four thirds earth' is therefore assumed for most link assess- ments and this is shown diagrammatically in Figure 27.14 for a 100 400 3600 300 2700 1 800 100 900 Distance in km C 150 Figure 27.14 'Four thirds earth' line of sight link diagram 27/12 Radiowave propagation Fresnel zone Direct path Indirect Receiver Fresnel zone Transmitter Figure 27.15 The Fresnel zone hypothetical link. This presentation has the advantage that trans- given by Equation 27.18, where R is the radius, and d, 1 and d2 are mission paths can be plotted as straight lines. the distances from point P to the ends of the path. The profile of the topology between transmitting and receiving antenna can also be plotted. It is desirable to avoid any obstructions (27.18) within the first Fresnel zone which is defined as the surface of an R2 = + d2 ellipsoid of revolution with the transmitting and receiving antennas at the focal points in which the reflected wave has an indirect path half a wavelength longer than the direct paths. Figure 27.15 illus- The height of the transmitting and receiving antennas above the trates this and the radius of the first Fresnel zone at any point (P) is intervening terrain and the roughness of the terrain has a marked effect on path attenuation and received field strength in the v.h.f. and dB (uv/m) u.h.f. bands. The Friis formula will generally yield results that are too optimistic i.e. attenuation is too low and field strength is too high. CCIR Recommendation 370 (CCIR, 1982) reproduced in Figure 27.16 shows received field strength values at 10 metres above ground level for a radiated power of 1kW for different transmit antenna heights. The field strengths are modified by cor- rection factors for the terrain irregularity between transmit and receive antennas which can increase attenuation by up to 18dB or reduce it by 7dB depending upon the topology. Average terrain irregularity is defined as differences in height above and below 50 metres in only 10% of the path length. Figure 27.17 (CCIR, 1982) gives attenuation co