[Antennas] Twin lead 450 Ohm

[Dave Kelley]

>Opinions Please...If 450 later line was suspended in
>the center of a ten" PVC pipe and buried would it have
>the properties of one suspended in Air?

It's a little difficult to say exactly how large the diameter
of the pipe needs to be to avoid significant losses, but
you're on the right track.

In a balanced, parallel-wire line the electric and magnetic
fields are most intense in the space between the wires
and taper off in intensity away from the wires.  To get a
sense of what the field structure looks like, think back to
when you were a kid in school.  If you were lucky, some
teacher somewhere placed a bar magnet under a piece
of paper and sprinkled iron filings on the paper.  With a
little agitation, the filings arranged themselves into a pattern
that indicated what the magnetic field around the magnet
looked like - crowded field lines between the north and
south ends of the magnet and less crowded farther away
from the magnet.  The cross section of the electric field
structure around parallel-wire line is similar.

So the question in your case is how strong the fields are
5 inches away from center of the ladder line.  My guess
is that the field strength is pretty low at that point.  The
amount of loss you'll get will depend on how much of the
electric and magnetic flux penetrates the lossy earth.
The presence of the earth all around the ladder line will
change the field structure slightly from what it would be
in free space, so this isn't a simple thing to analyze.

Probably the best advice is that offered by another 
responder:  Build it and see what happens.  Of course,
it will be a major pain to modify the installation if you
find there is too much loss, but at least you'll have your
answer.  Please share your results with us if you try it!

73,
Dave NB4J

--
David F. Kelley, Ph.D.
Assistant Professor
Electrical Engineering Dept.
Bucknell University
Lewisburg, PA 17837
dkelley@bucknell.edu