Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:

A conductor carrying RF currents and immersed in soil behaves like a
lossy transmission line. The bahavior depends on the parameters of
the soil (amongst other things), so the effect is different in dry
sand than in wet clay. Current entering the line will be attenuated
along the line. . .

Please permit me to amplify on what Owen said, because it looks like
it might be interpreted. Burying coax in soil has no effect on the
signal inside the coax -- the loss of the coax transmission line isn't
increased. The lossy transmission line Owen refers to is the line made
up of the outside of the coax and, for the other conductor, the Earth.
This is the transmission line which carries the common mode current
and the one which will be made lossy by burying the coax.

Roy, one of the lingering gaps in my post was that I spoke of a single
coax in a plastic conduit. As many readers will recognise, that is an
important qualification.

If you put the coax cable inside a metallic conduit, attenuation of
common mode current on the coax is much lower because it forms a coaxial
TL with the conduit. The next question is what is the CM current on the
outside of the conduit, and that requires examination of the end
conditions on the conduit.

The other issue, is that people often install multiple coaxs in one non
metallic conduit. If they are not bonded together, they may convey energy
with relatively lower attenuation by driving the coax outers in
differential mode.

I recall a project where understanding the configuration of underground
medium voltage and high voltage power entry to a comms facililty was real
important in modelling EMP impact. A neutral screened medium voltage
cable worked differently to three conductors in trefoil or flat. HV
cables were invariably armoured, and the armouring was relevant,
depending on how it was treated at both ends. In some situations, 30m of
underground power entry cleaned it of EMP, in others, it made almost no
difference (though still relevant in modelling).

Owen