Space-Diversity Engineering

Bell System microwave radio relay routes consist of links (paths, hops) that have an average length of about 26 miles. Transmission on most hops is along the line of sight, with antennas mounted on towers that are typically 250 feet high. In some cases, when line-ofsight transmission between towers is not practical, large reflectors on prominent points of terrain (passive repeaters) are used. Tower locations are selected to avoid ground reflections, but inevitable exceptions, such as transmission across a lake, do occur. Multipath propagation during anomalous atmospheric conditions can give rise to destructive interferences at the receiving antenna; the resultant signal fluctuates (fades) and may be reduced to practically zero for seconds at a time. The corresponding interruptions to service, if permitted to occur, would be unacceptable. Interruptions to operation can be avoided by switching from an unserviceable radio channel to a protection channel operating at a different radio frequency, since multipath fading, being an interference phenomenon, is frequency selective. Use of such protection (frequency diversity) has been restricted to conserve the frequency spectrum. 1 Space diversity is an alternative or additional form of protection from the effects of multipath fading. 2-8 Its effectiveness depends upon the fact that multipath propagation results in vertical structure of the electromagnetic fields at the receiving tower. Selection between two vertically separated antennas receiving at the same frequency is com103