s case the metalwork of the vehicle acts as the ground. This is normally a very efficient grounding system, especially at VHF and above where the bodywork extends for several wavelengths away from the base of the antenna. The other advantage is that the antenna can be mounted relatively high up. The best position from a radio performance standpoint is the centre of the roof of the vehicle; however, this is not always acceptable aesthetically. In this case mounting points on the wing usually give good performance as well. Antenna systems 109 Vertical element Quarter wave radials Feeder Figure 4.28 A ground plane antenna It is not always convenient to mount a vertical on the ground. It can easily be masked by nearby objects reducing its effectiveness. This is particularly true for VHF and UHF applications where the antenna is physically very small and mounting it on the ground would considerably reduce its effectiveness. In situations like this a ground plane system can be used. The ground plane is made out of a number of rods or wires which are normally a quarter wavelength long. Four rods are usually employed as being sufficient for most applications and a ground plane system may look like that shown in Figure 4.28. The feed impedance of a ground plane is low. To improve the match to 50 ohms the radials can be bent downwards. As the radials are bent further down, the antenna becomes more like a vertical dipole and its impedance increases towards a value of 73 ohms, assuming that there are no nearby objects. Alternatively the vertical element can be folded in the same way as a dipole, or a matching network can be used at the feed point to ensure a good match. Short antennas It is often necessary to design an antenna that is shorter than a quarter wavelength. With electronic equipment becoming smaller the same is often needed for any associated antennas. This is particularly true for cellular telephones where equipment is very much smaller than it was a few years ago, and also the antennas used on vehicles are requ