[GreenKeys] 88 mh toroids

[Jim Haynes]

Yeah, there's that guy on ebay who keeps trying to sell a set of
slug-tuned coils for RTTY use.  Before 88 mh toroids were discovered
by hams there were some T.U. designs that used TV width coils - slug
tuned coils.  There were others that used transformers or chokes -
the very early W2BFD T.U. used iron-core transformers or chokes with
the I laminations removed, putting two of them face-to-face to
create a coupled circuit that was a good bandpass filter.  But
when 88mh load coils became readily available there were all kinds of
T.U. filter designs that used them, both for input bandpass filtering
and for mark and space channel filters.

Must have been the summer of 1956 that I worked on installing an
intercity toll cable of 101 pairs.  Most of the pairs were unloaded
and were going to be used for N carrier operation, but a few were
for service to small towns along the route and were loaded.  The
loading coils we were using at that time were molded in epoxy.
No doubt by now the cable has been dug up and replaced by fiber.

What loading coils were all about is - the equations of transmission
on wire pairs show that losses can be reduced by adding series inductance.
That might be surprising, but it works.  However it only works up to a
point in the frequency spectrum; in effect it adds low-pass filtering
so that a pair loaded for voice frequency operation cannot be used for
carrier operation.  The situation could be improved by inserting coils
of less inductance closer together, but for cost reasons they want to
use as few coils as possible spaced as far apart as possible and still
preserve the frequency response of the line over the voice bandwidth.

An interesting case of loading came in a cable made for Western Union
for undersea use.  AT&T had developed Permalloy, a very high permeability
magnetic alloy.  They manufactured a cable for W.U. in which the
center conductor was wrapped in a thin Permalloy tape, so that the cable
was continuously loaded throughout its length.  This was first done about
1925 and was very successful.