[TowerTalk] Re: coax splices

[Ian White, G3SEK]

K5GW wrote:
>I have use N connectors exclusively in my 2m
>and 70 cm eme systems and can tell you that even with several cascaded N
>connectors through power dividers and phasing lines that they are almost
>lossless, approximately the same as an equivalent length of RG213.
>
I'd agree with that. 

>However, I must emphatically disagree with the idea that they are water proof
>as they certainly are not! I hate to think about the number of hours I have
>spent removing water from N connectors in these systems even though they were
>sealed using the best methods commonly available. I have made progress in
>better sealing them but there is still an occasional water problem. I hate to
>think how bad it would be without any sealing at all!

I had to scrap an all-coax feed system for the 8-yagi 432MHz EME array
because of water leaks in the damp British climate. There was a total of
13 N connectors... but which one had leaked this time? It drove me
crazy! Ironically the best solution was open-wire feed which is totally
exposed to the weather. 

The ultimate solution to water leaks is to pressurize the coax, which is
not as bad as it sounds. With 9913 or air hardline there is a continuous
air path through the coax (or up the inner for foam heliax of 7/8in and
above) and a small fish-tank pump will pressurize this to 2-3psi. 

There are some precautions necessary. First, the feedline system must be
sealed all the way along, and also at the far end because this is
intended as a statically-pressurized system and not a flowing-air
system. Also the feedline must not contain any major blockages to air
flow along its length. N connectors with removable pins and internal
parts will pass enough air through to maintain pressure along the line,
but 259/239 connectors will not (unless you make a 1/16in drillway
through the insulator, which also improves the impedance match :-)
Solid-dielectric cable such as RG213 is an almost total blockage.

The second precaution is that you need to dry the air. Otherwise you'll
get condensation and even freezing in the outdoor parts of the cable
run. Actually, this can happen anyway in non-pressurized systems - it's
not a fault of the pressurization. 

To dry the air, I use a 1in x 6in plexiglass tube filled with "self-
indicating" silica gel crystals. This starts out blue when the gel is
totally dry, and turns pink when the gel is no longer capable of
absorbing any more water. Two hours in an oven at 220F will restore the
gel to its original condition, so I bake up a batch and keep it in a
sealed storage jar, with another jar to hold gel that's ready for
recycling. The tube needs to be refilled about every two weeks in cold,
damp winter weather, but it can last for months in summer. 

The dried air is introduced through a side-arm soldered into the
hardline connector in the shack. Ideally this should be internally
sealed against leakage into the shack, using a very small amount of RTV. 

The benefit of this system is that you always have a positive pressure
inside the cable, so atmospheric pressure changes cannot suck in water
vapour. In my setup the pressure is maintained all the way out to the
preamp box, which is always totally dry.



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

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