Jim is so right! Expanding foam expands more than you think
once it begines to set up.
A few comments on PVC...
I've built several cellular, PCS, and 2.4 GHz co-linear antennas and
mounted them in PVC pipe. The PVC tends to really detune them, about
15%. That's when I use a 1/2" dia. PVC pipe, a 2" pipe with the elements
center down the middle doesn't seem to have any detuning effect.
PVC breaks down in sunlight, in spite of the UV inhibitors. Thin wall
stuff can get so brittle after only one Texas summer, you can't pick it
up to carry it around. Grey electrical PVC is good for outdoor use. Or
you can paint PVC with a non-metallic epoxy paint (use white) sold
as appliance touch-up spray paint by Krylon and Rust-Oleum. You need to
wipe of any mold release coating with weak paint thinner like kerosene
or turpentine and then prime with an epoxy primer (I think only Krylon
has the epoxy primer). Takes about two days to cure.
As for sealing my antennas...
Never use silcone glue. The voltile components in plastics (the oily
fog on your car's windshield in the summer) will act as a mold release
after a few months and the glue will peel off. People that build
aquariums out of plexglass and seal it with silcone glue (designed for
glass aquariums) find dead fish on the carpet after a few months.
Use 50-year caulk if you want to to weatherproof something.
J-B Weld epoxy is the best epoxy I've used (hey - Paul Harvey recommends
it!). Takes overnight to set. Don't bother with those 5-minute epoxies.
I use the epoxy to seal PVC antenna radome ends instead of an ugly looking
PVC cap. I stuff a bit of foam rubber into the pipe and then goop in a
thin layer of 5-minute epoxy or hot glue to seal the foam, and then pour
half an inch of epoxy over it until its level with the rim.
And finally, two words about "electrically inert" potting compounds -
they aren't! Manufacture assured me it would not alter the preformance
of a potted electrical circuit - it did. I potted several minature
switching power supplies I designed and they started generating parasitic
noise after the epoxy cured. Hooked up a digital logging scope and
discovered it lost its inertness 80% into its cure cycle.
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