s. Distance vertically polarized transmission over medium soil is 45 dB at 30 km, whereas a horizontally polarized signal is attenuated by nearly 95 dB. Vertical polarization is therefore almost always employed. Because currents are induced in the ground by vertically polarized ground wave trans- missions, attenuation is also dependent on ground conductivity and dielectric constant. Salt water offers low attenuation whereas desert sand or polar ice offer high attenuation and consequently reduced communications coverage. Terrain irregularities, such as hills and mountains, and rough (as compared with calm) seas also reduce coverage. Received, field strength at any distance can be calculated theoretically (Braun, 1986; Maslin, 1987); however, ITU-R published a series of graphs showing received field strength in microvolts per meter and in dBs relative to 1 microvolt for distances up to 1000 km. Fre- quencies from 500 khz to 10 MHz and homogeneous surface conditions from seawater to dry soil are shown. Some of the curves are shown in Figure 8.5 and are calculated on the basis of a 1-kW transmitter working into a short omnidirectional monopole antenna. The field strength values are proportional to the square root of the power and must be adjusted accordingly for different powers or for higher-gain or directional antennas. The ITU-R curves shown in Figure 8.5 assume propagation over homogeneous terrain, but discontinuities occur in paths over land and sea or over different types of soil, and the field strength over such a path can be calculated by a method developed by Millington (1949). An example of the field strength over a land and sea path is shown in Figure 8.6, and it is interesting to note that the field strength rises after the wavefront passes from land to sea. MF ground wave propagation offers the advantage of predictable but limited commu- nications coverage, which is largely independent of ionospheric conditions and diurnal or seasonal variations. Distances of up to 1000 km are achievable over seawater, b