I agree with Owen, something here doesn't compute. The only way I can
make sense out of it is if your line length is really an odd number of
quarter waves, and it's exceptionally lossy. As a first thing to do in
resolving the matter, I recommend measuring the velocity factor of the
main coax. For best accuracy, short circuit the far end of the line with
a low-inductance short circuit, and look for a low Z at the input. (You
can even use a scope or diode detector for this.) At low frequencies, a
single wire will do for the short; at higher frequencies, use multiple
wires radially extending from the center to the shield, or a metal
plate. At very low frequencies, the impedance will be low, increasining
with frequency. At some point it'll rise and become very high, then drop
again as frequency increases. Find the frequency where it's the lowest
-- this is the frequency where the line is exactly a half wavelength.
Physically measure the line and calculate the velocity factor. For line
constructed like you describe, the velocity factor should be around 84%.
A significantly lower factor probably means it has water in it. If it
comes out about right, let us know and we'll go from there.

Roy Lewallen, W7EL