I'm building a vertical antenna. The lowest portion of its element consists of 3/4" type M copper pipe, with end caps that will be soldered on and 3/8-24 screws poking through at both ends (so the bottom can screw into a 3/8-24 mobile mount, and the top can attach to Hustler mobile resonators like the RM-30 and RM-40).
So far, so good... mechanically and electrically, it's decent (I wouldn't want to depend upon a single 3/8-24 screw to secure the entire lever force of an 8-10 foot antenna, but I'll have a second strap a few inches higher to secure it against swaying and lever force, so the lower screw's main purpose is to provide electrical contact with the RF signal, support its vertical weight, and keep it from drifting horizontally). However...
I'm a Floridian, and I know from years of hard firsthand experience that any outdoor cavity that isn't airtight inevitably ends up with water trapped inside. Why? Humid air gets inside, condenses when it eventually gets "cold"(-ish), and the same seal that keeps rainwater out eventually drowns the very circuit it's naively meant to protect.
Is this an actual problem with antenna elements that have non-airtight cavities inside? Or is trapped water inside a hollow antenna element not actually a problem (because RF runs along the surface, or condensate is deionized & basically an insulator, etc)?
If it IS a problem, how big of a hole do I need to put at the bottom for water to drain out, keeping in mind that the copper end caps aren't very thick to begin with, and any hole I drill is going to have a negative impact on what little structural strength it has.
Putting the various dimensions into perspective, 3/4" type M copper pipe has an inside dimension of .8125". A 3/8" screw is 0.375". That means a perfectly-centered screw hole has at most .21875" of copper between the center hole and inside surface of the pipe.
A 2mm hole is approximately .08" wide (2mm x 1 inch/25.4mm), or approximately 1/3 of the area. Probably ok, though, if it's drilled near the edge of the area.
At 0.25", 1/4" hole would literally be wider than the distance between the 3/8" screw and pipe wall. A 1/4" hole would HAVE to be drilled through the side, instead (i.e., perpendicular to the 3/8-24 screw... through the 3/4" pipe itself and side of the end cap).