ed on the basis that, for a thin dipole, the current distribution along the antenna is approximately sinusoidal, as shown in Figure Generation of the electromagnetic wave 6/9 d-1 Cose Current element Current distribution Figure 6.10 The half wavelength dipole 6.10. In this figure the value of the current distribution is given by (6.45) Equation 6.39 and E0 by Equation 6.40. I = I m cos B 1 (6.39) The radiation resistance of the thin half wavelength dipole is given by the coefficient of 12 in Equation 6.46 and is, therefore, equal to (6.40) 73.1 Ohms. It is now possible to determine the r.m.s. value of the electric field in the direction of maximum radiation by combining Equations 6.41, 6.42 and 6.44, with 0 equal to zero, giving Equation 6.46 Using the same system of spherical co-ordinates defined earlier, where E is in Volts per meter, with P in Watts and d in meters. the far field radiation terms in free-space are given by Equations 6.41 and 6.42. (6.46) (6.41) 6.2.5 Radiation from a short monopole Figure 6.11 shows a vertical monopole on a perfectly conducting, (6.42) infinite, flat ground plane. The current distribution (given by Equa- tion 6.47), for a thin monopole is approximately sinusoidal and, as a result of the perfect image in the ground plane, the radiation The average power flux density through the surface of a surround- pattern above the plane is identical to that of a dipole of length 2H. ing sphere is given by the Poynting vector. The total power P radiated by the dipole is equal to the total power Current distribution (6.47) radiated through the surface of the sphere and is given by Equation 6.43. Thus, for a short monopole (H < 0.1 A), the current may be as- sumed to taper linearly from a maximum at the base to zero at the (6.43) top. The radiation pattern above the plane will be identical to that of the short dipole. However, since 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 ha