en permissible to results show that for very high reliability (i.e. for all rainfall rates assume the Earth flat and perfectly reflecting, particularly at the less than say 50 100 mm/h in temperate climates) any terrestrial link low-frequency end of the v.h.f. range. The space-wave field E at a operating at frequencies much above 30GHz must not exceed say distance d is then given by Equation 9.2. 10km in length. It is possible, however, to design a system with an alternative route SO that by switching between the two links the worst effects of localised, very heavy rain can be avoided. The (9.2) magnitude of attenuation in rain is shown in Figure 9.7(a) and the principle of route diversity is illustrated in Figure 9.7(b). For tem- perate climates, the diversity gain (i.e. the difference between the W is the power radiated from a N/2 dipole, and h and h and are attenuation, in dB, exceeded for a specified, small percentage of the heights of the transmitting and receiving antennae respectively. time on a single link and that exceeded simultaneously on two parallel links) varies as follows: 9/14 The lonosphere and the Troposphere 9.8.2 Beyond-the-horizon propagation Although propagation by surface or elevated layers (see Figure 9.6) cannot generally be utilised for practical communication circuits, these features remain important as factors in co-channel inter- ference. Considerable theoretical work, using waveguide mode theory, has been carried out on duct propagation and the results are 100 mm/h in qualitative agreement with experiment. Detailed comparisons are 50 mm/h difficult because of the lack of knowledge of refractive index Heavy structure over the whole path, a factor common to all beyond-the- 10 mm/h horizon experiments. Nevertheless, the theoretical predictions of Transmitter Receiver the maximum wavelength trapped in a duct are in general agreement with practical experience. These values are as follows: Alternative Repeater 1 mm/h route a (max) in cm Duct height in m PLAN VIEW f (GH) Normal surface duct