finite number of current generators of length dx driven by fields E(x). (6.58) Then, if the total impedance at the terminals is Z and the current volts generators have a normalised value of D(x) relative to the value at the terminals (x = 0), the elemental current generators will each contribute an amount E(x)D(x)dx/z to the current I. The current dis- Figure 6.16 gives the simple equivalent circuit for a half wave- tribution D(x) on a rod antenna is assumed to be analogous to the length dipole receiving antenna. For maximum power transfer R also equals 73 ohms, thus the power absorbed by the load is as in sinusoidal standing wave pattern obtained on an open circuited transmission line of the same length as the antenna, along which a Rotating electric field vector Direction of Propagation: normal to plane XOY Figure 6.14 Polarisation ellipse and circumscribing rectangles The space wave 6/13 volts VL volts Figure 6.16 Simple equivalent circuit for a half wavelength dipole Figure 6.17 Equivalent circuit of a loop antenna Equation 6.59 and the voltage across the load is with where B is given The equivalent circuit of the antenna is shown in Figure 6.17, with L being the self inductance, C the self capacitance, R the winding by Equation 6.60. resistance, C1 a tuning capacitor, and Z the load impedance. (6.59) This circuit will have a 'Q' value, such that the unloaded output voltage (V) at resonance is given by Qe volts. A sufficiently large voltage is only obtained from an air filled loop by making its area large. Large voltages are often achieved by (6.60) winding the loops on ferrite rods. In this way m.f. receivers can have antennas with cross sectional areas some thousands of times smaller than would be necessary if they were air filled. 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. 6.3.4 High gain rod antennas For the half wavelength dipole this is given by Equation 6.61. Frequently a simple dipole is inadequate for prac