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Message-ID: <5.1.0.14.0.20020114111441.00aaa760@gemini.herts.ac.uk>
Date: Mon, 14 Jan 2002 12:07:29 +0000
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: LF: Mystery Signal Partly Solved
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Dear LF Group,

Since it seemed the mystery carrier on 136.647kHz might be somewhere near 
Watford, only about 10 miles from my QTH, I went out on Sunday morning 
armed with a loop antenna and RX, a compass and my field strength measuring 
gear. After taking a few bearings, the source did indeed turn out to be 
near Watford - it comes from some overhead power lines.

Although there are many national grid lines in the area, I found that the 
signal clearly came from one particular line, which appears to be the main 
feed line for a sub-station at Patchett Green, beside the M1 motorway just 
east of Watford (next door to the Hare Krishna temple!), at 51 39' 39" N, 0 
20' 31" W. According to the ordnance survey map, it runs for about 32km, 
roughly on a bearing of 340degrees, terminating at a sub-station near 
Chalton, just north of Dunstable. The signal level underneath the lines was 
fairly constant up to 5km from the Watford end, so I assume it is actually 
propagating along the full length of line. The field strength is only of 
the order of 10mV/m directly under the wires, so there can't be much power 
involved - however, since it is several wavelengths long and on big pylons 
it is probably a reasonably efficient radiator. I suppose the ERP is of the 
order of milliwatts.

As to why there is a 136kHz signal going down a 400kV power line, I really 
don't know - could it be a power line signalling system like the ones used 
by utilities in the USA, which have been an obstacle for those campaigning 
for an LF amateur band there? I have never been able to detect any 
modulation on the signal, so it can't be used very much if it is.

Cheers, Jim Moritz
73 de M0BMU