The study of antennas is very extensive and would need several texts to cover adequately. In this chapter, however, a brief description of relevant performances and design parameters will be given for introductory purposes.
An antenna is a component that radiates and receives the RF or microwave power. It is a reciprocal device, and the same antenna can serve as a receiving or transmitting device. Antennas are structures that provide transitions between guided and free-space waves. Guided waves are confined to the boundaries of a transmission line to transport signals from one point to another , while free-space waves radiate unbounded. A transmission line is designed to have very little radiation loss, while the antenna is designed to have maximum radiation. The radiation occurs due to discontinuities (which cause the perturbation of fields or currents), unbalanced currents, and so on.
The antenna is a key component in any wireless system, as shown in Fig. 3.1. The RF/microwave signal is transmitted to free space through the antenna. The signal propagates in space, and a small portion is picked up by a receiving antenna. The signal will then be amplified, downconverted, and processed to recover the information.
There are many types of antennas; Fig. 3.2 gives some examples. They can be classified in different ways. Some examples are:
1. Shapes or geometries:
a. Wire antennas: dipole, loop, helix
b. Aperture antennas: horn, ...