 D is the distance in loss which is dependent on the particular propagation path under kilometres and P is the ERP in Watts. consideration. This loss prediction is complicated by the fact that the real world does not provide simple geometric shapes for the (6.65) boundary conditions, and nearly all calculation systems have been based on such simple geometries. It is, therefore, convenient to Some workers prefer to base an equation on the isotropic trans- stylise the real world into shapes which can be handled, and the mitting antenna. The difference is that 10 becomes 2.1dB following sections discuss these and the predictions based upon them. greater, because EIRP is used, which is compensated by the constant 76.9 becoming 74.8. 6.4.4 Knife edge diffraction 6.4.2 Free space path loss Fresnel's diffraction formula is still one of the most useful tools for A transmission path may also be characterised by the free space path prediction of the loss over a hilly terrain. A rigorous analysis for a loss, which is defined as the ratio received power Pr to transmitted path obstructed by many edges involves a multiple integral but, as this is difficult to solve, attempts have been made to get acceptable power P. Expressed in decibels, in a non-dissipative medium, this results from a sum of single integrals. It is now well established that is given by Equation 6.66. a method originally devised by Deygout (1966) gives the best results in most cases. It consists of taking each hill as if it alone = 17.7 + 2010g existed on the path, to establish which one gives the largest pre- 200g10-G-G, (6.66) dicted loss. This hill is then taken as a terminal to divide the path into two sub-paths, and the process repeated. Further subdivisions G and are the antenna gains relative to a half wavelength may then be made. The total diffraction loss is taken as the sum, in decibels, of the primary loss and the losses on each sub-path. dipole and L and the antenna feeder losses; subscripts t and r This process is illustrated by the profile drawn