anding wave pattern obtained on an open-circuited transmission line of the same length as the antenna, along which a wave is propagating with the speed of light. This statement lacks some rigor, but is accurate enough for most practical purposes and has stood the test of time. The current distribution, shown as a dotted line on Figure 7.15, is for a half-wavelength dipole antenna. In this case, if the field is considered constant along the antenna, the current is given by equation 7.57 where R is the load impedance and the intrinsic radiation resistance of the half-wavelength dipole is 73 ohms. (7.57) From equation 7.57 it follows that the open-circuited voltage, e, across the terminals is given by equation 7.58. (7.58) Figure 7.16 gives the simple equivalent circuit for a half-wavelength dipole receiving antenna. For maximum power transfer, R also equals 73 ohms; thus, the power absorbed by Current Distribution Fig. 7.15 Current distribution on a half-wavelength dipole. Chapter 7 Electromagnetic Waves E 2 Volts Fig. 7.16 Simple equivalent circuit for a half-wavelength dipole. VL Volts the load is as in equation 7.59 and the voltage across the load is E/T where B is given by equation 7.60. (7.59) (7.60) The effective capture area, or aperture, S, of an antenna is defined as the ratio of the received power to the incident power flux density. For the half-wavelength dipole this given by equation 7.61. (7.61) The Hypothetical Isotropic Antenna The concept of an isotropic receiving antenna creates conceptual problems because such an antenna has to be infinitesimally small. Nevertheless, an effective receiving aperture can be derived by taking that of a physically realizable antenna divided by its power gain relative to an isotropic antenna. Equation 7.61 gives the effective aperture of a half-wavelength dipole that has a maximum power gain of 1.64, relative to an isotropic antenna. The effective aperture of an isotropic antenna is therefore given by equation 7.62. (7.62) The Loop Antenna An application of Lenz's law 