This is a CW
carrier that has been present on 136.645 or thereabouts for as long as I
have been listening on LF from my present QTH. It is of the order of several
uV/m, so quite strong when the band is quiet, and often makes it difficult
to copy CW stations here. It is just a carrier, with no modulation I can
detect, and I have never known it to be QRT.
I D/F?d it a few
years ago to a 400kV overhead power line that runs from a switching yard
near Aldenham, Watford, just beside the
M1 motorway, to another switching yard north of Luton, also beside the
M1. The signal strength increases very rapidly under the lines, so I guess
the signal is transmitted differentially between two or more of the
conductors, and the radiated carrier is just ?leakage? from the transmission
line caused by imbalance of one kind or another.
I believe the range
9kHz to 148.5kHz is used in Europe for power line signaling - If you want to
find out whether a particular power line is carrying a similar signal, I
would recommend going /P and getting as close up to the power line as
possible ? the high signal strength will then make it relatively easy to
differentiate between power line signals and the other noises one hears on
LF. It should be fairly easy to follow the route of the cable, and see if
the noise is associated with it at several places ? LF noise seems to
propagate for a long way down most kinds of cable.