Mike,
The balanced line in theory cancels fields a short distance away from its own
conductors. However, if it runs close to a conductor, it might become
unbalanced in one side and then radiate. One way to improve its balance
against random outside influence is to twist the line periodically, thus
exposing both sides to any random influence. A twist every foot was about
what was done with older ladder line feed to early TV antennas.
If the soil is not very conductive, ie low iron or bauxite content, (not much
like iron or aluminium), and if you could run ladder line inside a plastic
pipe that does not accidentally have any filler material, like 6 inch sewer
drain pipe, and the climate is dry such that you would not have condensation
in the pipe, you might be able to do what you want. But if you bury the pipe,
and ground water intrudes later, accidentally, then you will have a conductive
element all around the ladder line, if the pipe is too small in diameter. The
Black pipe would need to be tested, by a small piece in a microwave oven, with
a cup of water beside it. Boil the water with the microwave oven, and check
to see if the pipe got hot. Make sure the pipe is on a paper plate or
blotter, in case it softens. Some colorants of black are done with carbon,
which is a semiconductive component, commonly used in resistors, so it would
not be desirable near the balanced line. Also, some plastics have incidental
metal fillers, or other fillers in the composition, that might conduct or be
RF lossy.
It is usually far less trouble to just treat ladder line like telephone lines
of the open wire type, and run it on short poles off the ground, and tall
enough for anyone to pass under it. Ladderline is happiest in dry air. To
mitigate moisture effects on plastic covered, or windowed parallel line, some
folks advocate waxing the plastic to repel water standing on its surfaces.
If I were running ladderline in the desert, I might try the underground pipe
bit, but would use spacers to keep the line from touching the bottom of the
pipe in case of condensation. Also, to keep the line equally spaced from the
pipe walls.
Hope you see the range of problems. Most underground electrical installations
sooner or later suffer from water engress problems. Running above your yard
would be so much easier.
73, Stuart K5KVH
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