ature of which can be calculated from ht = 0.078 (3.22) Fresnel Zone Once the variation of the signal in the atmosphere is known, the next step is to ensure that the maximum strength of the signal reaches the receiving antenna. This is possible if the designer makes sure that there is no obstruction between the transmitting and receiving antennas. This is done by keeping the first fresnel zone (FFZ) clear of any obstruction. As shown in Figure 3.41, the first fresnel zone can be defined as the volume contained in a three- dimensional ellipsoid between the transmitting and receiving antennas, in such a way that any ray drawn from the transmitting to the receiving antenna touching any point on the ellipsoid will be a half-wavelength longer than the direct ray between the two antennas. If the two lines originate from a point on the ellipsoid towards the centre of the transmitting and receiving antennas, then the combined length of the two lines will be one wavelength longer than the direct ray between the two lines. Thus, for subsequent fresnel zones, the combined length of the indirect signal will increase by a half-wavelength from the previous Mathematically, this can be represented as (3.23) Thus, the radius of the fresnel zone is given by ndid2 RnF = 12.75 (3.24) where n = number of fresnel zone (1,2,3, f = frequency (GHz) d, d1, d2 = distances (m) This shows that as the frequency increases, the radius of the fresnel zone decreases. Transmission Network Planning and Optimisation Ideally, the fresnel zone should be clear of all obstacles. A 100 % clearance criterion for the fresnel zone is followed in temperate climates. However, for lower frequencies of about 2 GHz and long paths, i.e. over 100 km, the criteria becomes stricter, i.e. k = 1 or k = 0.66. The impact of the variations of k-values will be given later in this chapter. 3.4.4 LOS Check As seen in the section above, a clear line-of-sight (LOS) is necessary for the microwave links to be approved and installed. The primary consideration is site selection. 