Some time ago I obtained a Decca loading coil on one of my visits to the
Crawley radio club. The Decca coil on page 64 of the LF Handbook illustrates
the general construction of one of these coils and the Decca loading and
matching system is shown on page 13. The Decca system used five transmitters
multiplexed (if this is the right word) into a single transmitting antenna
so the coils are quite complex, comprising several separate windings wound
using a low capacity basket
weave on the same former.
The purpose of this exercise was to see if this coil might be suitable for
amateur LF. For the coil to be of any use it would have to be completely
There seems to be two different types of construction. The one shown on page
84 of the LF book is an older type, which uses Perspex formers and are fixed
in place using an adhesive. This means that it is very difficult to
dismantle without damage. Furthermore the Litz wire is difficult solder -
you have to use methods as described on Page 84.
The later coils use a former ribs made of plastic using a sort of Lego
construction, which is held in place by the wire itself. Furthermore, the
Litz can be soldered by just dipping the end into a solder pot (after first
removing the plastic outer insulation).
The coil was easily dismantled in an afternoon and a finisher up with a
large plastic box of Lego bits, about 25kg of Litz and a basic coil former
comprising a hard plastic cylinder 449mm in diameter and 220mm long.
As you can see from the photo, the coil will have to be multi-layered. Is
there any performance penalty in using such a method of construction
compared with using the straight single layered coil?. One problem that
occurs to me is the potential gradient across the coil but I know how to
avoid any difficulties in this regard.
I wish to thank Dick, Rick and Andy for multi-layer coil formula. At the
moment I think that empirical experimenting is going to be the only way of
obtaining any useful information. Has anyone else got a Decca coil and have
they tried using it?
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