t Fresnel zone should be kept 100% clear. This means that there will be no reduction in signal level due to the terrain. If this first Fresnel zone were blocked, then the percentage of losses would be proportional to the percentage blocking in PRE-PLANNING IN TRANSMISSION NETWORK the Fresnel zone. In practice, transmission planning engineers try to keep some 'clearance' between the Fresnel zone and the highest obstacle in the radio path. 3.3.7 Microwave Link Planning This constitutes a very important part of the transmission planning process in networks that use microwave transmission. The aim of microwave link planning is to achieve the desired performance at lowest cost. The two major aspects of link planning are the link budget and propagation phenomena. Link Budget Calculations The aim of link budget calculations is to find out what path length would be suitable for getting the desired signal level. This received signal level is then used for calculation of the fade margin. Fade margin can be defined as the margin required for countering the effects of a decrease in the signal level (also known as 'fading') due to atmospheric conditions, etc. The first factor required in the calculation of the fade margin is the free-space loss, which is the same as the one introduced in Chapter 2. The phenomenon of free-space loss (FSL) stands true for microwave planning, except that now the distance changes from a few hundred metres to a few kilometres. FSL is dependent on the frequency of the microwave signal and the distance covered: FSL (Lfs) = 92.5 + 20 log + 20 log f (3.2) The second factor required is the gain of the antenna. The antenna should have sufficient gain SO as to receive the signal at a desired level. The gain of a microwave antenna is given as: (3.3) where G is the gain of the antenna in dB, A is the area of the antenna aperture, e is the efficiency of the antenna, and 2 is the wavelength (same units as A). Equation 3.3 can also be written as (for parabolic antennas): G = Da + 20 log f + 17.5 (3.4) where G