Dear LF Group,
Alan makes reference to the loading coils at the 16kHz VLF station
at Rugby; A while ago I got hold of a couple of old papers detailing
the original construction of the VLF stations at Rugby (1925) and
Criggion (1943).
The loading coils for both stations were wound as spirals on five
hexagonal wooden spiders, roughly 4m diameter. The spiders could
be moved to vary the inductance, which was about 2.5mH for
Rugby, and 5.4mH for Criggion. The wood is described as
"American whitewood", which the Post Office engineers apparently
found was a highly satisfactory material; no other insulation for the
windings was used. The antenna voltage and current was 220kV,
400A at Criggion, and 160kV, 600A at Rugby. The windings were
(HF) litz wire made up from 6561 strands of 36swg. The Q was in
the range 2000 to 4000. The Rugby transmitter building was
designed to minimise the amount of metal, etc. close to the loading
coils, while at Criggion, the loading coil was inside a reinforced
concrete building. This caused high losses until a Faraday shield
was installed by lining the walls with a mesh of copper wires. The
Rugby transmitter building burned down in 1943, so we are not the
only ones with these problems!
Both these stations are still there (obviously modernised a bit), and
worth a look if you are passing by. Rugby is by the side of the A5,
and Criggion is on the Welsh borders a few miles east of
Welshpool. The road to Criggion village actually passes under the
VLF antenna, so you get a good view there.
It is interesting to read the discussion at the end of the Rugby
paper - some audience members state that the VLF station had
effectively been rendered obsolete by the HF beam systems then
coming into being - yet, 75 years later, GBR is still there on 16kHz,
and it is the HF systems that have disappeared!
Cheers, Jim Moritz
73 de M0BMU
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