on system availability due to probability of system outage from increased System Gain may be used to allow longer hops. This may microwave propagation phenomenon. seem obvious, but what is not SO obvious is that longer hops can Also, the area of coverage determines the amount of microwave result in less microwave equipment required to cover a given dis- equipment required which, since no equipment is perfectly reliable, tance, and that less equipment results in higher reliability. The strongly affects the system availability due to outages from equip- inherent and unavoidable failure rates of cascaded equipment adds. ment failure. The Mean Time Between Failure (MTBF) of the basic multi-hop All of the above, i.e., signal-to-noise ratio, distortion, Bit Error microwave system is the inverse of this total failure rate as in Rate, and system availability are components of transparency, Equations 50.15 to 50.17 where n is the number of similar micro- which, as previously discussed, is the most desirable overall char- wave radios in series. acteristic of the microwave portion of a system. These components are determined by the microwave equipment specifications, and just (50.15) as importantly, by microwave system design. (50.16) 50.4.3 Important equipment specifications Many microwave equipment specifications are interrelated, as far as their effects on system performance are concerned, and most of MTBF, a 1 (50.17) them are of importance. The most important of the equipment specifications, if any can be SO identified, is System Gain. System Gain's importance is that it is a major controlling factor in system If longer hops are not needed or desired, it may be possible to use design, system performance, system availability and system cost, smaller antennas. In the USA the FCC, however, places a limit on i.e. almost everything of interest. the smallest antenna that can be used. Acommonly used formula for Prior to defining System Gain and delving deeper into the subject, dish antenna gain is as in Equation 50.18 or 50.1