(ERP). The numerical power gain of a half-wave dipole relative to an isotropic source is 1.641 (2.15 dB). The computation of the intrinsic or directive gain of an antenna requires a complete knowledge of the three-dimensional radiation pattern. Fortunately, for many practical cases a sufficiently accurate approximation to the three-dimensional pattern can be obtained from the radiation patterns measured in two mutually perpendicular planes. These patterns rep- resent cross-sections of the three-dimensional pattern and are normally measured in the horizontal plane and the vertical plane. Under these conditions, the patterns are called the Horizontal Radiation Pattern (HRP) and the Vertical Radiation Pattern (VRP), respectively. They can also be referred to as the H-plane and E-plane radiation patterns, respectively, for a vertically polarized antenna and, vice versa, for a horizontally polarized antenna. It is not uncommon for transmitting antennas to have the main lobe, or beam, of the VRP tilted down below the horizontal to reduce the power wasted in radiation above the horizon. In such cases, the HRP is measured for O equal to the beam tilt angle, not 90 degrees. Figure 7.12 shows a vertical radiation pattern for a typical high-power UHF television broadcasting antenna, and Figure 7.13 shows the associated HRP. The maximum intrinsic gain or directivity can be computed by performing an integration, provided the functional Fig. 7.12 Vertical radiation pattern of a typical UHF broadcast antenna. Degrees below Horizontal Generation of Electromagnetic Waves Relative Field Fig. 7.13 Horizontal radiation pattern of a typical UHF broadcast antenna. form of the radiation pattern is known. The maximum effective gain can then be derived by subtracting all the dissipative losses and the maximum effective radiated power then obtained for any particular value of transmitter power. The ERP for any direction can be derived by measuring the relative field coefficients r and S from the radiation patterns and computing as in equa