ve signal path is made a straight line, SO changing the Earth's radius in the process. Transmission planning engineers have to consider another parameter, known as the k- factor. This factor is the one used to calculate the antenna heights and is defined as the ratio of the effective radius of the Earth, ka, to the actual radius of the Earth, a: k = ka/a. (3.11) This k-factor can also be derived using the above analogy and using equation 3.10, because this factor is dependent upon the curvature of the microwave signal which in turn is depen- dent upon the pressure, humidity and temperature at that point on the Earth's surface. As these factors all vary with height, it can as well be said that the k-factor also varies with height: k =157/[157+(dN/dh)]. (3.12) During standard conditions of temperature and pressure, the value for dNIdh is considered to be -40N/km. Thus, the k-factor turns out to be 4/3. This is the reason why transmission planning engineers plan microwave links using the k-value of 1.33, From the k-value, it is easy to predict the trajectory of the microwave signal if the path profile is known. Using the concept of the path profile and the trajectory of the microwave signal, along with the concept of the Fresnel zone, the minimum distance between the highest obstacle and Fresnel zone can be determined. This then leads to the calculation of antenna heights on two sites. Pressure, humidity and temperature all change in the 24-hour cycle of the day. This leads to a continuous variation of radio refractivity, N, and thus the k-factor during the day. TRANSMISSION NETWORK PLANNING AND OPTIMISATION k =1.33 k ==-1 Earth Surface Earth Surface k = 0.8 Earth Surface Earth Surface Figure 3.10 Variation of the k-factor Transmission planning engineers should make themselves familiar with the known climatic changes in the city/region under consideration. In practice, the Earth's curvature remains constant SO only the trajectory varies with the value of k, but it is easy to understand and analyse the phenomenon if 