as horizontally polarised and, when the plane is perpendicular to the surface of the Earth, the wave is defined as vertically polarised. 6.2.9.2 Circular polarisation Degrees below the horizontal A wave is said to be circularly polarised where the electric field vector rotates in the plane normal to the direction of propagation such that the locus of the extremity of the vector describes a circle. Figure 6.12 Vertical radiation pattern of a typical u.h.f. The agreed convention for defining the sense of rotation of the broadcast antenna electric vector is in terms of the direction of rotation for an observer looking in the direction of propagation i.e. towards the receiver. Clockwise rotation is often called right-hand circular polarisation power UHF television broadcasting antenna and Figure 6.13 shows and counter clockwise, left-hand circular polarisation. The electric the associated HRP. The maximum intrinsic gain or directivity may field vector rotates at the angular frequency w of the wave and be computed by performing an integration, provided the functional describes one revolution in the period 2r/w. form of the radiation pattern is known. The maximum effective gain may then be derived by subtracting all the dissipative losses and the maximum effective radiated power then obtained for any particular 6.2.9.3 Elliptical polarisation value of transmitter power. The ERP for any direction may be derived by measuring the A wave is defined as elliptically polarised where the electric field relative field coefficients r and S from the radiation patterns and vector rotates in the plane normal to the direction of propagation computing as in Equation 6.55, where is the maximum ERP. whilst its amplitude varies such that the locus of the extremity of the vector describes an ellipse. The vector rotates by one revolution P=12P (6.55) during the period of oscillation 2r/w of the source, though the angular velocity is not constant throughout the revolution, as it would be for circular polarisation. 6/12 Electromagnetic wave