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To prevent problems associated with condensation, most users of waveguide and coaxial cable fill their transmission lines with a gas. The gas is typically dry air or dry nitrogen that has been pretreated to a condensation temperature (dew point) that is far below the lowest temperature that the lines are expected to reach. The gas pressure is maintained at a value greater than the anticpated variations in atmospheric pressure; that is, the inside pressure never falls below the outside pressure, which ensures that high-moisture air cannot enter the transmission line. |
Andrew automatic dehydrators are used to reduce the
moisture content (and, therefore, the dew point) of air and
maintain a pressure greater than atmospheric pressure. For
transmission lines with typical leakage rates, the dehydrator
should normally run once every 24 hours for no more than 72
minutes and no less than 10 minutes. Figure 4 is a simplified diagram of dehydrator operation. The purge-gas stripping method of regeneration is used, where moisture is removed from the air at high pressure and returned to the atmosphere at low pressure. This is illustrated in Figure 5. In operation, the pressure sensitive switch closes whenever the transmission line pressure falls below 3 psi (21 kPa). This action applies power to the compressor motor, the solenoid timing motor, and the purging cylinder solenoid valve. |
