Hi Peter,
Peter Voelpel wrote:
> <snip>
>
> 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.
It likely won't but that shouldn't be a problem unless it's subject to
freeze, thaw cycles.
> 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'").
>
>
On that I've taken out coax and found the braid deteriorated as far back
as 3 to 6 feet from connectors.
> 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).
>
Although barometric pressure changes are small, temperature changes are
not. Coax in the direct sun can get too hot to handle or above 50C/122F
and pretty chilly such as 5 or 6C or ~ 40F in a single day. Empty
gallon cans in my shop subjected to normal daily changes this time of
year will pop and bang as the pressure changes due to temperature. That
is with the temp changing from 62 to 72F . The setback thermostat keeps
the temp at 60-62F and I run it up to 72 when working out there.
I have a number of 5 gallon gas cans. With only a gallon removed a
temperature change from 40 to 70 produces enough pressure that you can
hear it whistle out the vent when it is opened.
Over time with the daily temperature cycles the coax will repeatedly
draw in and expel air. Although not a great amount at any one time
hundreds of cycles will eventually produce liquid water. It's basically
the reason commercial, air core coax is purged with low pressure, dry
Nitrogen. Whether the freely breathing 9913 is an advantage or
disadvantage in this case could be argued. IOW the more air volume the
more it is affected. As 9913 is like, or rather is a tube with a center
conductor, air can move freely through it. I would think, but don't know
for sure, the free breathing 9913 would also readily expel moisture
except when liquid condensation has formed.
Having said all this, under normal conditions it might take years for
liquid water to actually form in an unbroken piece of coax that can
breathe freely. The problems would arise with high RH coupled with wide
temperature swings or a number of consecutive days with high RH.
> 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.
Don't forget the capillary effect. In a tube the maximum amount of water
that can be lifted is about 34', but it can go much higher in a small
capillary. Then there is the coax braid which can "wick" up the 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.
>
>
Or sealed coax with a leak can act as a pump, drawing in water as it
cools. More than likely it would just expel the moist air as it heated.
Now think of that sealed piece of coax with a nick in a heavy rain.
It's being cooled and drawing in liquid water which is unlikely to ever
get out. I had that happen to a piece of suspended coax from the remote
antenna switch to the balun on one of the sloping dipoles. I had one
other piece do the same thing. both cables had the jackets abraded. In
the one case I could blame the roof of the shop and some very strong
winds. In the second case I couldn't make the coax reach the edge of the
roof, so it's possible I missed the spot in the inspection and put it up
that way This is why at least one *protected* end should be able to
breathe.
My experience has been that if water can get in, it will.
For instance although not from breathing, the AV640 that had the wasp
nests was completely cleaned and put back up. Water got into the
matching network and made its way around the center pin on the chassis
connector. From there it got into both the inside of the 9913 and the
braid. Inside the shop was a splice to LMR-4 00 using a pair of
PL-259's and a double female connector at the end of the 60' of 9913.
The system was working fairly well and then abruptly the amp started
kicking out due to SWR, but the antenna analyzer said the coax and
antenna were good. So I started out with just a few watts and again it
was OK. I slowly brought up the power. Some where around 300 to 400
watts the SWR became erratic. To me that said, "water" in capital
letters and that splice was the logical place to look. Unfortunately
the splice was about 3' away from the junction box, in the 2" EMT. So I
had to open up the patch panel, disconnect the LMR-400 , pull back about
3' to the upper junction box and then go to the lower box where the
cable comes in the shop and pull it back there as well. When I took the
splice apart, water ran out. It was no more than a couple of spoon
fulls, but there was the water. So the 9913 was replaced with a short
piece of LMR-600 from another job.
How did the water get into the matching network that was supposedly
sealed except for the drain holes on the bottom that had incidentally
let in the Wasps? I don't know for sure as there are several
possibilities and none to which I could pin it down. It's possible that
it leaked around the cover or around one or more of the screws holding
the enclosure to the mast/ground, or around the feed through to the
antenna. Then there is the possibility of the drain hole itself. The
box is about 6 X 6 X 3 (108 cu in") and black so it gets really hot in
the sun. We had some very heavy rains which would have cooled the box
from the neighborhood of 120F to less than 60F or roughly 49C to 15.5C.
It wouldn't have had to draw in much water to reach the center pin of
the chassis connector. As the system had been back up several weeks and
we'd had a series of heavy rains this could have pumped in a fair amount
of water. However all of this is hypothetical except for the water that
was actually in the coax.
73
Roger (K8RI)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|