pends on the atmospheric conditions. Multipath Diversity techniques can be used to prevent multipath fading. Space, angle and frequency diversity are the three main types of diversity techniques. Frequency diversity is recommended in cases where frequency selective fading is expected. However, as the spectrum is usually a problem, this technique is not used very much. Space diversity used in cases such as signals reflected from water/reflection surfaces are causing degradation of received power. In such cases, angle diversity, i.e. tilting of antennas, is also used. Angle diversity is generally used in conjunction with space diversity, but it can also be used alone in cases where microwave towers do not have space for a diversity antenna. The improvement due to space diversity can be calculated as I = [1 - exp (-3.34 x 10-4. 0.87 X (A-V)/10 (3.54)* where Po = Pw X 10 A/10 (100 (3.55)* V = |G1 - G2| (3.56)* Transmission Network Planning and Optimisation water surface Figure 3.55 Space diversity = fade depth (dB) for the unprotected path = percentage of time the fade depth A is exceeded in the average worst month = fading occurrence factor = vertical separation (centre-to-centre) of receiving antennas (m) = frequency (GHz) = path length (km) G1, G2 = gains of the two antennas (dBi) However, microwave engineers should try to avoid a path over the water surface. If unavoidable, it is best to do a reflection calculation to find the point of reflection on the surface of the earth. During the design phase itself if the point of reflection falls onto a water body, then the transmission and receiving antenna (and tower) should be placed at the point where the reflection point falls over the water (shown in Figure 3.55). However, if the water surface is totally unavoidable at the point of reflection and the link performance is degrading, space diversity should be used. In this method, at the receiving end, another antenna is placed at a distance d from the main antenna. This is called the diversity antenna. The distance at