of -157 N units/km, the signal will trace a path similar to the curvature of the earth. However, when the refractivity gradient becomes less than this, the signal becomes trapped in the atmospheric layers, causing ducting. Fronts, subsidence, radiations nights, etc., cause the formation of fronts. However, the type duct, its thickness, height and layers within also have an impact on the received signal. Transmission Network Planning and Optimisation 3.5.3 Overcoming Fading As mentioned above, the fading phenomenon usually causes a reduction in the signal strength. Thus, it becomes quite important for microwave engineers to introduce the aspects that counter fading during the design phase itself. It is quite a tedious process to optimise a microwave link, especially when a link is installed. k-fading Once the link is 'up and running' it is very difficult to change the height, position, etc., of the microwave tower without losing the traffic (and revenues) for the period of time that the changes are taking place. With weather being the most unpredictable aspect of microwave link planning, the best a microwave engineer could do is during the link designing phase itself. Precautions against k-fading can include getting the required clearances by inputting various values of the k-factor. Also, if a choice is to be made between taller and smaller towers, then the former should be chosen. This is because a taller tower will give more flexibility of space (to move antennas) and protection against phenomena like ducting, etc. Based on the ITU recommendation and researches, there are design principles that a microwave engineer can follow to choose antenna heights. Links under 15 km (climatic conditions: cold and temperate) The first fresnel zone is free of obstacles for k = 1.33. An additional check may be done with small k (e.g. k = 0.5) to get at least zero clearance. Links between 15 km and 30 km (climatic conditions: cold and temperate) The first fresnel zone is free of obstacles for k = 1.33. An additional check may b