e signal in free space to its velocity in another medium is known as the refractive index (n) of that medium. This term signifies the refractivity of the medium: n = Vis / Vm (3.8) where Vfs is the velocity of the signal in free space and Vm is its velocity in the other medium through which it is passing. The Earth's atmosphere is a mixture of gases. The refractive index of the atmosphere is dependent upon three factors: pressure, humidity and water vapour. As we move up higher into the atmosphere, all these three change. On a mountain you can feel the change in pressure and humidity and, of course, it becomes cooler. Thus, the refractive index of the atmosphere, which is normally considered to be close to unity, has to be calculated taking these three effects into account. The value is approximately 1.00045. Another term used is radio refractivity, N, which is defined as: v = (n - 1)*106 (3.9) PRE-PLANNING IN TRANSMISSION NETWORK Microwave Signal Microwave Signal Earth Surface Earth Surface Figure 3.9 Microwave signal trajectory Upto 300 GHz, this equation is also written in terms of pressure, humidity and tempera- ture as: N = 77.6(P/T)+ 3.73*106*(e/T2 (3.10) where e is the partial pressure of water vapour (millibars), T is the absolute temperature in kelvin, and P is the pressure (millibars). Owing to changes in the pressure, temperature and humidity at every single point in the atmosphere, there will be changes in the radio refractivity at each point. We can assume that a small change in height, dh, will bring about a small change in radio refractivity, dN. Thus, the path of the signal is not a straight line but is in the shape of an arc. Hence, in reality, the trajectory of the signal in relation to the surface of the earth is as shown in Figure 3.9(a). This kind of situation (i.e. two arcs, one for the microwave signal and one the Earth's surface) is complicated to analyse mathematically. To make the analysis easy, one of the two arcs is made into a straight line as shown in Figure 3.9(b), where the microwa