lf that for the short dipole of length 2H with the same current at its centre. Thus, in free space, Equation 6.48 may be obtained. Evaluation of this integral is dealt with by Jordan and Balmain (1968, pp. 330-331) and (for 10 given by Equation 6.45) gives (6.48) Equation 6.44. P = 73.17 watts (6.44) Once again the radiation resistance is given by the term in square brackets. 6/10 Electromagnetic waves d-1 Cos Current distribution Perfectly conducting infinite plane Image Figure 6.11 Vertical monopole over a ground plane The maximum field strength at a given distance from the monop- E = Y30 P (6.52) ole, in the far field, occurs at the surface of the conducting plane and is V2 times that for a short dipole radiating the same total power, because all the energy is confined to a hemisphere. By choosing 6.2.8 Antenna gain and effective radiated power units of power in kilowatts, distance in kilometres and r.m.s. electric field in mV/m, the maximum electric field strength is given by Equation 6.49. The term antenna gain defines the degree to which an antenna concentrates radiated power in a given direction, or absorbs incident power from that direction, compared with a reference antenna. The E - 300 VP (6.49) logical reference antenna is the isotropic radiator, in view of its lack of directional properties, though the half-wavelength dipole is also often used. Antenna gain is most easily derived by considering all The quantity 300NP is called the Cymomotive Force (CMF). antennas as transmitting antennas. If the r.m.s. field strength in free space produced at a distance d is 6.2.6 Radiation from a quarter wavelength given by E, the total power radiated by the antenna is given by the monopole surface integral as in Equation 6.53. The quarter wavelength monopole above the conducting surface (6.53) may be compared to the half wavelength dipole with the same base current: it has the same radiation pattern above the surface, half the total radiated power and the maximum field strength is given by Equation 6.50, where E is 