"Spike" wrote in message
...
On Tue, 1 Feb 2005 10:40:22 -0000, Airy R.Bean wrote:

This is thin coax which has been installed for about 22 years.

I was intrigued to find that both the braid and the inner conductor
were blackened, despite being on a section of the cable that could
not have had water ingress.

The problem is not '(liquid) water ingress' but diffusion of water
molecules from the atmosphere through the cable sheathing.

Perhaps is time to replace with some larger diameter coax?

The cable sheathing may not be that much thicker than that of the
'thin' coax, and so will not significantly slow the diffusion, and in
addition you will have mechanical problems due to the much heavier
coax.

Your thin coax probably blackened years ago.

You can try to reduce the effect by covering the cable sheathing with
a light coating at regular time-intervals of something water-resistant
such as wax furniture spray or WD40 - not as '(liquid) water' is the
problem, but that the light fractions of the wax or oil will tend to
fill the pores in the sheathing material and so delay the diffusion
effect. You could move to the Arctic or a desert of your choice ;-)
--
from
Aero Spike
=================================

A not-very-well-remembered true story.

Shortly after WW2, a British Post Office cable ship was repairing (or
stealing for reparations) a coaxial telephone cable in the North Sea,
somewhere off the Heligoland Bight. It had been manufactured and laid by our
European friends, the Germans, during the war.

The cable was polyethylene plus air insulated. The inner conductor was
insulated by a thick, circular, polyethylene string or rod, slowly
spiralling around it. Plenty of air space. There may not have been an outer
conductor - the return conductor could have been the North Sea water. It's
OK up to a few KHz. But the whole thing was protected by a thick
polyethylene sheath in contact with sea water. Polyethylene was a very
uncommon material in those days.

( It has always amazed me how much time communications engineers had to
spare in experimenting in the middle of a world war. The Russians
experimented with future, peacetime, high-voltage DC, single-wire power
transmission while the population was dying by the million from murder,
starvation and disease. I obtained some of my programs' formulae for
calculating the resistance of ground electrodes from a translated Russian
book of the time.)

The above-mentioned unbroken cable was laid across the deck of the ship. The
cable jointer, not knowing quite what to expect, had his tools around him,
including a lighted parafin blow lamp.

The cable jointer, a skilled workman, cut into the sheath with a saw. And
immediately there was a hissing noise and a very hot flare of pale flame.
Whether the jointer was injured or not is lost in history. ( I was either
in Hong Kong at that time or was walking the darkened pavements in Hiroshima
marvelling at the lighter-coloured shadows cast on the paving slabs by the
first of the WMD.*)

It later transpired in the laboratory that when water is under relatively
high pressure, such as at the bottom of the North Sea, when in contact with
a thick polyethylene membrane it dissasociates into hydrogen and oxygen.
The very light element diffuses through leaving heavy oxygen on the outside.
Thus, after a few years, the air-space on the inside becomes filled with
high pressure hydrogen. If there's any copper on the outside then
presumably it becomes blackened by copper oxide.

A tongue-in cheek WARNING - When digging up very old air-spaced coax from
soggy soil in your back yard it is advisable to erect no-smoking notices.

( * Actually, the RAF, of which I was a member, had already incinerated more
people than both atomic bombs just in one fire storm on one city. Churchill
might have been knighted for services rendered. But Air Marshall Bomber
Harris was never decorated with a medal or honoured with a statue. He,
partially responsible, with 10,000 air-crew plus 100,000 ground staff, took
the ashamed blame. After all, there are very few problems in finding
100,000 people to manufacture and operate and transport 5 million people to
the gas chambers.)

I'm on South African red wine tonight. It says on the bottle it should be
drunk within 12 months of purchase and within two days of opening. I have a
corkscrew. No problems!

Help yourself to a program.
----
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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