"Ron" wrote in message
news
On Wed, 31 Dec 2003 02:51:26 +0000, Dave Woolf wrote:


Just wonder what other's might do to keep moisture from getting into the
semi-hollow 9913 type low loss coax. It would seem inevitiable with
changes in temperature and pressure that moist air would eventually work
its way into the coax and then condense.



I'd think it makes sense to seal the top, but let the connectors at the
bottom
breath, that would alleviate moisture ladden air from being drawn in at
the top. Especially during rain storms! Another idea would be having the
top sealed, and the bottom allowed to breathe through a plastic jar of
desiccant.

Pete

Uncle Peter wrote:

"Ron" wrote in message
news
On Wed, 31 Dec 2003 02:51:26 +0000, Dave Woolf wrote:


Just wonder what other's might do to keep moisture from getting into the
semi-hollow 9913 type low loss coax. It would seem inevitiable with
changes in temperature and pressure that moist air would eventually work
its way into the coax and then condense.



I'd think it makes sense to seal the top, but let the connectors at the
bottom
breath, that would alleviate moisture ladden air from being drawn in at
the top. Especially during rain storms!

That will certainly help.

Another idea would be having the
top sealed, and the bottom allowed to breathe through a plastic jar of
desiccant.

Unfortunately the desiccant doesn't last, because it absorbs moisture
from the air in the shack. However, you can easily regenerate silica gel
by baking it in a moderate oven for a few hours.

Even better is "active" pressurization - keeping a positive pressure
inside the line at all times. A very simple, low-cost method is to use a
small fish-tank aerating pump in the shack, with the far end of the line
sealed (exactly as Pete recommends) so there is *no* through flow. The
positive pressure is very effective at keeping moisture out.

In the shack, the air is connected by soldering a small brass tube
through the side of a normal coax plug. PL259/SO239 connectors will
leak, so you have to tape over them; most types of N connectors are
already well sealed.

Positive pressure is even more useful if you have a sealed box at the
far end of the line, containing preamplifiers, relays etc. In the long
term it keeps outdoor electronics in far better condition than the
alternative method using a vent hole in the box, which exposes
everything inside the box to atmospheric moisture.

To make sure the air pressure gets through into the box, you may need to
drill small holes through the insulators of some in-line connectors.
Only small holes are needed because there's very little flow. For a
deluxe job, pressurize the system with dry air, by connecting the pump
through a tube of silica gel desiccant (regenerated every few months as
above).

I can't remember who first suggested this method, but I'd like to thank
him! It worked very well indeed for me, for more than 10 years.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek