[Lowfer] Coil question

[Clint Turner]

$0.02

One of the best thought-out, highest-Q LowFER loading coils that I have
seen was that used by Mark Mallory, WB7CAK on the MPM beacon.

This beacon was in operation in the Salt Lake City, Utah area in the
late '80s.  Since it was a relatively long time ago, details are
somewhat sketchy, but a few details also appeared in the Western Update
at the time.  I'll drag the details from my brain as I can remember
them.  (If there is serious interest, I could peruse my archives and
maybe even pester Mark himself...)

Mark started off by analyzing things inside-and-out, consulting
classical texts such as Watt (LF Engineer's Handbook) and Cornell (need
I say more?) as to the best way to obtain highest Q, as well as did a
few experiments.

His ultimate conclusion was that a "short, fat" coil was best - about
1.5 times as big in diameter as tall.  (I'd have to check on that, but
that's what I remember it looking like.)

The form was made out of wood, which was varnished (to prevent moisture
ingress.)  The top and bottom were solid pieces, I believe, and an ODD
number of dowels were used.

The wire was, as I recall, 100/40 litz with the "Nyleze" insulation.
Mark said that he ordered as much wire as the minimum order from the
wire company (K&R - Chicago, I believe) would buy him - and it was
plenty, apparently.

The wire was wound in basket-wave style so that no two adjacent turns
paralleled each other - they only "crossed" each other   between
dowels.  I don't recall how many "rotating polygons" there were in the
winding pattern (i.e. how many dowels were skipped between each
"securing" dowel.  Does that make sense?)  The idea of this was to
absolutely minimize the self-capacitance of the coil itself (leaving as
much as possible to the antenna...)  Once it was wound, I believe that
the top portion was slid over the dowels.  (I don't know how the wire
was secured, whether it was glued or held in by notches.)

I'm reasonably sure that his measured Q was well over 1000 (the actual
measured figure appeared in the Western Update, I believe.)  The coil
adjustment was done using a variometer.

With such an absurdly high-Q, it turned out to be difficult to keep the
system tuned:  The mere presence of Fog would detune the antenna enough
to be annoying.  For actual operation, Mark built a simple circuit that
used a voltage detector, a current detector, a 1496 (or was it an Exar
chip?) that detected the phase of the voltage and current with respect
to each other.  When these drifted apart (a "fixed" offset being taken
into account) the output signal drove a stepper motor driver assembly
attached to the variometer and kept the antenna in tune.

Interestingly, after a short time of operation, it was apparent that the
tuner could only be enabled during key-down positions (it seemed obvious
- after the fact) or else the tuner would go bonkers during key-up,
trying to tune-out (as mismatch) whatever random noise appeared on the
detector.  I don't know if he used a simple threshold detector or had an
"enable" line on the tuner.

What happened to this coil?  Mark eventually moved to the Bay area and I
know that the loading coil was used, for a time by Jim Erikson and (I
believe) Herb Vanderbeek.  It was (sadly) destroyed by a lightning
strike (I think) a number of years ago - but it would probably be best
to ask someone who was there:-)

As of a few years ago, Mark still used the SWR bridge device on an HF
loop antenna (and I had the pleasure of using it, too.)  For the
morbidly curious, a picture of the loop's twin brother (without the
automatic matcher system) may be found at:

http://www.ussc.com/~turner/fc.html

Clint
KA7OEI
"CT"