antennas are widely used, especially at VHF and UHF. As the antenna element is in the vertical plane, power is radiated in all directions around it. Coupled to this the antenna has a low angle of radiation, meaning that little signal is wasted by radiation in an upwards direction away from the earth. In view of their radiation pattern these antennas are ideal for mobile applications. Their all-around radiation pattern means that the antenna does not need to be repositioned as the vehicle moves. The use of these antennas is not restricted to mobile stations. Fixed 108 Newnes Guide to Radio and Communications Technology Voltage Vertical antenna element Current Feeder Earth connection Figure 4.27 A quarter wave vertical antenna stations too can use them when an omnidirectional pattern is required. Most medium wave broadcast stations use vertical antennas to enable them to cover all directions away from the antenna. Although a vertical antenna can be made a variety of different lengths, they are often made a quarter wavelength long. This means that they are fed at a current maximum point as shown in Figure 4.27. A vertical is an unbalanced antenna and is fed with coaxial feeder as shown. The centre conductor of the feeder is connected to the vertical element, while the screen is connected to a ground. The grounding system must be very good. A single spike into the earth may have a resistance of 100 ohms or more in some cases. As the feed impedance of a vertical of this type may be only 35 ohms, the earth resistance easily becomes the dominating factor absorbing most of the power supplied to the system. Broadcast stations using vertical antennas require the optimum performance and will ensure that a very good earth system is installed along with the antenna itself. Apart from a good earth spike at the base of the antenna radial systems consisting of wires a quarter wavelength or more are buried just below the surface of the ground. For mobile applications the antenna is mounted onto the bodywork of the vehicle. In thi