I've had buried coax runs at two locations. Both used unperforated PVC
pipe, 100 mm (approx. 4") in diameter.
First location:
Sloping ground, around 25 m (80') to the tower, good drainage. Five RG213
lines.
Second location:
Relatively flat ground, five pipes with runs up to 70 m (220'), and
extremely poor drainage with a high water table (manholes often fill up with
water after rains). One 7/8" hardline run, seven 1/2" hardline runs, around
a dozen RG213 runs.
Both produced the same results. There was always water in the pipes
(presumably due to the condensation mechanism that has been mentioned). At
the second location, I know for a fact that some lines, including some
RG213, lie in water all the time.
However, I measure cable loss annually, just before the contest season.
There was no measureable deterioration in loss at the first location, after
8 years. The second location is only 4 years old, and there's no
measureable deterioration yet.
Incidentally, I used to measure loss with a dummy load and a wattmeter. I
now use the MFJ259, and a calibration chart. They provide a chart for
unterminated lines (i.e. infinite SWR), which I've checked against the
original wattmeter readings. They tally very well.
I now have become even lazier. I no longer terminate the antennas with open
or short circuits for these measurements. Instead, I measure with an
antenna connected, and using a range of frequencies different to that
favoured by the Yagi. As long as you stay off its working range and
secondary resonances, its SWR is close enough to infinity to provide a very
good approximation. Of course, a better-than-infinite-SWR match will reduce
the reflected power, making the cable look more lossy than it really is. In
other words, if you obtain the original (unterminated) reading over a range
of frequencies, you can accept those readings even if there is an antenna
connected to the feedlines.
The advantage is that I can now measure the integrity of all the feedlines
in the station within a few minutes, and without having to leave the shack.
I'm sure that both the assertion that there is little detriment from leaving
the cables submerged in water, and the Heath Robinson technique for checking
line loss, will leave purists cringing, but they work. It's good enough for
me.
Chris R. Burger
ZS6EZ
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