In the '60s I worked on heavy ground radar, which produced peak powers
of several megawatts. The waveguides for those antennas were routinely
kept slightly pressurized with dry air. In the set I worked on the most,
the air was pumped through one of two canisters of silica gel dessicant,
while the other was being heated and purged of accumulated moisture. The
roles of the two canisters was automatically and periodically reversed
to assure a continuing supply of dry air. A small hole at the antenna
end of the feedline allowed a little air flow to occur. In some climates
at least, the waveguides would internally arc if the dry air supply failed.

Roy Lewallen, W7EL

Irv Finkleman wrote:

Russ wrote:

It's probably "dry" N2. It is used to prevent water absorbtion and O2
absorbtion of the RF. It can also be used to detect leaks and to keep
out atmospheric water.

Russ


Exactly! Very common in some of our shipboard antenna tuning units as
well -- at least back in the 70's.
Irv VE6BP