[time-nuts] GPS antenna in silicon/RTV encapsulation

[nuts]

I'd be inclined to look at radome construction. 
> http://www.mpdigest.com/issue/articles/2008/may/mfg/default.asp
The E-3 AWACS is mostly S-2 glass, but they need the strength. For a
radome sitting outside, you might be able to do better.

The advantage to S-2 glass is you can buy it easily, especially if you
have a Tap Plastics handy. It works well with the Marine Resin they
sell, so you know it will last a long time outdoors. 

You need to cure the resin in a vacuum, though not a particularly good
vacuum. There are probably guides on the internet, but here is the
basic scheme to make a fiberglass composite structure. 

Make a form of the final product.

Cover the form in mylar. Tap Plastics sells the mylar too. The resin
won't stick to the mylar.

Cut the cloth to cover the form. Where you overlap, leave at least an
inch. You need both the S-2 glass and the thin fiberglass mat. The mat
is a very loose weave that can hold more resin than the glass.

The basic construction is a sandwich of cloth, mat, then cloth. You can
add more layers, but you always want mat between the cloth, and cloth
on the outside.

You will need more mylar to cover the last layer of cloth. This is to
keep the air out. It helps to have this cut ahead of time because the
resin will be hardening as you work. 

Mix the resin with hardener. 

Brush the mylar on the form with resin. The resin on the form is needed
to hold the cloth onto it.

Place the cloth on the form. Brush on resin. 

Cover with mat. Brush on more resin. 

Cover with the final layer. Apply more resin.

Apply the mylar. Use masking tape to keep it attached.

Wait two days. 

Note that the form has to be "destructible", that is you need to pull
it away from the radome. Generally you use cardboard.

An alternative scheme, though I don't suggest it for a radome, is to
use a foam core. You can bush directly onto the form. You need to at
least a layer of mat then cloth on both sides of the form. Apply the
mylar and wait two days. The form is part of the final structure, so
there is nothing to remove. 

Generally for antennas in radome, all you do is have a "weep" hole at
the bottom the antenna to allow air exchange.