se and at right-angles to each other power is therefore 34.7 W. Notice that, because in this which propagate as an electromagnetic wave. case R = Zin, maximum power is radiated into free space. Besides being an effective radiator, the dipole antenna is widely used as a VHF and TV receiving antenna. It has 2.1.5.3 The half-wave dipole antenna a polar diagram which resembles a figure of eight (see Figure 2.1-13). Maximum sensitivity occurs for a signal Most antennas can be analysed by considering them to be arriving broadside on to the antenna. In this direction the transmission lines whose configurations and physical di- 'gain' of a dipole is 1.5 times that of an isotropic antenna. mensions have been altered to present easy energy An isotropic antenna is a theoretical antenna that radiates transfer from transmission line to free space. In order to or receives signals uniformly in all directions. Basic features of radio communication systems CHAPTER 2. 1 TX line End-fire Receiver input direction Fig. 2.1-14 Folded dipole antenna. At low frequencies where a greater length is required, the antenna is often supported by poles. Broadside direction This antenna is favoured for use in low frequency transmitting stations, in portable radio receivers, in Fig. 2.1-13 Polar pattern of a half-wave dipole. mobile radio-telephones, and for use on motor vehicles because it has a circular polar receiving pattern, i.e. it The gain is a minimum for signals arriving in the transmits and receives signals equally well in all di- 'end-fire' direction. Gain decreases by 3 dB from its rections around its circumference. This is particularly maximum value when the received signal is +39° off the important in mobile radio-phones and in motor vehicles because a motor vehicle may be moving in any direction broadside direction. The maximum gain is therefore 1.5 and the half-power beam-width is 78°. The input with respect to a transmitting station. To minimise in- impedance of a half-wave dipole antenna is about 72 O. It terference from the eng