This article originally appeared in the October 1994 issue of Broadcast Engineering magazine. O perational transmission line is critical to broadcast operations, however, it is often overlooked or neglected until it is too late. Performance of air dielectric transmission lines can be greatly reduced when the lines are filled with ambient air, moisture in the air can condense on the inside of the transmission lines. To prevent electrical performance degradation transmission line should be maintained under dry gas pressure. Unfortunately, many of today's transmission lines are not pressurized.
Joint seals have a calculable leak rate, which increases with age.
Without positive dry air pressure, breathing allows moist air to enter. As the temperature drops, it will condense and cause corrosion, which increases attenuation. As the path arcs over, failure occurs. Water deposited on horizontal insulators and on the bottom surface of horizontal runs of waveguide causes increased reflected power. Design considerations Condensation in transmission lines depends on the amount of moisture in the contained air, expressed as dew point or relative humidity, and the outside ambient air temperature.