es follow the surface of the Earth 3. Ionospheric propagation, where radio waves are refracted by ionized layers in the atmosphere 4. Tropospheric propagation, where transmission is "line of sight," with some atmospheric refraction occurring Chapter 8 Radio Wave Propagation 5. Scatter propagation, where natural phenomena such as tropospheric turbulence or ionized meteor trails, are used to scatter radio waves The Radio Frequency Spectrum The radio frequency spectrum is divided into a number of bands that have been given des- ignations such as LF, MF, HF, and the like, for ease of reference. These are shown in Figure 8.1 and are presented on a logarithmic scale. The microwave band, usually taken to be from 300 MHz to 30 GHz, has been subdivided into a number of subbands, which have been given letter designations such as X band (8 to 12 GHz), but differing definitions are used and can cause confusion. GHz in the range of 30 to 100 are commonly referred to as the millimetric band. Allocations have been made up to 275 GHz by the ITU, but there is little activity other than experimental work above 100 GHz. Frequency and wavelength are shown in Figure 8.1. As radio waves propagate 3 X 108 meters per second in free space, frequency and wavelength are related by equation 8.1, where a is wavelength in meters and f is frequency in cycles per second. Axf=3x108 (8.1) Wavelength 3 X 104 M 3 X 103 M 300 M Meters Frequency 10 kHz 30 kHz 100 kHz 300 kHz 1 MHz 3 MHz 10 MHz 30 MHz Microwave Millimetric 0.3 M 3 X 10-2 M 3 X 10-3 M 30 MHz 100 MHz 300 MHz 1 GHz 3 GHz 10 GHz 30 GHz 300 GHz Visible Light 3 X 10-4 M 3 X 10-5 M 3 X 10- 6 M 3 X 10-7 M 300 GHz 1 THz 10 THz 100 THz 1000 THz Fig. 8.1 Radio frequency bands and designations. Free-Space Propagation Free-Space Propagation Free-space propagation is seldom a practical situation, but it occurs when both transmitting and receiving antennas are situated away from the influence of the Earth's surface or other reflecting and absorbing objects, including the transmitter and receiver the