[time-nuts] wwvb weak on east coast especially when the pre-amps under water.

[Arnold Tibus]

Am 14.05.2012 21:54, schrieb Hal Murray:
> magnus at rubidium.dyndns.org said:
>> I had the same problem with a GPS antenna at work. Somebody had put the
>> manufactures label over the porus plug that should have vented out any
>> water... but it didn't so I had too high water-level inside the antenna. 
> How does water get in?
>
> I'm not doubting that it does, just trying to understand the mechanism.
>
Let me try to explain it.
The mechanism is as follows:

Air does contain water in form of gas (vapour) which does condense to
water when
the temperature drops down (eg. at night). The problem is coming up on
tight boxes
having a pressure leak eg. due to defective sealing, cracks or via non
hermetic cable
connections. Changing air pressure do always pump in fresh and humid air
and the
condensed water remain on the bottom inside the box and may as well
penetrate into
the wire mesh/ braid of coaxial cables. The copper will start "modering"
and turn black.

The only solutions I think:
Apply air pressure tight boxes having a breathing hole an the bottom,
mount the
box that no rain and water can penetrate from the top or sides. If the
hole is big enough,
eg. 2mm, no pressure difference is possible and no pumping effect will
occur.
(If the hole is too wide, small animals may penetrate).
Or,
when using a pressure tight box, it must be stiff and sealed to
withstand under all
temperature conditions more then 1 bar/ 100 kPa. Do not forget that all
feed
throughs must be of real hermetic type, normal coaxial connectors are
not tight!
Don't route cables directly in, because no cable braid or mesh is vapor
tight.

I hope this will help,
we had never problems that way.

Arnold