Transmit power, PTX Received power, PRX Figure 3.44 Microwave link where G and G are the gain of the transmitting and receiving antennas respectively. When converted to dB, the above equation will be Pr(dB)=P(dB)+G(dB)+G,(dB)+201 = log (-2010g(d) (3.28) The last two terms in Equation (3.28) are called the path loss in free space or the free space loss (FSL). The first two terms, i.e. P and G, combined are called the effective isotropic radiated power (EIRP). Thus, FSL(dB) = EIRP +G,(dB) - P,(dB) (3.29) Thus, the free space loss can be given as LdB=92.5+201og(f)+20log(d) = (3.30) where f is the frequency in GHz and d is the distance in km. Now consider the microwave link shown in Figure 3.44. The hop loss is the difference between the gains and losses on a microwave link. The gains constitute the antenna gains on either side while the losses constitute the sum of free space loss and losses due to extra attenuation and atmosphere (water vapour and oxygen): Lh=LdB+LexLat-(G+G) (3.31) where Gt, G = antenna gains at transmitting and receiving ends respectively Ldb = free space loss Lex = extra attenuation loss (e.g. radome loss) Lat = loss due to water vapour and oxygen When the signal travels from the transmitting to the receiving antenna, the signal power reaching the receiving antenna Prx is calculated as Prx = Pix - Lb (3.32) Transmission Network Planning and Optimisation Once a microwave link is designed and installed, it is there 'forever'. Hence, during the design stage itself, the microwave engineer should make sure that even under worse conditions, the signal received at the receiving antenna is above the threshold. Thus, the concept of fade margin (FM) comes into the picture, which is defined as the difference between the received signal and the receiver threshold: (3.33) The fade margin is necessary as it gives the necessary guard band to prevent link failure against unpre- dictable atmospheric conditions. The lower the frequency of operation, the longer is the hop length and the, higher the probability of 