Microwave Radio Obstruction Fading

The design and the transmission performance of terrestrial line-ofsight microwave radio paths is greatly influenced by two clear-air propagation phenomena, obstruction and multipath fading. Both types of fading are caused by anomalous stratification of the atmosphere. In the case of obstruction fading, atmospheric stratification temporarily changes the transmission path to such an extent that it becomes blocked by the terrain. In the case of multipath fading, a different type of atmospheric stratification temporarily creates multiple transmission paths that cause destructive interference of a number of waves at the receiving antenna. Satisfactory transmission performance is attained when the height of microwave radio towers is sufficient to reduce the occurrence of obstruction fading to a tolerable value, and when diversity protection is used to eliminate transmission impairments caused by multipath 785 fading. Methods for the estimation of multipath fading and the associated transmission performance have been developed, but a similar body of knowledge does not exist for obstruction fading.1"6 One reason is that the variation of the occurrence of obstruction fading is much larger than that of multipath fading, which implies that the cost for establishing a data base for obstruction fading by direct measurement would be prohibitive. However, obstruction fading can be related to meteorological variables for which a data base already exists. Recent work on this approach has resulted in a method that permits estimation of obstruction fading for any location in the United States.