From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Wed, 31 Oct 2001 10:32:12 +0000
Subject: Re: LF: Multi-layer loading coils
In-reply-to: <000a01c16187$aec96d00$49c928c3@ericadodd>
Message-ID: <E15ysc1-0004SW-00@gemini.herts.ac.uk>
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Dear Peter, LF group,

I also got one of the Decca loading coils - mine was from the scrap 
pile at Puckeridge, and was of the old type with the difficult-to-
solder litz and the former solvent glued together, and so not 
possible to dismantle without breaking many parts. Mine had 12 
wires in parallel and L of a few hundred uH, so not very good for 
my antenna, which needs about 4mH to resonate on 136k.

In the end, I just used the wire from the Decca coil and scrapped 
the rest. As a former, I found a "sectional manhole" made by Osma 
was very suitable. This is a brown PVC cylinder about 500mm 
diameter and 350mm long, with external ribs. It is meant to form the 
walls of manholes for drains, and several sections can be slotted 
together for a bigger former if needed.

About 6 turns of the Decca litz wire fit between each pair of ribs, 
forming a winding 3 layers deep by 2 long. about 80 or so turns 
altogether gave me the required inductance, which has a Q of 
about 700. I tried several experiments and found that in all practical 
cases, single layer windings gave higher Q than multi-layer ones, 
and have the advantage that there is the maximum seperation 
between the two ends, which may have upwards of 20kV between 
them when running 1.2kW. However, a single layer coil using the 
thick Decca wire would have been very large - about 2 or 3 metres 
long using the sectional manholes - so the configuration above, 
which is effectively 14 x 6 turn windings in series, each 3 deep, is 
a reasonable compromise between Q, L, and size, and keeps the 
voltage between adjacent turns below 1kV if wound carefully. This 
has been working happily for the last year or so. I am using a 
second sectional manhole wound with more turns of smaller, 
ordinary stranded wire for 73k, which stacks on top of the 136k 
loading coil.

As others have pointed out in the past, big loading coils only give 
minor improvements in antenna efficiency - but the main reason for 
having one is so that it does not melt, catch fire, etc when running 
high power. This one barely gets warm at the 1.2kW level.

Cheers, Jim Moritz
73 de M0BMU