 is the gain of the antenna in dB, Da is the diameter of the antenna in metres, and f is the frequency of operation in GHz. Based on the above, the hop loss Lh can be calculated as: LhLfs-G-G+Lext+Latm (3.5) where Gt is the gain of the transmitting antenna, G- is the gain of the receiving antenna, Lext is extra attenuation (due to radome, etc.), and Latm stands for atmospheric losses due to water vapour and oxygen. The received signal level P can then be calculated as: Prx = P, - Lh. (3.6) Mathematically, the fade margin can be described as the difference between the received signal power and the receiver threshold (Rxth): FM = P x - Rxih (3.7) TRANSMISSION NETWORK PLANNING AND OPTIMISATION Example 2: Calculation of fade margin Calculate the fade margin of a microwave link whose dimensions are as follows: Hop length = 10 km Frequency = 15 GHz Antenna diameter = 0.6 m Transmit power : 20 dBm Extra attenuation = 0 dB Atmospheric attenuation = 0 dB Receiver threshold = -75 dB Using equations 3.2 to 3.7: FSL (Lfs) = 92.5 + 20 log (10) + 20 log (15) = 136.02 dB Gain of antenna (G and G,) = 20 log (0.6) + 20 log (15) + 17.5 = 36.58 dBm Hop loss (Lh) = 136.02 - 36.58 - 36.58 - 0 - 0 = 62.86 dB Received signal level (Prx) = 20 62.86 42.86 dB Thus, fade margin (FM) = -42.86 - (-75) = 32.14 dB Propagation Phenomena In this section, we try to look into the factors that would affect the received signal level at the receiving antenna. As seen above, there are losses in the received power between the transmitting and the receiving antennas and this loss has been termed 'free-space loss'. Apart from this, there are many other factors such as weather conditions, rain, terrain profile, etc., that need to be considered before the link design can reach the implementation stage. First, however, the principles of propagation should be understood. When a signal passes from one medium to another, the trajectory and velocity of the signal change, owing to the refraction that takes place in the atmosphere. The ratio of the velocity of th