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Date: 17 Jul 2001 14:51:59 +0100
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From: "Andrew Talbot" <ACTALBOT@dstl.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: ELF Antennas - ground loops tested.
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I did hear from a non attributable source in the academic world that at one
time there was a proposal in this country to use the third rail of the
railway network as an antenna for ELF/ULF communications.   But he was not
able to recall if any actual tests were ever made of this medium.

For readers outside the UK (and those living in the north of the country who
have overhead powered electric trains) the third rail supplies power for
the railway in the Southern part of the UK.  It is a system that dates back
to the 1950s (I think) and runs at about 700 Volts DC - presumably at a few
thousand Amps for the larger commuter trains.    Not very efficient, there
are transformers and rectifier houses every few km along the track.  Return
current is via the normal tracks at ground potential.   The network provides
a well insulated mass of metal which could be either loaded against ground
or turned into massive horizontal loops.  This assumes that the difficulties
of providing the electrical isolation between the railway power, the railway
signalling and transmit signals could be organised.   The presence of trains
on the track would also presumably affect the tuning !

I also heard that a test was once made (sometime in the 1960s) using a spare
power station, running at a frequency other than 50 Hz, loaded into a
section of high voltage power grid shorted to ground at the far end.  Wonder
what they did about phase balance ?  I don't know how successful these tests
were, but at least they did actually happen.

Presumably other ideas for antennas were also tried back then.

The fact that we do not use ULF/ELF for submarine comms suggests that either
the tests were unsuccessful, or, more likely, we did not need the sort of
communications that ULF/ELF would give, ie.  world wide, very low data rate
with deep submerged vessels.   

Readers of Tom Clancy's "The Hunt for Red October" can work out that the US
do indeed have something like this !   The commemorative transmission from
GBR a few weeks ago also formally clears the fact that we use 16kHz for our
comms links - hopefully I will no longer have to use the word allegedly when
stating this.

It would certainly seem that loops were considered the way to go at low
frequencies.  There are no apocryphal tales of km high metal towers with
megavolts on them !

Andy  G4JNT


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