opagation 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 The radio frequency spectrum costs) and link availability. This chapter examines radio wave propagation in the frequency The radio frequency spectrum is divided into a number of bands bands from v.l.f. (10kHz) to the millimetric band (100GHz), and the which have been given designations such as 1.f., m.f., h.f. etc. for influence of the earth, the atmosphere and the ionosphere on such ease of reference. These are shown in Figure 27.1 and are presented transmissions. It is assumed that propagation is in the far field i.e. on a logarithmic scale. The microwave band, usually taken to be several wavelengths from the antenna, where the electric and mag- from 1GHz to 30GHz has been subdivided into a number of sub- netic components of the wavefront are at right angles and normal to bands which have been given letter designations such as X band the direction of propagation. The process by which energy is laun- (8GHz to 12GHz), but differing definitions are used and can cause ched into the propagation medium in the near field is described in confusion. 30GHz to 100GHz is commonly referred to as the mil- Chapter 17, on antennas. limetric band. Allocations have been made up to 275GHz by the Radio waves may be propagated in one or more of five modes, ITU, but there is little activity other than experimental work above depending upon the medium into which they are launched and 100GHz. through which they pass. These modes are: Frequency and wavelength are shown in Figure 27.1. As radio- waves propagate at 3x108 metres per second in free space, fre- 1. Free space propagation, where radio waves are not influenced quency and wavelength are related by Equation 27.1, where a is by the earth or its atmosphere. wavelength in metres, and f is frequency in cycles per second. v.l.f. WAVELENGTH METRES FREQUENCY 10KHz 30KHz 1