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Feed Assembly - Moisture

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Ex Member

Jul 31st, 2010 at 6:04pm  

I occasionally find moisture bubbles on the inside of the lens cap of the feed assembly. Now in my country its normally hot during the day, cold at night, so i am assuming this is condensation at work.

My real question is, since the cables and connectors are all covered in 3M waterproofing tape, along with silicone, how can this be avoided, in addition to  how can this be fixed once it has already started?
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USN - Retired
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Reply #1 - Aug 1st, 2010 at 4:34am  
Do you pressurize your waveguide? If yes, you've got an issue with the pressurization system. If no, you've likely got a bad seal at one or more of the waveguide joints. Disassemble and dry everything thoroughly, obtain new seals, lubricate them with <u>non-hardening</u> silicone dielectric, reassemble. That's about all you can do to minimize condensation in non-pressurized WG.

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« Last Edit: Aug 1st, 2010 at 2:06pm by N/A »  
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Eric Johnston
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Reply #2 - Aug 1st, 2010 at 11:49am  
If you go for the totally sealed idea (as greg mentions above) with greased gaskets or rubber 'O' rings then it is important to dry out the initial air filling. Put the entire feed system in a polythene bag plus a bag of dried silica gel for a few hours and then with your arms/hands entering polythene bag carefully assemble in dry air atmosphere. Some feed horns already come with seals at both ends with dry air inside.

Marginally sealed systems (plain flanges) do leak air slightly - both ways, in and out.  At night the air inside cools down and contracts, sucking in cold moist night air. In the day the heat expands the air inside and the inner air escapes.  What matters is where is the leak.  If the leak is at the top the least dense dryer air preferentially escapes, leading to a build up of moisture inside. This explains why significant quantities of liquid water can sometimes be found inside apparantly sealed systems.  If the leak is at the bottom then the heavy damp air falls out. Some feed designs actually have pin hole to allow deliberate air flow - but beware of insects getting inside if the hole is too large (1 mm suggested).

Pressurised systems: These typically have a dehydrator and air pump. There is an air input pipe and flowmeter to the feed system and a 'calibrated adjustable leak' at the far end of the feed system. Slight air flow helps keep the whole system dry. The problem is long term monitoring and maintenance of the dehydrator and pump and long term maintenance of staff skills.

Note that in very hot weather, the amount of water in the air can still be quite high.

Best regards, Eric.
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