far from the Elliot, R.S. (1981) Antenna Theory and Design, Prentice Hall, body as possible and as high as possible. Englewood Cliffs, N.J. Galindo, V. (1964) Design of Dual Reflector Antennas with Arbi- 17.5.5 HF communications trary Phase and Amplitude Distribution, IEEE Trans., AP- 12, HF antennas are used in the range of frequencies from 2 to 30MHz James, G.L. (1986) Geometrical Theory of Diffraction for Electro- for mobile communications and some fixed communications. Space magnetic Waves, Peter Peregrinus (IEE), London. Johnson, R.C. and Jasik, H. (Eds) (1984) Antenna Engineering precludes more than a brief mention of the types of HF antennas. Handbook, McGraw-Hill, New York. Surveys can be found in (Rudge, 1986; Johnson, 1984). HF antenna design is constrained by the ionospheric propagation characteristics King, R.W.P. (1956) The Theory of Linear Antennas Havard Univ. Press. Cambridge, Mass. which change both daily, seasonally and with the sun-spot cycle. King, R.W.P., Mack, R.B. and Sandler, S.S. (1968) Arrays of Cy- Antennas can receive either the sky wave reflected from the ionos- phere or the ground wave if transmitter and receiver are close lindrical Dipoles, Cambridge Univ. Press, New York. together. The wavelength in the HF band is such that antennas are Lamb, J.W. and Olver, A.D. (1986) Blockage Due to Subreflector Supports in Large Radiotelescope Antennas, Proc. IEE, 133(H) usually only a fraction of a wavelength in size and this in turn means that the local environment around the antenna will have a major 43 (1). impact upon the performance. This is particularly true for antennas Lo, Y.T. and Lee, S.W. (1988) Antenna Handbook, Van Nostrand, New York. mounted on vehicles, ships and aircraft. The analysis of the antenna must take account of the environment by techniques such as wire Love, A.W. (1976) Electromagnetic Horn Antennas, Selected Re- print Series, IEEE Press, New York. grid modelling (Mittra, 1975). This is computer intensive and line- Love, A.W., Rudge, A.W. and Olver, A.D. (1986) Primar