ions that vary with time. The types of antenna employed to generate such waves are many, depending on the radio frequency to be transmitted, the radiation pattern required to fulfill the needs of the system, and the environment in which the antenna is required to operate. The following sections deal with several fundamental antenna elements from which the performance of many practical antennas can be derived. Radiation from an Infinitesimal Linear Element The electromagnetic field equations for many complex antenna systems can be derived from a knowledge of the fields produced by an oscillating current in an infinitesimal linear element (Jordan & Balmain, 1968; Balanis, 1982). For such an element, the current can be assumed to be constant throughout its length. Consider a current element of length 81 carrying a current I cos wt, located at the origin of a spherical coordinate system in a homogeneous, isotropic, nonconducting medium, as shown in Figure 7.8. The formulation of the electric and magnetic field magnitudes gener- ated by such a current element is made more succinct by the introduction of two variables, as in equations 7.26 and 7.27, from which equations 7.28 to 7.30 are obtained. Fig. 7.8 Radiation from an oscillating current in an infinitesimal linear element at the center of a spherical coordinate system. Generation of Electromagnetic Waves (7.26) (7.27) H(--) (7.28) E==ny(1-a+a2) (7.29) E,=2nYo:(1-o.)cot6 (7.30) Terms in a and a2 are known as the induction and electrostatic fields, respectively. They are also known as the near-field terms, representing energy that ebbs and flows between the field and the current source. At d = 16A, a is equal to 0.01 and can effectively be set to zero. This leaves us with the radiation field terms. The total power radiated (Prad) by the current element can be obtained by integration of the radial Poynting vector over a spherical surface with the element at the center, to give equation 7.31, where I is the peak current and I is the r.m.s. current. (7.31) From equatio