rrection factors for field strength for various terrain height differences. Frequencies at 900MHz and 1800MHz are used for comparatively short distance "cellular" services, and communications will often be limited by multi-path interference cause by single or multiple re- flections, particularly in urban environments. The only practical solution is to try different positions for the transmit antenna to minimise this problem. Multi-path interference can also be a prob- lem on microwave links for TV outside broadcasts where trans- missions may be from moving vehicles or aircraft. This can be minimised by using circular polarisation, as the reflected signals will undergo a polarisation reversal which will enable the receive antenna discriminate against them. 1000 km As the propagation medium is continuously changing, the figures Distance from transmitter derived from CCIR curves are based on statistical probabilities, generally 50% signal levels for 50% of the time, and therefore diversity transmission and a larger link margin may be required to increase channel availability. Figure 27.16 Received field strengths for different transmit antenna heights, v.h.f. and u.h.f. bands Over the horizon transmissions 27/13 h = 500m 250 km 300 Distance from transmitter Figure 27.17 Attenuation correction factors for different terrain roughness Over the horizon transmissions Tropospheric scatter and meteor scatter communications both provide over the horizon communications on a regular and predict- able basis. Tropospheric scatter relies on small cells of turbulence The four thirds earth radius does extend transmission beyond the in the troposphere between 2km and 5km above the Earth surface horizon in normal conditions, but a natural phenomenon known as which scatter incident radiation, as illustrated in Figure 27.18. ducting can extend this considerably. Ducting happens in stable Frequencies in the lower part of the microwave range from 1.0GHz weather systems when large changes in refractive index with height to 5.0GHz are gen