[Lowfer] Coil question

[John Hoopes]

Hi Clint,

I remember reading that particular Western Update that talked about the west
coast gang's camp-out/outing. It had photos of the gang and a picture of
Mark and his coil which he brought along for show-and-tell. Man...was that
thing big. I thought he was from Texas for a moment! HI. That was one beauty
of a coil though.

73
John/AB4MS/JDH

----- Original Message -----
From: Clint Turner <turner@ussc.com>
To: <lowfer@mailman.qth.net>
Sent: Thursday, February 14, 2002 3:55 PM
Subject: Re: [Lowfer] Coil question


> $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"
>
>
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