TABLE OF CONTENTS An antenna is any structure or device used to collect or radiate electromagnetic waves. Antennas allow the transformation of electromagnetic waves propagating along a transmission-line into electromagnetic waves propagating in free-space. This transformation may be performed in both directions. A transmitting antenna transforms a transmission-line wave into a free-space wave, whereas a receiving antenna converts part of the free-space wave into a transmission-line wave. An antenna usually is a reciprocal device and, in principle, each antenna may be used for receiving as well as for transmitting electromagnetic waves. However, the choice of the type of antenna and details in their construction depend on the special application of the antenna. An antenna is designed to achieve a certain radiation pattern (i.e., angular distribution of the radiated power). Furthermore, volume, weight and mechanical stability play a role. In the case of transmitting antennas, the power of the transmitted signal also has to be considered. With decreasing wavelength the antenna dimensions decrease as well. Due to this circumstance the scope for antenna design at higher frequencies is larger. Figure 13.1 shows some antenna types. One of the most common antenna types is the dipole antenna shown in Figure 13.1(b). The dipole antenna usually is formed by two straight wire segments and excited by a source inserted between these segments. Using the mirror principle, we can put one wire segment in a position normal to a conducting plane. This yields the monopole antenna according to Figure 13.1(a). This antenna type...
