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[TowerTalk] Force 12 loading coils

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Subject: [TowerTalk] Force 12 loading coils
From: "Jim Thomson" <jim.thom@telus.net>
Date: Tue, 5 Jan 2016 03:00:02 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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