gives the open-circuited voltage of a loop antenna as in equa- tion 7.63, where N is the number of turns in the loop and A is its area. e =wuHNA (7.63) The Space Wave Fig. 7.17 Equivalent circuit of a loop antenna. The equivalent circuit of the antenna is shown in Figure 7.17, with L being the self- inductance, Cc the self-capacitance, Rc the winding resistance, CT a tuning capacitor, and ZL the load impedance. This circuit will have a Q value such that the unloaded output voltage (Vu) 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 winding the loops on ferrite rods. In this way MF receivers can have antennas with cross-sectional areas some thousands of times smaller than would be necessary if they were air filled. High-Gain Rod Antennas Frequently a simple dipole is inadequate for practical reception purposes. More gain is required, together with the ability to reject unwanted signals coming from directions other than that of the wanted signal. This is often achieved with a Yagi antenna consisting of a driven element with a passive reflector and directors (Yagi, 1928; Balanis, 1982). The reflector is more efficient if it consists of a mesh of rods effectively forming a back screen. Gain can also be achieved by having several dipoles spaced and combined to give directivity. An additional requirement can be that the antenna have a wideband response as well as gain relative to a dipole; this is neatly achieved with a log-periodic antenna (Balanis, 1982). The terminated voltage (V) expressed in decibels relative to 1 uV, for a practical receiv- ing system with antenna gain, G dB, relative to a half-wavelength dipole and attenuation of the feeder system, L dB, is given by equation 7.64, assuming perfect impedance matching to the load resistance, R. Vu=20logE-L+G-120-20B1073 (7.64) The Space Wave Earlier expressions were derived for field strength in free space. It is convenient to convert these to f