e done with small k (e.g. k = 0.7) to get at least zero clearance. Links under 15 km (climatic conditions: tropic, hot and dry climates), where the following rule can be used in addition to the above: At least zero clearance should be obtained for k = 0.3.6-2 Links above 15 km (climatic conditions: tropic, hot and dry climates), where the following rule can be used in addition to the above: At least zero clearance should be obtained for k = 0.5. While choosing antenna heights, it is better to choose higher antenna heights than lower ones. The reason is that once the tower is installed, it is easier to lower the antenna. On the contrary, if the tower is chosen to be lower and later a need to increase the antenna height arises (due to, for example, ducting), the tower would need to be taken off and a new one installed, which would result in a loss of traffic, time and the huge costs associated with replacing the tower. Rain plays an important part in link budget calculations above frequencies of 10 GHz. At these frequencies the link lengths start to become shorter as compared to ones at lower frequencies, such as 2 GHz. There are some rules of thumb for protection against rain: The microwave loop should have a diameter more than 3 km. The angle of separation between two links should be more than 60°. Links that belong to the same loop should not cross each other. Microwave Propagation Figure 3.54 Protection against rain Now consider a loop of microwave links. When heavy rainfall takes place near site B shown in Figure 3.54 and the angle between line BA and link BC is very small, fading due to rain may take place on both the links at the same time, resulting in loss of traffic generated at site B. However, if the links are widely separated, the probability of the traffic loss becomes less. The same is true of the loop having a wider area and links from the same loop crossing paths. Microwave planning engineers should remember that the rules of thumb given above only reduce the probability of fading, but much also de