Date: Mon, 4 Jan 2016 15:24:25 +0100
From: "Peter Voelpel" <dj7ww@t-online.de>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Force 12 loading coils
With wider turn spacings, capacity between turns is lower and coil Q higher.
Centre loading is more efficient then base loading on shortened antennas.
Silver plating of coils with cool-amp is for the birds.
73
Peter
## With wider turns spacing, UH drops like a rock. Then you have to add a
helluva lot
more turns to get the UH back to where it was originally. Then you just added
additional total capacitance, so you end up back to square one. Also, the coil
assy
will end up being a LOT longer..( like almost double).....1st from the 50%
increased turns spacing, then again
from the additional wide spaced turns required to get the uh back up to
normal. The only way
around that dilema is to increase the OD of the coil assy, which then makes for
a huge coil.
Look at the latest version coils used by F12 for their retrofit kits, they are
massive with their
3/8 inch spacing between the 1/4 inch al tubing. Then to top it off, they
still did not make em big enough
uh, hence the new tip dimensions are insane vs oem. 60-70 inch long tips
made from 3/8 inch tubing is crazy.
For fixed coils, the spacing between turns should be equal to the tubing
diameter.
## The problem with center loading...half way out each ele, is the UH has to
be DOUBLE vs
loading next to the boom. The advantage of loading half way out is the Z is
higher.
A good compromise is to insert the loading coils 1/4 the way out on each
side.
F12 does this on their 340N style eles... which vary from 52-59 ft in length.
Optibeam does the same thing..as does Jk ants. The problem with either a
tornado drive, or
relay switched coils...half way out each ele half is.... you get RF into the
control wiring which is
a real pita to resolve.
## On my 80m rotary dipole, I wanted to be able to use it across the entire
band. F12
used these puny 12 gauge THNN coils, wound butt tight, in air, 6 of them, 3
per leg, +
3 x DPST 30A relays, to switch the 8 segments. The Seco tornado drive uses
a
pair of compressible 6-12 uh coils, made from 1/4 inch OD plastic coated Cu
tubing, installed
right at the feedpoint to allow for entire band coverage. The LL wires were
replaced with 16 ft long
capacity hats...aka.. T bars, to provide for the main loading. The 6-12 uh
coils, provide the balance of the
loading, and allow for 3200-4100 khz operation. I could have used large 30
uh coils, half way out each side, instead
of the T bar scheme, but still would require the tornado drive to cover the
entire band. The tornado drive can be
built with greater or less UH, but you still end up with a max to min ratio of
2:1. To cover the entire 80m band, I require
a 6 uh spread. Unlike a roller coil, that will go down to zero uh, the
compressible coil scheme has limitations, with its
2:1 max-min ratio. I cant do a 0-6 uh spread on a tornado drive. The
smallest that will work is a 6-12 uh setup.
Another scheme I was going to use was 8 x SPDT vac relays and tubing coils at
the feedpoint.
But even that wont cover the entire band, just the cw and center phone band.
The ant is only 65 khz wide.
Another scheme I cooked up was to use a motor driven vac cap at the feedpoint
to raise the resonant freq. But the
cap was large and heavy assy, so dropped the install of it. In the end, the
seco was used, simple, and with 10 x pre-sets,
plus manual tuning between pre-sets, plus digital turns counter, its makes for
easy qsy across the entire band. 1/4 inch OD
Cu tubing is a lot better than 12 ga THNN wire wound butt tight.
## where I used 4 x SPDT vac relays was at the feedpoint of the 40m yagi, DE
only. That provides for 4 x
segments, which is ample..and overkill, to ensure flat swr across the entire
40m band. Each coil is just 7 turns
of 1 inch wide silver plated cu strap, with a 1.5 inch ID. C between turns is
nothing with strap coils, wound flat....
like a 10m strap coil in a linear amp. Like 2 x knife edges facing each
other. Very little spacing required between turns.
## I silver plate all my cu tubing, strap, etc, simply cuz I hate the look
of bare copper...drives me nuts..esp inside a linear amp.
I have loads of cool-amp goop so no big deal to use it. I have also used
cool-amp silver plating on mech relay contacts..like the
30A P+B and deltrol relays..makes a HUGE difference. F12 supplied deltrol
30A DPST relays with its 40m switch box, and
the damn contacts are high resistance, right out of the box..which makes for
bad news on RX. I have also used it on contactors,
same deal. We used it at work in the telco for cu buss bars..where they bolt
together..and that’s for –52 vdc use. Power
companies uses it all the time on buss bar connections etc..and that’s at 60
hz. Its also a good interface between Cu and AL surfaces.
Sure the coils don’t require it, but Id at least use it at the extreme ends of
the coils, where the connections are made. Plan B
would be to use solid AL tubing,and heliarc weld the ends...like JK ants
does.
later... Jim VE7RF
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