[Lowfer] 8 kHz VLF loading coil idea

[W2MXW]

-----Original Message-----
From: Bill Ashlock <ashlockw@hotmail.com>
To: lowfer@mailman.qth.net <lowfer@mailman.qth.net>
Date: Sunday, August 10, 2003 5:53 PM
Subject: Re: [Lowfer] 8 kHz VLF loading coil idea


>Jon,
>
>Always exciting to work in unchartered territory!

Yes, it is exciting, and long overdue. I have been mulling over the idea of
transmitting on VLF in fits and starts for a long time, I decided I want to
just do it already!

>>I think that the loading coil required at VLF is the most difficult part
of
>>the whole system, especially making the coil easily tunable.
>
>This would be the easy part from my perspective, although this coil will
>have to have a reasonably high Q, maybe 100. Why not use a ferrite coil?
You
>would have to gap the ferrite to eliminate the saturation tendencies of
>ferrite, however. The gap size could be the tuning feature.

I actually had thought of that first; I have a nice big color TV flyback
which could be modified for adjustable airgap tuning. However, I had
a couple concerns, besides the mechanical logistics of making the core
adjustable: A) Will it have enough inductance? (I don't know what
these typically spec at.) I'd need probably around a Henry, maybe more since
tuning can only be done by decreasing inductance. I guess I'd have to
measure it with a cap and sig. gen. B) Will it take 50 watts? (or
thereabouts.) The secondary is wound with what looks to be Litz (from what I
can tell of the bare 'ground' end) and it is pie-wound in nature so it may
have a fairly high Q. But I wonder if that fine wire can handle the current
that may be pushed through it? Truthfully, I'd rather
use it than a MOT, as long as it could do the job. But I got the 'pet' idea
of the saturable reactor, and you know how we all cling to 'pet' ideas! :-)
I like the thought of tuning electronically as opposed to mechanically.
The thing about making a custom coil is I really don't feel like winding a
bazillion turns, even with hi-mu ferrite, so I'm trying to find a ready-made
option.

>How much open land do you have? You will be faced with the typical ground
>radial problems and the need to have a lot of capacity in the top hat. ...
>and your site has to be pretty free of trees.  No relief from the 'tree
>effect' at these low freqs, as the trees still look electrically like a
huge
>distributed ~10K resistor and any small capacity coupling into them will
>kill your antenna current due to reduced system Q.

Yeah, that is the downside to a vertical...and I do have plenty of trees.
The silver lining is they could make good supports for capacity hat umbrella
elements (if I decide to go that route). I guess you just have to work with
what you've got. I do have room for a pretty big cap hat, tree issues aside,
and can arrange it so that at least none of them will be 'inside'/under the
hat.

>Have you considered a loop antenna placed over the top of any trees you may
>have? I admit that my first attempt at operating a loop at 9 kHz was
>discouraging but have some areas to work on before I give up.

I have thought about a loop too, and may very well still try it, especially
if you or someone else gets good results so there is something concrete in
terms of design to work from. For now though a vertical just seems to have a
certain appeal, despite the fact that it will probably be the least
efficient of the three options (earth current, loop, and vertical).

>One of these
>includes finding a low-cost, high Q, 10uf resonating cap, or bank, able to
>handle some 20A and 400vac. ....That won't bow up. <G>

Have you considered one of those Maxwell pulse-power oil caps, or a
power-factor correction cap? Those should really take a beating. Don't know
about the Q though...

>At least the
>transmitter is simple.

Yep, TX is the easy part. I am using a pair of 6L6GBs in Class AB1 P-P, with
untuned output (just an output tranny) basically a simple audio power amp. I
chose a linear mode for the finals because, while I realize Class C would be
fine since we're not reproducing music or voice and would give more output
power to boot, it'll also produce a lot of harmonics. Linear operating mode
plus push-pull will help keep that to a minimum, but I'll still be using a
LPF after it, before the antenna tuner. Don't wanna get into trouble for
causing interference to above 9 kHz.
I can pretty much plug in any
oscillator/exciter I want to the final, purposely designed it that way, but
I think my main osc. is going to be a fundamental xtal-controlled job (yes,
I have an honest-to-gosh VLF
xtal).
Sorry for the long delay in responding, that's what happens when I get so
much email (not always devoid of spam, unfortunately  :-( that I often let a
day or two go by without checking it...

>Good luck!

Thanks!!

73, Jon