In the microwave spectrum, it has long been recognized that refractive effects tropospheric scatter and ducting are not only significant but may dominate propagation characteristics. In the propagation of millimeter waves, however, precipitation atmospheric gases, water vapor, and particulates are generally recognized as the dominant factors ducting effects have not previously been investigated. A unique experiment has been designed to assess the effect of the evaporation duct on over-the-horizon signal propagation at a frequency of 94 GHz. Results from five months of measurements made near the Southern California coast strongly suggest that the evaporation duct significantly enhances the received field on a path that is nearly twice the line-of-sight range. Thin evaporation ducts 3 m are observed to increase the received field to levels more than 50 dB above diffraction. Predictions for thicker ducts 18 m indicate that the signal can approach levels of free space minus the atmospheric absorption. An evaporation duct is created above the air- sea interface by the rapid decrease of moisture with increasing height. The strength of the duct is described by the thickness of the trapping layer, which is the height above the surface where the modified refractivity, M, is a local minimum. The thickness, or duct height, varies between 0 and 40 m with a worldwide mean height of about 13 m however, because evaporation ducts are leaky, they affect radio and radar terminals above as well as within the duct.
Presented at the International Geoscience and Remote Sensing Symposium (1987), Ann Arbor, MI, 18-21 May 1987.