purpose of a tower-top amplifier

"Dana" hath wroth:

From the Andrew Corp data sheets on waveguide and coax cables:

Dry air is normally used for pressurizing. Dry nitrogen may also be used. When pressurizing equipment is connected to a gas port on the waveguide connector, or whenever pipe fittings are reassembled, threads must be covered with sealing tape to ensure leakproof connection.

Pressurization is needed in air-dielectric cables because changes in temperature can cause condensation of moisture from outside air that enters the cable. This moisture can seriously impair the efficiency of system operation. Connecting a pressure source of dry air or nitrogen (dehydrator) to the cable at slightly more than atmospheric pressure will correct this condition since the moisture will be removed and air will then leak from instead of into the cable.

At one community repeater system I maintained in the early 1970's, I used a compressed air drying filter and a bicycle pump. I ran the pressure up to about 7 psi. It would leak down to about 2 psi in 3 months. I missed re-pressurizing the coax one winter and got to spend a back breaking day purging the coax. I built a small pressurization system from an air brush pump and eliminated the bicycle pump ordeal. Refridgerated dessicating system are nice, but you can get away with much less if you don't mind purging the coax every few years.