Dear Roger, LF Group,
Also, you do wonder with the network of gas pipes, water pipes,
electricity
pylons, phone lines etc criss-crossing vast swathes of countryside just
how
far "utility assisted" (for the want of a better term) earth mode VLF
comms
could travel.
VLF signal propagation via utility pipes and cables is widely used by
contractors as a method for pipe and cable location, and there has been
equipment commercially available for many years - see for example the quite
informative brochure at
http://www.radiodetection.com/doclib/RDTheoryManual_complete08_en.pdf
The equipment comprises a detector, which can detect magnetic fields due to
mains frequencies, ambient signals from VLF utilities that couple to buried
conductors, or a VLF or audio signal from a "Genny" low-power transmitter
unit which can be directly or inductively coupled to the buried conductor.
I'm told by people who use these things that plastic pipes are supposed to
be installed with a tracer wire to make location easier, but this doesn't
always happen ... different buried conductors can also couple to each other,
and wild goose chases are apparently not unusual!
I don't know about buried conductors, but overhead telephone and power lines
are the norm around my QTH, and when I have made mobile field strength
measurements there have certainly been observable effects on 136k and 500k
signals. In particular, overhead lines that are aligned in a radial
direction from the transmitter produce peaks and nulls in the apparent field
strength when the measuring receiver is close to them, corresponding to a
standing wave pattern along the line. This means you have to be quite
careful about choosing sites to make FS measurements - there seems to be
much less effect when the overhead line is at right angles to the direction
of the transmitter. However, the overall results of the FS measurements
indicate that this kind of coupling only has a very localised effect near to
the coupled conductor - it certainly does not seem to enhance the overall
radiated signal at these frequencies, although VLF might well be different.
Cheers, Jim Moritz
73 de M0BMU
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