inable down to the skin depth. Deygout, J. (1966) 'Multiple knife-edge diffraction of Micro- Electricity undertakings have measured conductivity, for 50Hz, to waves', IEEE Trans. Antennas and Prop., AP-14, (4) pp. 480- a depth of about 20m. This gives a good guide for values which can be used in the m.f. band. Various methods are quoted in published literature for the determination of ground conductivity (CCIR, Hacking, K. (1970) UHF Propagation Over Rounded Hills, Proc. 1990e). Also a method developed in Finland (Koskenniemi and Inst. Elect. Engrs., 117, (3), pp. 499-511. Laiho, 1975) appears to be accurate, and can be measured quickly Hufford, H.A. (1952) An integral equation approach to wave propa- from an aircraft. gation over an irregular surface, Q.J. Appl. Math., 9, pp. 391- In the UK, ground conductivity can be as high as 20mS/m (milli- Siemens per metre) but is more likely to be 10mS/m in fertile regions, and about 1mS/m in the mountainous regions. If the ground Jasik, H. (1961) Antenna Engineering Handbook 1st Ed., McGraw conductivity changes by a small amount over the path in question, Hill, Chapter 6, 'Loop Antennas' by Blass, Judd. a mean value will normally produce a reasonable result but, for large Jordan, E.C. and Balmain, K.G. (1968) Electromagnetic Waves and variations, calculations must take account of the different charac- Radiating Systems, 2nd. Ed., Prentice Hall, pp. 317-321. teristics in different segments of the path. A useful method for dealing with this problem was proposed by Millington (1953). Koskenniemi, O. and Laiho, J. (October, 1975) Measurement of Effective Ground Conductivity at Low and Medium Frequen- cies in Finland'EBU Technical Review 153, pp. 237-240. References Laws, J.O. and Parsons, D.A. (1943) The relation of raindrop size to intensity Transactions of American Geophysical Union, 24, Aboramowitz, M. and Stegun, I.A. (1964) Handbook of Mathemati- pp. 452-460. cal Functions, Dover, Chapter 7. Longhurst, R.S. (1967) Geometrical and Physical Optics, 2nd. Ed., Balanis, C.A. 