round Conductivity It is necessary to add a few words about the way a conductivity figure is ascribed to the Earth for the present purposes. In the case where the ground is uniform and composed of similar material at all depths and composed of material having a conductivity that remains constant independent of frequency, the meaning of the term is fairly clear. However, the geologic structure of the real Earth has considerable variations in the constituent materials near the surface. To determine the depths to which the characteristics of the Earth's surface are significant, the "skin depth" of the current flow must be calculated. This skin depth varies between 70 m for low frequencies and poor ground conductivity to 20 m for high frequencies and more conductive ground. For the sea, with its greater conductivity, the skin depth at medium frequencies is less than 1 m. The effective conductivity for land is best taken as a mean value averaged over the values obtainable down to the skin depth. Electricity undertakings have measured conductivity, for 50 Hz, to a depth of about 20 m. This gives a good guide for values that can be used in the MF band. Various methods are quoted in published literature for the determination of ground conductivity (CCIR, 1990e). Also, a method developed in Finland (Koskenniemi & Laiho, 1975) appears to be accurate and can be measured quickly from an aircraft. Chapter 7 Electromagnetic Waves In the United Kingdom, ground conductivity can be as high as 20 mS/m (milli-siemens per meter) but is more likely to be 10 mS/m in fertile regions and about 1 mS/m in moun- tainous regions. If the ground conductivity changes by a small amount over the path in question, a mean value will normally produce a reasonable result, but for large variations calculations must take account of the different characteristics in different segments of the path. References Balanis, C. A. (1982) Antenna Theory Analysis and Design, Harper & Row. Bremmer, H. (1949) Terrestrial Radio Waves: Theory of Propagation, Elsevier