f metal like those seen on automobiles. Some may be quite sophisticated and give higher levels of performance, but at a higher level of cost and possibly larger in size. Simple antennas may also be able to perform a satisfactory function. For each application it is necessary to understand the requirements and use this information to select the optimum type. Gain, directivity, frequency of operation, bandwidth, efficiency and many more attributes all need to be considered when making the choice of antenna. In addition to this some may be horizontal while others may be vertical, either for operational considerations, or possibly for mechanical considerations. As a result a vast number of different types of antenna may be seen for differing applications. The simplest type of antenna is one that is very short compared to a wavelength. However, these antennas are very inefficient and seldom used. Possibly the most common type of antenna is the dipole. This can be used on its own or as a building block in a more complicated antenna. Dipole The most widely used form of the dipole is the half wave version, although it can be any odd multiple of electrical half wavelengths. The half wave version consists of a half wavelength section of wire, with the feed point in the centre of the antenna as shown in Figure 4.15. To understand more about the operation of the antenna it is necessary to look at the voltage and current distribution along the wire. From Figure 4.16 it can be seen that they vary sinusoidally along the length of the antenna. The voltage has a maximum at either end, and a minimum Half wavelength Quarter Quarter wavelength wavelength Feeder Figure 4.15 A half wave dipole antenna 96 Newnes Guide to Radio and Communications Technology Current Voltage Feeder (a) Half-wave dipole Current Voltage Feeder (b) Three half wavelength dipole Figure 4.16 Voltage and current distribution of dipole antennas in the middle where the feed point is taken. The current follows the opposite pattern, falling to a minimum at the end a