You have two types of air dielectric hardline...
rigid which resembles conduit. a small pipe supported by nylon bushings "bullets" inside a large pipe.
flexible which more resembles coax or heliax, as mentioned above in the quote from nr6ca.org and pictured below.
flexible is normally found at cell sites in diameters under two inch while rigid is normally found at broadcast stations in diameters over three inches.
Of all the systems that I have installed using air dielectric cable, only one was ever just coax and connectors, and surprisingly it had no problems of moisture intrusion.
The majority of systems used a dehydrator that was connected to the connector at the radio.
The dehydrator would repeatedly pressurize the cable to roughly 4psi then slowly purge the air through silica beads to remove any moisture.
A few of the systems used tanks of nitrogen gas that kept a constant pressure on the coax so that no moisture could enter.
I've heard of and read about "old timer" hams that fabricated their own air dielectric out of whatever they could get their hands on... copper pipe, galvanized fence rail, conduit etc etc.
I never found out how they dealt with moisture other than to have a small drain hole in the "jacket".