h in order to decrease the propagation outage time. to squelch is 0.01%. This corresponds to a path availability (or Another solution is available. reliability) of 99.99%, or an outage time of 53 minutes per year. For Consider the situation shown in Figure 50.13. A single transmit- a system consisting of a single microwave hop, this may seem ting antenna is used, and the two receiving antennas are usually acceptable. Consider, however, a system of 20 tandem hops, each spaced vertically 30 to 60 feet. Each receiving antenna is connected with a 53 minute outage per year. Murphy's law says that no two to a separate receiver and the receiver baseband outputs are then paths will fade simultaneously SO that a propagation outage time of connected to provide a combined signal (Figure 50.14). The desired 20 times 53 minutes per year, or 17.67 hours per year, might be microwave signal is received by both antennas, but should a cancel- expected. This would be unacceptable to most system users. lation at one antenna occur due to multipath or a reflected signal, a About 20 years ago W.T. Barnett and A. Vigants of Bell Telephone simultaneous cancellation on the other antenna is not likely, due to Laboratories developed new methods of calculating propagation the different path length of the reflected signal. The formula for unavailability based upon extensive experimental and theoretical calculating the improvement to be expected by using space diversity work. These methods take into account the frequency and path as in Equation 50.6, where I is in decimal form; S is the vertical length as well as climate and topographic factors. The formula for antenna spacing in feet; F is the lower fade margin associated with propagation unavailability (outage) is given as in Equation 50.5. the receiving antenna with the longer transmission line. (a) ) ( F ) (50.5) (50.6) U is the unavailability in decimal form. a is a topographic factor and equals: 4 for very smooth terrain including water; 1 for average Typical values of 1SD are in the ran