Preface

Compact antennas are a subject of growing interest from industry and scientific community to equip wireless communicating objects. Although a lot of effort has been made in the de‐ sign of mobile terminal antennas, major advances are needed for emerging applications such as pervasive wireless sensor networks. The need for high performance small antennas and RF front ends is the challenge for future and next generation mobile devices.

In addition to being compact, major keys for antennas designers are the implementation of multi-functionality and broadband properties.

This book brings together the rapidly growing body of knowledge on compact antennas into a single comprehensive volume. The Progress in Compact Antennas book is designed to meet the needs of electrical engineering and physics students at the senior undergraduate and beginning graduate levels, and those of practicing engineers.

Information contained in this book is the result of ongoing research investigations on com‐ pact antennas. I would like to acknowledge the work of all authors who have contributed to the realization of this book.

I am also grateful to the staff of InTech, especially Ms. Iva Lipović, for her interest, support, cooperation, and production.

> **Laure Huitema** Xlim Research Institute, University of Limoges, France

**Chapter 1**

**Compact Antennas — An overview**

Additional information is available at the end of the chapter

Antenna size reduction is restricted by fundamental physical limits [1-3], in terms of trade-off between radiation performances and impedance bandwidth. Miniaturization of devices leads to the reduction of antennas which becomes one of the most important challenges [4]. Limi‐ tations in terms of bandwidth and efficiency suggest an analysis with respect to fundamental limits [5]. Although interests are often focused on the impedance bandwidth, many studies deal with the radiation quality factor Q. Some papers [6] have been concluded that the impedance bandwidth BW equals 1/Q. The minimum Q value reachable by an infinitesimal electric dipole, or similarly by the azimuthally symmetric TM10 spherical mode, has been

Hansen and Best [7] have shown that the lower bound on Q, deriving from Chu's analysis, is

where a is the minimum radius of the sphere enclosing the antenna and k is the wave number

The Figure 1 shows that it is very difficult to have a wide bandwidth (low Q-factor), while reaching a good efficiency for miniature antennas (k.a around 0.2). Thus, the miniaturization

Since many years the scientific literature addresses some approaches concerning miniaturiza‐ tion techniques. The goal is to decrease the electrical size of the radiating element. This chapter will draw up a survey of compact antennas in practical settings and the most common

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Qlb* <sup>=</sup>*η*( <sup>1</sup> (*ka*) <sup>3</sup> + 1 *ka* )

of antennas implies them to suffer of both limited efficiency and low bandwidth.

depending on the expense of efficiency as shown by the equation:

L. Huitema and T. Monediere

http://dx.doi.org/10.5772/58837

**1. Introduction**

investigated thoroughly.

miniaturization techniques listed below:

(k=2π/λ).
