tion in milliradians (mrad) is (3.38)* where the path length d is in kilometres. The percentage of time Pw that fade depth FM (dB) is exceeded in the average worst month is (3.39)* where f is the frequency in GHz. Example. Error performance calculations for a radio relay system based in ITU-R 530 for a 15 km hop operating at a frequency of 7.4 GHz. The parameter values are given in table 3.11. 3.4.6 Repeaters The path between the transmitter and receiver is a straight line clearing all obstacles. As already seen, a microwave signal requires more than just an optical line-of-sight clearance to allow for both fading and for certain interference characteristics of the beam itself at microwave frequencies. To achieve desired clearance, the antenna systems on both ends can be increased in height, which of course results in increased costs. Another way to overcome the obstructions is by the use of passive repeaters or radio reflectors. As microwaves follow most of the rules of conventional optics, a system of 'mirrors' similar to an optical mirror system can be constructed to reflect the beam over or around an obstruction. Passive repeaters are used when a tower cannot provide clearance over an obstruction. For example, if two sites are separated by a hill or very tall building, the microwave beam may have to be redirected at one or more intermediate points to get it around or over the hill/building. Although active repeater stations could be used at these points to amplify and retransmit the signal, passive repeaters may be used to merely change the path direction without amplification. As passive repeaters add no signal amplification and require no power and very little maintenance, they can be located in places where access is very difficult. A passive repeater is a highly efficient device used to re-radiate intercepted microwave energy without the introduction of additional electronic power. Passive reflectors may be used in microwave systems in both the near field and far field. Double passive repeaters are used w