ious types of paths where the path characteristics are obvious and the path is short. A Fade margin typical microwave survey report generated by the survey contractor Receiver sensitivity MMMMMM will contain most of the site and path data needed for FCC licence applications and FAA approval in the case of towers near an airport. In the case of potential reflection points observed along the path, Figure 50.9 Path calculations it is possible to design a path SO as to minimise the possibility of signal cancellation by a reflected signal. Calculation of the potential reflection areas that would cause cancellation may be made in Similarly, the antenna gain and transmission line loss are shown advance of the actual field work, minimising the need for extensive at the receiver site. Notice that the signal at the receiver input is field investigation along the path. significantly higher than the receiver sensitivity, or minimum ac- ceptable receiver input signal. This is intentional and the difference 50.1.4 Propagation may be as large as 40dB or 50dB. The difference is known as fade margin. Once the physical characteristics of a microwave path have been Microwave signals, even under normal atmospheric conditions determined we can turn our attention to the determination of an- where none of the previously mentioned phenomena such as earth tenna systems and propagation availability (reliability) that might bulge, ducting, reflections, etc., occur, may still be subject to vari- be expected. ation due to an effect called multipath fading. Multipath fading Calculation of the path loss, or attenuation of the microwave occurs when the microwave energy is diffracted or scattered along signal between the ends of the path is the first step. In order to make the path SO that part of the signal arrives at the receiving antenna out the formula as generally applicable as possible, the path loss calcu- of phase with the desired signal. Figure 50.10 shows this effect with lation assumes the use of a theoretical point-source or isotropi