher
insulation value (MOTs are rated for about 2 kV and it's likely the
secondary wdg. will have much more than that across it at resonance.) Maybe
a potential trafo rated for higher voltage would be better. A pole pig would
be great but is overkill for this power level. The point is, something
capable of taking some kV and with a bit more inductance than required
(likely around or a bit over 1 Henry for 8.7 kHz and the size of antenna I
can install). I don't know what the inductance of the typical MOT is but I
can find out. Hopefully it will be what I need, as I have a lot of them!
Anyway, the HV secondary would be used as the loading coil. The 120 volt
primary would be used as the bias winding, with DC applied via the
rectified, filtered output of a Variac (we don't want modulation, although
purposefully producing same in this manner might be in order for future
experiments!) The Variac would be used to control the DC bias in the primary
wdg. of the tranny, thus (hopefully) saturating the core in a controlled
manner and permitting reduction of the inductance to the value needed for
resonating the antenna. The Variac would of course have to have a current
limiter, probably a choke or capacitor, in its supply line.
This would be like "fine-tuning", so much better than a tapped transformer
(which would be hard to get anyway, in a HV style.) And any cap used with a
tapped coil for fine-tune, were that method to be used, would need to be a
big vacuum job to take the volts. A real pain, to say the least.
My question is: will this saturable-reactor idea work, or am I barking up
the wrong proverbial tree? Am I missing something obvious (or sublime?) Will
it take so much current to bias the trafo that my electric meter will spin
at Warp 9.5? I really want to find an "elegant" (and compact) solution to
the loading coil problem at VLF without having to build a helix house 3
stories tall and winding the coil out of copper tubing! (I know I can do
earth current, which doesn't need a load coil, and I probably will in
future, but right now I really wanna use a vertical!! :-)
Thoughts? Comments? Criticisms? Input? Thanks!!

73 de Jon W2MXW

P.S. Sorry for such a lengthy post.