s are such that complete trapping occurs only at centimetric wavelengths. Partial trapping may occur for the shorter metric wavelengths. Over land the effects of irregular terrain and of thermal convection (at least during the day) tend to inhibit duct formation. For a ray leaving the transmitter horizontally, the Figure 9.7 (a) Attenuation in rain; (b) the application of route diversity to minimise effects of fading vertical gradient of refractive index must be steeper than 157 parts in 106 per kilometre. Even when super-refractive conditions are absent, there remains considerable variability in the characteristics of the received signal It tends to decrease as the path length increases from 12km, for usable in the 'scatter' mode of communication. This variability is a given percentage of time and for a given lateral path separ- conveniently expressed in terms of the transmission loss, which is ation. defined as 10 log (P/P), where P and are the transmitted and It is generally greater for a spacing of 8km than for 4km, though received powers respectively. In scatter propagation, both slow and an increase to 12km does not provide further improvement. rapid variations of field strength are observed. Slow fading is the 3. It is not strongly dependent on frequency in the range 20- result of large-scale changes in refractive conditions in the atmos- 40GHz for a given geometry. phere and the hourly median values below the long-term median are distributed approximately log-normally with a standard deviation The main problem in the v.h.f. and u.h.f. broadcasting and mobile which generally lies between 4 and 8 decibels, depending on the services (apart from prediction of interference) is to estimate the climate. The largest variations of transmission loss are often seen on effect of irregularities in terrain and of varying antenna height on paths for which the receiver is located just beyond the diffraction the received signal. The site location is of fundamental importance. region, while at extreme ranges the variations