ine of the vehicle and for maxi- mum receiving height, rod aerials are frequently turns out that the input impedance and the radiation mounted on the roofs of vehicles. These aerials are also resistance of a dipole antenna are about the same. often mounted at an angle of about 45° to the horizon to enable them to be receptive to both horizontal and ver- 2.1.5.4 Folded dipole antenna tical polarisation transmissions. The folded dipole (Figure 2.1-14) is a modified form of the dipole antenna. The antenna is often used for VHF 2.1.5.6 Single loop antennas FM receivers. The impedance of a folded 2/2 dipole is Another type of antenna which is frequently used for approximately 292 Q. This higher input impedance is TV reception is the single loop antenna shown in advantageous for two main reasons: Figure 2.1-16. This loop antenna usually has an electrical it allows easy connection to 300 O balanced lines; length equal to approximately 2/2 at its operating its higher impedance makes it more compatible for frequency. It is popular with TV manufacturers because use in directive aerials (particularly Yagi arrays); it is comparatively cheap and easy to manufacture. The which will be described in Section 2.1.6. 2.1.5.5 The monopole or vertical rod antenna The monopole or vertical rod antenna (Figure 2.1-15) is antenna basically a coaxial cable 14 whose outer conductor has been removed and connected to earth. It is usually about Receiver 2/4 long except in cases where space restrictions or other input electrical factors restrict its length. At high frequencies, the required 2/4 length is short and the antenna can be made self-supporting by the use of hollow metal tubing. Fig. 2.1-15 Rod or monopole antenna. 14 A typical example of a coaxial cable is the TV lead which connects your television set to the antenna. CHAPTER 2.1 Basic features of radio communication systems Single Multi-loop antenna Input to Input to receiver receiver Fig. 2.1-16 Single loop antenna. Fig. 2.1-17 Multi-loop antenna. antenna's input impedance is appro