 pattern above the plane is identical to that of a dipole of length 2H. Current distribution I = =Imsin[B(H-1)] (7.47) Thus, for a short monopole (H < 0.1A) the current can be assumed to taper linearly from a maximum at the base to zero at the top. The radiation pattern above the plane will be identi- cal to that of the short dipole. However, because power can only be radiated into the hemisphere above the reflecting plane, the total power radiated (for a given value of current at the base of the monopole) is half that for the short dipole of length 2H with the same current at its center. Thus, in free space equation 7.48 can be obtained. (7.48) Once again the radiation resistance is given by the term in square brackets. The maximum field strength at a given distance from the monopole, in the far field, occurs at the surface of the conducting plane and is 2 times that for a short dipole radiat- ing the same total power, because all the energy is confined to a hemisphere. By choosing units of power in kilowatts, distance in kilometers, and r.m.s. electric field in mV/m, the maximum electric field strength is given by equation 7.49. E=300VP (7.49) The quantity 300VP is called the Cymomotive Force (CMF). Generation of Electromagnetic Waves Radiation from a Quarter-Wavelength Monopole The quarter-wavelength monopole above the conducting surface can be compared to the half-wavelength dipole with the same base current: It has the same radiation pattern above the surface and half the total radiated power, and the maximum field strength is given by equation 7.50, where E is in mV/m, P in kW, and D in km. E=313,P (7.50) The Isotropic Radiator The hypothetical isotropic radiator, which radiates power equally in all directions, is widely used as a reference for comparing the performance of practical antennas. Such a radiator located at the center of a large sphere of radius, d meters, and radiating a total power, P watts, produces a uniform power flux density, EAH, over the surface of the sphere, as in equation 7.51. (7.51)