RF and Microwave Engineering: Fundamentals of Wireless Communications

Chapter 7

Antennas

Antennas are used to emit and receive electromagnetic waves that can propagate through free space. A transmit antenna converts power that is delivered to its circuit terminal into electromagnetic waves (see Figure 7.1a). A receive antenna captures power from an electromagnetic wave and provides it at its circuit terminal (see Figure 7.1b).

Figure 7.1 (a) Transmitting antenna converts transmission line waves to free space waves. (b) Receiving antenna captures energy from the free space waves and invokes waves on the transmission line.

Figure 7.1 shows a wireless communication link with two antennas. A transmission line wave arrives at the circuit terminal (port 1) of antenna 1. Ideally the reflection coefficient associated with that port is zero and all power is accepted by the transmit antenna. The antenna produces an electromagnetic wave that propagates through free space. The mathematical treatment of the electromagnetic field in the vicinity of the antenna (nearfield) may be quite complex. For practical purposes it is commonly sufficient to describe the electromagnetic field in some distance of the antenna (farfield). In the farfield we see spherical wavefronts as discussed in Section 2.5.4. Therefore, farfield antenna parameters are commonly given in spherical coordinates (r, ϑ, φ). The receive antenna (antenna 2) is positioned far away from antenna 1 and ...