ssed in Wm-2, i.e. Two related characteristics of electromagnetic waves are used as a method of referencing the waves. They are the 1 mW cm -2 = 10 W m-2 = frequency (already discussed above) and the wavelength. The latter is denoted by the symbol lambda (A). The Electromagnetic waves propagated in free space have relationship between these two characteristics involves the electric and magnetic fields perpendicular to each consideration of the velocity of propagation of radio other and to the direction of propagation, as represented in waves. Figure 2.5-4 and are known as transverse electromagnetic The velocity of propagation of all electromagnetic waves (TEM waves). waves (c) is constant in a given homogenous medium The basic nature of an electromagnetic wave can be and in free space has a value of 2.997 925 X 108 s-superscript(s) physically illustrated by holding two pencils with their but the approximate figure of 3 X 108 ms-1 is used unsharpened ends touching and the two pencils being in practical calculations. This figure is also used for air mutually at right angles to each other and held SO that one but does not apply to propagation in other media. The is parallel to the ground and one pointing vertically to relationship between frequency and wavelength is represent the planes illustrated in Figure 2.5-4. If now C = fN a third pencil is added, mutually at right angles to the other two, it will indicate the direction of propagation as where the wavelength (A) is the physical length of in the figure. The vertical pencil point represents the one cycle of the propagated wave, as shown in electric field (vertically polarised wave) and the second Figure 2.5-4. pencil the magnetic field. For electromagnetic waves in free space, where f is in The plane of polarisation of a wave is, by convention, hertz (Hz): that of the electric field, i.e. the polarisation in Figure 2.5-4 is vertical. This convention has the advan- 1(m) = tage that for vertical polarisation the antenna will also be vertical (e.g. a simple rod ant