Millimeter wave attenuation in moist air: Laboratory measurements and analysis

Experiments were performed with a millimeter wave resonance-spectrometer capable of measuring absolute attenuation rates alpha (dB/km) by water vapor up to saturation pressures. Vapor (e) and air (p) pressures were varied at constant temperature and set frequency. Anomalous absorption behavior (i.e., high rates alpha, extreme temperature dependences, hystereses in pressure and temperature cycles) could be identified as being caused largely by instrumental condensation effects. Uncorrupted data at 138 GHz display, in addition to air-broadening, a strong self-broadening component. Based upon these results, a practical atmospheric millimeter wave propagation model was formulated that predicts attenuation, delay and noise properties of moist air over the frequency range of 1 to 1000 GHz and a height range of 0 to 100 km. The main spectroscopic data base consists of 48 O2 and 30 H2O local absorption lines complemented by continuum spectra for dry air and water vapor. Model input relies upon distributions of meteorological variables along an anticipated radio path. These variables are pressure, temperature, and relative humidity for moist air; suspended droplet concentration for haze, fog, cloud conditions; and a rain rate. In special cases, trace gas concentrations for ozone and carbon monoxide and the geomagnetic field strength can be added.