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
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