Broadband Antennas

Introduction

Antenna is an indispensable part in wireless communication systems and its design is becoming more important due to the advent of various wireless devices and applications. The rapid growth of antenna design industry can be attributed to the fast growing mobile communications market. However, the demand for high data rate and the arrival of spread spectrum technology are the catalyst for greater research in broadband antennas which were previously employed mostly for radar and tracking applications. Typically, there are two categories of broadband antennas: the one having wideband over a continuous range of frequencies and the other whose wideband is realized by the sum of multiple discrete narrow bands. In this book chapter, our discussion is focused on the former category only.

The bandwidth of an antenna can be defined in terms of one or more physical parameters such as impedance, polarization and radiation pattern. The fidelity of these parameters over a frequency range is generally interpreted as the bandwidth of the antenna. Generally, a satisfactory impedance bandwidth is the primary consideration while designing an antenna. The impedance bandwidth is generally specified in terms of returnloss (S parameter: S11) or a voltage standing-wave ratio (VSWR) over a frequency range. An antenna must be well-matched over the entire operating frequency range and this condition is verified by a specific level of VSWR and S11. The impedance bandwidth is inversely proportional to the quality factor (Q) of the antenna and is given by

An antenna should demonstrate VSWR = 2 or S11 = -10 dB within the operating range. The condition might be stringent for some applications that would require VSWR = 1.5 or S11 = -15 dB.

Although there is no specific definition of broadband antenna, generally an antenna that has a bandwidth of 50% or above is considered a broadband antenna. In some references, however, one octave of bandwidth (70.7% bandwidth) has been suggested as broadband. While the dual-polarized planar antenna in (Cui, Li, & Wang, 2013), having a bandwidth of 50%, has been suggested as broadband antenna, the ESPAR antenna proposed in (Alshami, Aboulnour, & Dib, 2009), which has a bandwidth of 40%, has also been touted as broadband antenna. On the other hand, the circularly polarized antenna in (Zhang et al., 2011), designed to operate in the L band, having a bandwidth of only 33% has been termed as broadband antenna as well.