As with light, radio waves propagate outwards from a source of energy (transmitter) and comprise electric (E) and magnetic (H) fields at right angles to one another. These two components, the E-field and the H-field, are inseparable. The resulting wave travels away from the source with the E and H lines mutually at right angles to the direction of propagation, as shown in Figure 1.2. Radio waves are said to be polarised in the plane of the electric (E) field. Thus, if the E-field is vertical, the signal is said to be vertically polarised whereas, if the E-field is horizontal, the signal is said to be horizontally polarised. Figure 1.3 shows the electric E-field lines in the space between a transmitter and a receiver. The transmitter aerial (a simple dipole, see page 16) is supplied with a high frequency alternating current. This gives rise to an alternating electric field between the ends of the aerial and an alternating magnetic field around (and at right angles to) it. The direction of the E-field lines is reversed on each cycle of the signal as the wavefront moves outwards from the source. The receiving aerial intercepts the moving field and voltage and current is induced in it as a consequence. This voltage and current is similar (but of smaller amplitude) to that produced by the transmitter. Note that in Figure 1.3 (where the transmitter and receiver are close together) the field is shown
spreading out in a spherical pattern (this is known more correctly as the near field). In practice there will be some considerable distance between the transmitter and the receiver and so the wave that reaches the receiving antenna will have a plane wavefront. In this far field region the angular field distribution is essentially independent of the distance from the transmitting antenna.