-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Kim Elmore
Sent: Sonntag, 21. Dezember 2008 23:31
To: towertalk@contesting.com
Subject: [TowerTalk] Ew: PL-259s Going Open?
As a bit of additional info, I haven't direct-buried any 9913: i's all in
conduit. That said, there's no guarantee that the conduit has remained
perfectly dry. That said, I have seen no evidence whatsoever that the coax
itself has suffered any water ingress, and I've even attempted to draw water
through the coax like a straw (to quote Yukon
Cornelius: "Nothin'").
Now, a comment: I've heard stories of changes in barometric pressure and/or
temperature causing air-dielectric cable to "breathe." I've found no
evidence of that in may case, and I've looked pretty carefully. As a
meteorologist, I can say that atmospheric pressure changes by only about
3.5% from highest to lowest pressure and under isothermal conditions, this
will cause a volume change of 3.5%. As for ingress via water vapor, there is
actually very little water in the air. Under saturated conditions (100% RH)
at 20 C, the mixing ratio is 15 g H2O per kg of dry air (about 15 cc). At an
RH of about 50%, there's about 8 g H2O per kg dry air (about 8 cc). A kg of
dry air at STP has a volume of about 1 cubic meter. I don't know what the
contained volume of 9913 is, but I'll bet it takes a very long length to
contain 1 cu m. To actually condense out all the water would require cooling
the air to a very cold temperature (about 0 F would do most of the job).
Given that the induced volume change is very small, water ingress due to
vapor drawn in by pressure changes (or temperature changes, for that matter)
is probably negligible. Now, I'm obviously neglecting any hygroscopic
concerns. But, if an open end of the coax effectively sits in water and the
other end is well sealed, it could act as a straw and draw water into the
open end. However, there's a limit to how much water could be drawn in, so
not even this effect could fill a tube with water. In fact, if you drew a
vacuum on the open end, you could support a water column of only about 34
feet. So, to get much water into the coax, there must be a couple of leaks
someplace, with a way for air to escape and be displaced by water. Under
that circumstance, you could completely fill the coax with water.
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My experience with air dielectric cable is different.
When not run vertically condensed water will collect and short the cable at
the lowest point of the horizontal run if not pressurized.
It took 5 years to happen on a 150m run of 7/8" cable at DF0CG/DR1A, of
course some hours before the start of a contest.
73
Peter
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