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Pressurizing coax transmission lines
If you have a broadcast transmission system that uses air dielectric transmission lines, hopefully you are already aware of the need to keep pressure on those lines.
There are at least two good reasons it’s important to pressurize coaxial cables properly. First, it keeps moisture out and therefore prevents arcs inside the cable. Second, it improves the power-handling capacity of the coax by increasing the breakdown voltage between the inner conductor and the shield.
Rolin Lintag wrote in TV Technology, “When designing a cable-pressurization system, the first issue to consider is the behavior of air, which naturally expands when heated and contracts when cooled. For example, coax air pressure that is at 1 psi during the cool of the evening can rise to 5 psi in the heat of the afternoon, depending on the volume of the air inside the coax. Also, large coax cables, such as 4- to 6-inch diameter cables, can contain a significant volume of air.
“A large volume of air is good,” he continued, “because if there’s a leak in the cable, it takes longer for the air to leak out. But such a large volume of air also can experience drastic changes in air pressure between a hot day and a cold night. The important thing is not to exceed the maximum air pressure specified by the cable’s manufacturer.”
You can pressurize the line with a nitrogen tank that directly feeds the gas barrier input of the coax. Some stations use a dehydrator as the main supply of air. The dehydrator removes the moisture from the air before feeding the line. Never use a standard air compressor as this will feed ambient air that contains moisture into the line.
This tip appeared in the Alabama Broadcasters Association’s “Engineering Academy Monday Morning Coffee and Technical Notes.” Info at https://al-ba.com/wp2/ under “Engineering.”
