1
Introduction

The twenty-first century is the wireless century. In the near future, it is very likely that most electronic devices will include some wireless functionality. If we look at the job market, known brands which seem to have nothing to do with antennas, such as Microsoft, Google, Amazon, and so on, are all recruiting engineers with antenna knowledge. On the other hand, there are not that many antenna engineers out there. The root cause of the shortage of antenna engineers can be traced all the way back to the university. The cornerstone of antenna engineering is electromagnetics (EMs), which is a quite abstract class and involves a lot of mathematics. The world unveiled by EMs is a four-dimensional one, which includes three spatial dimensions and one temporal dimension. To most students, the many new concepts introduced in the class are counterintuitive and confusing. As a logical consequence of natural selection, the EM major is removed by most students from their list of favorites.

People like to think of antennas as a black box of magic. The explanations given by antenna engineers are always so vague that it seems they never give people a definitive answer. It is easy to come to the conclusion that even designing a simple antenna requires years of experience. The truth is that if there was an appropriate book which presented all the required information, most electronic engineers who have studied some EM theory in university could design antennas. You do not need any mathematics to design an antenna. What you need is an understanding of how an antenna works. Of course, if you want to be an exceptional antenna engineer and design antennas with extreme constraints, a solid knowledge of EM theory and years of experience are still necessary.

This book provides a comprehensive discussion of the state-of-the-art technologies of antenna design for mobile communications. The book covers all the important aspects an engineer might need when designing an antenna, which includes how to make a fixture, how to design various antennas, how to optimize match circuits, and carry out different measurements.

It is recommended that the book is read in its entirety. However, for engineers who only want to design a single-band antenna in the shortest time possible, Section 1.6 will provide enough knowledge to kick-start a simple antenna project.

The book has six chapters, and the chapters are arranged as follows:

Chapter 1 provides an overview of most antenna design technologies used in mobile devices. Before anyone starts to design an antenna, it is very helpful for him or her to understand the following: (1) What can be done? (2) What kind of freedom do we have? Both topics will be briefly discussed here. Based on readers' feedback from the book's first edition, a practical example is added in Section 1.6. The section can also serve as a gamebook which can divert readers to different sections if they want to explore more.

Chapter 2 describes different matching techniques used in antenna design. In real-world engineering, antenna matching circuits are widely used, probably in at least half of all devices. The popularity of the matching network is due to two reasons: (1) it gives the engineer more freedom, one more parameter to play with when making design trade-offs; and (2) the value change of a matching component is quite a quick process, which can be a last-minute change. On the other hand, an antenna modification needs at least several days of lead time. The chapter discusses single-band matching, multiband matching, and advanced matching techniques. Complementary software written by the author will be provided to provide practice matching techniques (see the web address on the back cover).

Chapter 3 introduces different external antennas, including both stubby and whip-stubby antennas. The external antenna dominated the cell phone antenna design. The market share of external antennas has been consistently decreasing in the past decade, but it is still a very important antenna configuration. Many basic techniques used in external antennas, such as multimode single-radiator multiband antennas and multi-radiator multiband antennas, are also used in internal antennas.

Chapter 4 introduces different internal antennas. The internal antenna is the current fashion. Under the internal antenna category, there are several different concepts, such as folded monopole, inverted-F antenna/planar inverted-F antenna (IFA/PIFA), loop, and ceramic antenna. All of these will be discussed in the chapter.

Chapter 5 introduces important issues related to engineering antenna measurement. Besides the passive antenna measurement, which is familiar to most electronic engineers, active measurement will also be discussed. Some details, which are key to accurate measurement, such as how to make fixtures and use a choke, will all be covered in the chapter. Various antenna measurements in the production line are also covered in the chapter.

Chapter 6 is about the various regulations which are important to antenna engineers. These can be split into three topics: (1) specific absorption rate (SAR), which is about the radiation to the head and body; (2) hearing aid compatibility (HAC), which is about electromagnetic compatibility (EMC) with hearing aids; and (3) EMC, which is about the EMC with other devices.

1.1 The Evolution of Mobile Antennas

There is some argument about who invented the first mobile communication system, because for some people mobile communication also means vehicle communication. However, when referring to the first commercial handheld cellular phone, the answer is Motorola DynaTAC 8000X [1], without any doubt, which was introduced in 1983, as shown in Figure 1.1.

Figure 1.1 Sleeve dipole antenna on a Motorola DynaTAC 8000X (1983).

(Source: Reproduced with permission of Motorola.)

The antenna installed on a DynaTAC 8000X is a sleeve dipole antenna [2], which now is an obsolete design in the mobile phone industry but still widely adopted by various wireless LAN access points, such as the one shown in Figure 1.2. Sleeve dipole antenna is the best performing antenna ever installed on any cellular phone; however, this is also the largest cellular phone antenna. The length of a sleeve dipole is about half the wavelength at its working frequency. At 850 MHz, the antenna itself needs a length of 176 mm. At the dawn of the personal mobile communication era, those dimensions look quite reasonable when compared to a vintage cellular phone. For instance, the dimensions of a DynaTAC 8000X are 330 mm × 44 mm × 89 mm, without the antenna.

Figure 1.2 Sleeve dipole antennas on a wireless LAN access point. Linksys WAP55AG.

(Source: Cisco, Inc.)

With the significant improvement in cellular technology and the aggressive shrinkage of the size of phones, soon the size of a sleeve dipole was no longer proportional to the phone. Unlike dipole antennas, a monopole antenna [3] on a ground plane has only a length of a quarter of a wavelength, which is 88 mm at 850 MHz. Shown in Figure 1.3 is a Motorola MicroTAC 9800X sitting on a charger. The phone is a flip phone and has a microphone located inside the flip. The thin wire on the top of the phone is a monopole whip antenna.

Figure 1.3 Whip antenna on a Motorola MicroTAC 9800X (1989).

(Source: Reproduced with permission of Motorola.)

A sleeve dipole, such as the one shown in Figure 1.1, has an integrated choke which retains most radiation current within the antenna; thus, the antenna is insulated from the phone and also from a user's hand on the phone. However, a monopole antenna must use the metal inside a phone as part of the antenna's radiating structure. Some portion of radiating current must flow over the phone. Putting one's hand on the phone absorbs some energy, and thus decreases the overall antenna performance. Although the performance of a whip monopole antenna is inferior to a sleeve dipole, it is still better than all other members of the family of cellular phone antennas. The whip antenna is the second largest one in the family.

In fact, the antenna used on the MicroTAC 9800X is a retractable antenna. A retractable antenna is a combination of a whip antenna and a helix stubby antenna. When the antenna is extended, it functions as a whip monopole and provides good performance. When the antenna is retracted, it functions as a stubby antenna and still has acceptable performance. The retractable antenna has the best of both worlds, as it is a low-profile solution and is still capable of providing good performance when needed.

Obviously, the mechanical structure of a retractable antenna is quite complex, as it involves moving parts and multiple radiators. A stubby antenna, as shown in Figure 1.4, eliminates the whip in a retractable antenna. From the performance point of view, a stubby antenna is not as good as a retractable one. However, stubby antennas dominated the cellular phone market at the end of the past century. The reason for the wide adoption of stubby antennas is the significant improvement in cellular networks. As the number of mobile phone users exploded, the density of base stations also increased dramatically. That means the distance from any user to the nearest base station is much shorter...