 (1943) Radio Engineers' Handbook, McGraw-Hill, 675-682. Tsukiji, T. and Tou, S. (1980) On Polygonal Loop Antennas, IEEE Trans. Antennas Propag. 28(4):571-575. Van Der Pol, B., and Bremmer, H. (1937) The Diffraction of Electromagnetic Waves from an Electrical Point Source Round a Finitely Conducting Sphere, with Applications to Radiotelegraphy and the Theory of the Rainbow, Phil. Mag. 24(1):141-176; 24(2):825-863. Radio Wave Propagation Mark Holker Hiltek Ltd. Introduction Electromagnetic energy radiates outward from the source, usually an antenna, at approxi- mately the speed of light and is attenuated and influenced by the medium through which it travels. Radio communications necessitate launching RF energy into the propagation medium, detecting its presence at some remote point, and recovering the information con- tained within it while eliminating noise and other adverse factors introduced over the transmission path. An understanding of radio wave propagation is therefore essential in the planning and operation of radio communications systems to ensure that communications can be established and that there is an optimum solution between costs (capital and running) and link availability. This chapter examines radio wave propagation in the frequency bands from VLF (10 kHz) to the millimetric band (100 GHz) and the influence of the Earth, the atmosphere, and the ionosphere on such transmissions. It is assumed that propagation is in the far field (i.e., several wavelengths from the antenna), where the electric and magnetic components of the wavefront are at right angles and normal to the direction of propagation. The process by which energy is launched into the propagation medium in the near field is described in Chapter 9. Radio waves can be propagated in one or more of five modes depending on the medium into which they are launched and through which they pass. These modes are 1. Free-space propagation, where radio waves are not influenced by the Earth or its atmosphere 2. Ground-wave propagation, where radio wav