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
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