or the transmit antenna to minimize this problem. Multipath interference can also be a problem on microwave links for TV outside broadcasts, where transmissions might be from moving vehicles or aircraft. This can be minimized by using circular polarization because the reflected signals undergo a polarization reversal, which enables the receive antenna to discriminate against them. Because the propagation medium is continuously changing, the figures derived from ITU-R curves are based on statistical probabilities, generally 50 percent signal levels for 50 percent of the time, and therefore diversity transmission and a larger link margin might be required to increase channel availability. Over-the-Horizon Transmissions dB (uV/m) Fig. 8.16 Received field strengths for different transmit antenna 1000 km heights, VHF and UHF bands. Distance from Transmitter Ah = 500 m 300 m 150 m 100 m Fig. 8.17 Attenuation correction factors for different 300 km terrain roughness. Distance from Transmitter Over-the-Horizon Transmissions The four thirds Earth radius does extend transmission beyond the horizon in normal condi- tions, but a natural phenomenon known as ducting can extend this considerably. Ducting happens in stable weather systems when large changes in refractive index with height occur and cause propagation with very low attenuation over hundreds of kilometers. The Chapter 8 Radio Wave Propagation Scatter Volume 2-5 km Up to 800 km Fig. 8.18 Tropospheric scatter communications link. propagation mode is similar to that in a waveguide. Ducting is not a very predictable form of propagation and therefore cannot be used commercially. Tropospheric scatter and meteor scatter communications both provide over-the-horizon communications on a regular and predictable basis. Tropospheric scatter relies on small cells of turbulence in the troposphere between 2 km and 5 km above the Earth's surface, which scatter incident radiation, as illustrated in Figure 8.18. Frequencies in the lower part of the microwave range from 1.0 to 5.0 GHz