[amsat-bb] Re: 18 el cp yagi

Stan, W1LE stanw1le at verizon.net
Thu Jan 21 11:23:28 PST 2010

Good Afternoon Bob,

Sealing may not be the answer. There are many ways water/moisture can enter.

Water ingress can happen at gaskets, coax penetration points, DC 
penetration points,
connector flanges, mounting screws, cover screws, etc. Coax can even 
"hose" water inside.

I suggest limiting the moisture entry AND allow it to drain out the 
bottom. This vent hole will
also allow the pressure to equalize, inside vs. outside.
If the enclosure breathes there is no pressure or vacuum buildup inside.
Vent hole can be small, just big enough to keep the bugs out. ~1/8" is 
on the big side.
Let gravity work for you, put the vent hole on the lowest point.

One scenario includes a hot summer day and the air inside the sealed 
enclosure expands.
A thunder storm comes along with much cooler temps and rain. The 
enclosure is cooled and doused.
A vacuum builds up inside and any moisture is sucked in thru any and 
every possible entry.
This vacuum may exist overnight > 8 hours, so it has a long time to "work".
The moisture  and acid rain starts to corrode everything inside.
Electricity tends to accelerate the corrosion.
With a functioning vent hole, the sucking and blowing is controlled and
no significant differential pressure from inside to outside.

So, what is the solution in a rain/wet environment ?

Let the item breathe, allow air to circulate.
Keep the rain/snow from falling onto the item, cover it up.
If you use an enclosure be sure to vent it.

Avoid dissimilar metals that will only corrode, no plated steel hardware 
or phono connectors.
Use sealed/potted relays.

For items outside where weight is not a problem, I will design a 
connection point so I can put a
bucket or waste basket over it to keep the rain off. An open bottom, so 
everything can breathe.
The plastic container must be UV proof. A black plastic waste basket of 
different sizes is my favorite.

My old KLM 22 element 2M Xpol has an open polarity select PCB on the 
antenna boom,
but I sealed up the relay and I removed all plated steel hardware and 
connector, direct solder wires.
Then I gave it all a few coats of a clear coat spray.

Do not use RTV that smells like vinegar (acetic acid),it only 
accelerates corrosion.
I have used the 3M 5200 series sealants with success.

Most of the Landwehr mast mounted preamplifiers directly exposed to the 
weather will leak.
Probably why they are no longer on the market and have sought other 
business opportunities.

Document performance so you can compare later test results.

Stan, W1LE    FN41sr     Cape Cod

Bob- W7LRD wrote:
> Hello 
> I am refurbishing a 18 element CP yagi.  I am not sure of the manufacturer.  I opened up the relay switching box and found a lot of moisture (Yes I live near Seattle) in the box.  Lots of yuck and rust, on non stainless steel hardware..  The 12V connector is a simple phono plug, which should never be used outside.  Before I reinvent the wheel has anyone been here before?  Replace the phono plug, sealing everything.  In the past I have used a material called 3M5200, a marine sealant, nasty but almost permanent.  Suggestions please. 
> 73 Bob W7LRD 
> Seattle 
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