Chapter 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 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 (EM), which is a quite abstract class and involves a lot of mathematics. The world unveiled by electromagnetics 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 and it seems that 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 were an appropriate book which presented all the required information, most electronic engineers who have studied some electromagnetic 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.

This book has six chapters and they 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: (1) what can be done?; and (2) what kind of freedom do we have? Both topics will be briefly discussed here.

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. This chapter discusses single band matching, multi-band 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 used to dominate the cell phone antenna design. The market share of external antenna has been consistently decreasing in the past decade, but it is still a very important antenna configuration. With the adoption of MIMO (multiple-input and multiple-output) technology in handsets, the external antenna might see its renaissance in the near future. Many basic techniques used in external antennas, such as multi-mode-single-radiator, multi-band antennas, and multi-radiator multi-band 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, IFA/PIFA, loop, and ceramic antenna. All of these will be discussed in this 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 this chapter. Various antenna measurements in the production line are also covered in this 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 with hearing aids; and (3) Electromagnetic Compatibility (EMC), which is about the electromagnetic compatibility with other devices.

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, Sections 4.1 and 5.1 and will provide enough knowledge to kick-start a simple antenna project.

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.

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 dipoles are 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.

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.

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 last 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 than previously. As the path loss between a cellular phone and a base station tower is directly proportional to the distance between them, a shorter distance means less strain on the antenna's performance. Inside a stubby antenna, the metal radiator can be a helix made of metal wire, a meander-line made of flexible printed circuit board, or a sheet metal stamping part.

The next antenna to enter the market was the internal antenna. The phone shown in Figure 1.5 is not the first phone to adopt an internal antenna, however, it is one of the most successful phones with an internal antenna. Nokia sold approximately 160 million Nokia 3210 during the phone's whole life span. When tested in free space or next to a phantom head, an internal antenna can achieve a similar performance to a stubby antenna. In everyday use, internal antennas are more vulnerable to hand blockage by the user. It is quite a natural gesture for a user to put his or her fingers on top of the antenna and bring the speaker closer to his or her ear.

From the mechanical point of view, the internal antenna is better than the external antenna, as it eliminates the through hole and mating features necessary to accommodate an external antenna. A phone with an internal antenna normally has better performance in drop tests, wearing tests, and various other mechanical tests. Because an internal antenna is totally concealed in the phone, the phone user has little chance to abuse it. Some people have a habit of playing with the item in their hand when they are sitting in meetings or are idle in front of their desks. I have seen some colleagues unconsciously extend and retract their antenna's whip hundred of times in a single meeting.

All traditional internal antennas are located on the upper part of a phone. In a normal talking position, the distance between the top internal antenna and the user's head is quite small. To eliminate the influence of a user's head on the antenna's performance and also decrease the harmful radiation emitted toward the head, a ground layer must be placed beneath the antenna to increase isolation between the user's head and the antenna. However, the ground layer decreases an antenna's bandwidth. To compensate, the antenna size must be increased. The Motorola Razor V3 was the first phone to adopt a bottom internal antenna. It was a brave act. According to the conventional wisdom of that time, an antenna in the bottom would be held in the center of a user's palm; a bottom antenna might have good performance in the lab but could not provide acceptable performance in real use. That conventional wisdom was proved wrong by the Motorola V3. The Motorola V3 has become another legend in cellular phone history. It sold more than 110 million. By relocating the antenna to the bottom, the antenna is away from the head. The ground layer, which is required by top internal antennas, can be eliminated. Furthermore, the antenna's thickness and volume can both be significantly decreased, as shown in Figure 1.6. The Motorola V3 was the slimmest phone when it was released. Since then, many slim phones have adopted bottom internal antennas, and most big players in the cellular phone market have their own versions of bottom antenna phones. The new wisdom is that whenever you need to design a slim phone, it is better to put the antenna on the bottom.

Thousands of models of cell phones have hit the streets since 1983. It is almost impossible to list them all. To get more comprehensive information, the Internet is a good resource. Some posts [4] show chronicles of cellular phones. Some web sites [5] are dedicated to the phones' news. For more detailed information about certain phones, which are sold in the United States, go to the FCC web site [6].

1.2 How to Quantitatively Evaluate an Antenna

After designing an antenna, we cannot say whether it is good or bad by simply looking at it. We must find a way to quantitatively evaluate it. In cellular antenna's designs, the frequently used parameters are the reflection coefficient, the voltage standing wave ratio (VSWR), efficiency, gain, and bandwidth. The contents of this section are only a brief review of frequently used parameters. More comprehensive materials and detailed deductions can be found in some classical textbooks [3, 7, 8, 9, 10].

From the circuit point of view, an antenna is a single port device. A transmission line can be used to feed the antenna, as shown in Figure 1.7. An input signal takes the form of an incident wave traveling along the transmission line. It flows from the signal source toward the antenna. Assuming the amplitude of the incident wave is V_incident. At the antenna port, some of the energy carried by the incident wave is radiated by the antenna. In the meantime, the residual energy is reflected at the port and travels back along the transmission line. The amplitude of the reflected wave is V_reflected.

The reflection coefficient is given by:

(1.1)

Clearly, all the reflected energy will be wasted. When designing an antenna, our goal is to minimize the reflection at the antenna port. A perfectly matched antenna can radiate all energy, thus its reflection coefficient is 0. When a device reflects all the energy back, its reflection coefficient is 1.

In microwave theory, the S-parameter matrix is used to quantitatively describe a multi-port network. The S stands for scattering. A one-port network is a special type of multi-port networks, its S-matrix degenerates to a single element, . For an antenna, the definition of is identical to the reflection coefficient.

(1.2)

In engineering, the is often used in the decibel (dB) scale.

(1.3)

The is defined by the ratio of the voltages of incident and reflected wave; while the is defined by the incident and reflected power. That is the reason why the coefficient in Equation (1.3) is 20. As the of any antenna is a value less than 1, the is always a negative value. The absolute value of is called the return loss (RL):

(1.4)

Although the definitions of , , , and the return loss are somehow different, they are all deduced from the incident wave and the reflected wave. The other commonly used parameter, voltage standing wave ratio (VSWR), is directly defined by the standing wave formed by the superposition of the incident and reflected waves.

(1.5)

The VSWR is the ratio of the amplitude of a partial standing wave at an antinode (maximum voltage) to the amplitude at an adjacent node (minimum voltage), in an electrical transmission line. Although the VSWR's physical meaning might seem less straightforward than , the VSWR is the only parameter that could be easily measured when the microwave and antenna technology was still in its infancy. Today, the VSWR is still widely used, especially in the antenna business. The correct format of VSWR is X : 1, such as 2 : 1, 3 : 1, and so on. A VSWR 2 : 1 means the maximum voltage is twice as much as the minimum voltage.

As the and are formed when the incident and reflected waves are constructively and destructively superimposed respectively, Equation (1.5) can be rewritten as:

(1.6)

The relation between VSWR and , or return loss, is a one-to-one correspondence. The return loss of 10dB is a commonly used specification for antennas. The corresponding VSWR is approximately 2 : 1.

Bandwidth is another important parameter used to describe antennas. Whenever we give an antenna's bandwidth, we must give the criteria that define the bandwidth. As shown in Figure 1.8, the antenna has a −10 dB bandwidth of 70 MHz. However, you can also claim that the antenna's bandwidth is 132 MHz, if one uses −6 dB as the criteria. Different companies might use different criteria to measure their antennas, it is our responsibility to pay a little more attention to the details.

A well-matched antenna does not necessarily mean it is a good antenna. Efficiency is the parameter which tells us how well an antenna can radiate. The efficiency is given by:

(1.7)

Where the is all the power radiated, the is the total available power from the signal source. Efficiency is a value between 0 and 1. In the antenna business, the efficiency in dB is also commonly used.

(1.8)

A dB-efficiency of −3dB means 0.5 or 50% efficiency in the linear scale, which is still pretty good value for real antennas.

In the cellular antenna's world, the gain is not an important parameter, because it is mostly decided by the position in which an antenna is installed and the size of the grounding structure. The antenna element itself does not have too much to do with deciding the gain. The commonly used units for gain measurements are dBi, dBd, and dBic. These are normalized to isotropic linear polarized antenna, dipole antenna, and isotropic circular polarized antenna respectively. More information about gain can be found in Chapter 5.

1.3 The Limits of Antenna Designs

As antenna engineers, we are under consistent pressure to shrink the size of the antennas and still provide better performance. There is an elegant art to communicating with team members and managers from other disciplines when explaining that a limit in antenna design does exist. For each kind of antenna there is a boundary, which regulates an antenna's size and its performance. As a new engineer, the easiest way to get a feeling for that boundary is by measuring various phones designed by different companies. Also a much quicker way to learn new design techniques is by reverse engineering using existing antennas on the market.

In 1948, L. J. Chu published a paper [11] which quantified the relationship between the lower boundary for the radiation quality Q of an electrically small antenna and its physical size relative to the wavelength. This lower boundary is now known as the “Chu” limit. Shown in Figure 1.9 is a schematic diagram of a vertically polarized omni-directional antenna. The sphere with radius r is the minimum one which can enclose the antenna. The lower boundary for the radiation Q is decided by . The boundary given by Chu is based on a simplified model and is considered as the strictest one. Several boundaries based on more realistic scenarios [12–16] have been proposed since then. However, Chu's limit is still the one that is most referred to.

Bandwidth can be derived from Q by assuming that the antenna is a resonant circuit with fixed values. The normalized bandwidth between the half-power frequencies is [12]:

(1.9)

Equation (1.9) is a good approximation when Q 1. Otherwise, the representation is no longer accurate. Shown in Figure 1.10 is a figure presented in [12]. The x axis is . The y axis represents the quality. Different curves are single mode Q for various antenna efficiencies.

With a fixed efficiency value, say, 100%, when the sphere's radius increases, the radiation Q decreases, which also means the maximum achievable bandwidth increases. Of course, the bandwidth predicted by the curve can never be achieved in an actual implementation. Various studies [17–20] have been done to approach the limit.

Another thing that can be observed in Figure 1.10 is that a lossy antenna, which has lower efficiency, always has a wider bandwidth. In the real world when the bandwidth of an antenna is abnormally wide, this is not good news, because most of the time it is due to unwanted loss.

As antenna engineers, we do not really evaluate the achievable bandwidth based on figures and formulas given in references. It is very difficult to define the minimum sphere to enclose the antenna in a cellular phone. It will be demonstrated later that all metal structures, including the ground, in a phone can give off radiation. If we define a sphere that encloses the whole phone, the bandwidth calculated by the Chu limit can be so wide that it is meaningless. From time to time, there are claims that the Chu limit has been surpassed. In most cases, the sphere used in calculations only encloses the antenna element itself. As the ground is also part of the radiator, by excluding the ground from the sphere, the achievable bandwidth is artificially narrowed, and that is why those antennas have wider bandwidth than the theoretical limit.

When designing a cellular antenna, many factors, such as the nearby battery, the speaker under the antenna, the metal bezel on the phone, and so on, all play a role on determining the achievable bandwidth. With the accumulation of experience, eventually one can estimate the achievable bandwidth more accurately.

1.4 The Trade-Offs in Antenna Designs

To be a good antenna engineer not only means designing an antenna with the best performance, it also means having a profound understanding of the possible trade-offs in antenna designs. Among them, some trade-offs are the same ones which are applicable in all engineering disciplines, such as the trade-off between the design time and performance. Designing an antenna is a project with a time constraint instead of an open-ended art creation. The thought of designing a perfect antenna might do more harm than good. As well as those common-sense trade-offs, there are some that are particular to antennas:

• Bandwidth trade-off. In the last section, the bandwidth limit of a single band antenna was discussed. Most phone antennas used today are multi-band antennas. A similar limit also applies to their combined bandwidth. If an antenna is well designed, whenever the bandwidth of one band increases, the bandwidth of the other bands must shrink. To fully understand the design technique of one kind of antenna, we need to find out how to trade-off bandwidth between different bands. For example, if the specification for an antenna is 50% efficiency across all bands, and the efficiency of the antenna designed is 60% at the lower band and 40% at the high band, your work hasn't finished yet. The unbalanced performance tells everybody that you have not really mastered the design skills of this antenna.
• Trade-off between complexity and performance. By introducing more freedom into an antenna's design, it is possible to achieve better performance. However, the marginal improvement of each incremental variable is regressive. As a new antenna engineer, try to avoid using complex designs in the beginning. It is quite easy to be drowned by a large amount of variables. One should start from simple designs and assess the impact of each design variable. For many applications, an antenna with a handful of variables is good enough.
• Trade-off between manufacture consistency, tooling time and cost. Better manufacturing consistency means less antenna variation and better antenna performance. However, better consistency also means longer tooling time and higher cost. There is no manufacturing solution that can provide all the benefits, otherwise it would already have been part of the antenna's manufacturing process. Many different manufacturing processes are available; one should understand the advantages and the disadvantages of each of them. Taking the processing of internal antenna as an example, there are metal-stamping, flex circuit, Double-Shot Molded Interconnect Device (DS-MID), Laser Direct Structuring (LDS), and so on. The metal-stamping technology is the cheapest and can be adjusted quite quickly if the parameter that needs to be adjusted is known and already included in the tooling design. The flex circuit technologies has better consistency than the metal-stamping, however, it is a little more expensive and it takes a longer time to implement a design change. Both DS-MID and LDS technology have the best consistency, because antennas are part of the plastic structure instead of separate parts. Both of them are based on a technique called selective metallization. The DS-MID process begins with the application of a shot of plateable thermoplastic resin in an injection-mold cavity. Next, the cavity is changed and a second shot of nonplateable thermoplastic resin is molded around the first shot to create a circuit pattern from the plateable material. Depending on the antenna shape, the two resins can be reversed in shot order. After two shots, a part has its intended geometry with select plateable surfaces exposed. These surfaces are then plated with a layer of copper. The DS-MID takes the longest lead time, because any modification to the antenna pattern requires tooling changes. The LDS is a relatively new process. The thermoplastic resin used in LDS process is nonplateable after the molding process and can be transformed to plateable by using a laser beam to active it. The LDS process literally draws the antenna pattern onto the plastic. The pattern can be adjusted quite easily by uploading a new pattern file to the laser. Similar to the DS-MID, a plating process is required to deposit copper onto the part's surface.
• Trade-off between total radiated power and radiation exposure. Higher total radiated power and lower radiation to the human body are a pair of contradictory requirements. Ideally, we should first design an antenna to have an on-phantom efficiency as high as possible, then choose an appropriate conductive power level to meet the human exposure specification. In reality, there are three constraints: the conductive power, the total radiated power, and the human exposure. An antenna with very good efficiency might give you trouble later on. In the whole design process, one should always keep these three specifications in mind and check their status constantly.

1.5 Mobile Communication and Band Allocations

The radio frequency (RF) electromagnetic spectrum is an aspect of the physical world which, like land, water, and air, is subject to usage limitations. Use of radio frequency bands in the electromagnetic spectrum is regulated by governments in most countries, in a spectrum management process known as frequency allocation or spectrum allocation [21]. Although countries are working on a universal frequency allocation plan, the existing frequency allocations are still country dependent. In the United States, the spectrum from 0 Hz to 1000 GHz was allocated by the Federal Communications Commission (FCC) [22]. The US Department of Commerce has a color chart of frequency allocation, which covers 3 kHz to 300 GHz [23]. In most countries, the spectrum allocation plan is not a static one and is being continuously revised.

Most bands used in the design of mobile phones are given in Figure 1.11. However, this is not a complete list.

There are some ambiguities when referring to bands and their respective technology. Only the band names are given in Figure 1.11 Depending on the different countries, the exact frequency range of each band might vary slightly:

• CDMA band: also known as AMPS band or 850 MHz band, 824∼894 MHz;
• GSM band: also known as 900 MHz band, 880∼960 MHz;
• GPS band: 1575 MHz;
• DCS band: also known as 1800 MHz band, 1710∼1880 MHz;
• PCS band: also known as 1900 MHz band, 1850∼1990 MHz;
• UMTS band: also known as 3G band or 2100 MHz band, 1920∼2170 MHz;
• WLAN band: also known as Bluetooth band or 2.4 GHz band, 2400∼2480 MHz.

Strictly speaking, using those abbreviations to name bands is not appropriate. Some of them are based on specific technologies. The following are brief introductions; more comprehensive information can be found on their respective websites and in other books [24].

• AMPS is the abbreviation of Advanced Mobile Phone Service. It is an analog standard used by the first cellular communication network. Motorola DynaTAC 8000X is based on AMPS technology. It is obsolete in most countries. The United States was one of the last to shut down AMPS services. The final date of use was February 18, 2008.
• CDMA is the abbreviation of Code Division Multiple Access [25]. In the cellular business, this means the IS-95 standard or the cdmaOne standard. The technology itself is band independent. In the United States, CDMA systems are deployed in both 850 MHz and 1900 MMHz bands.
• GSM is the abbreviation of Global System for Mobile communication [26]. Both GSM and CDMA are second generation (2G) cellular communication standards. In the global market, GSM is the most influential standard. About 80% of the global mobile market used this standard in 2009 [26]. The GSM technology itself is also band independent. GSM systems are deployed in different bands, such as 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz bands.
• GPS is the abbreviation of Global Positioning System [27]. GPS is a receiver-only technology. It can extract positioning and timing information from signals transmitted by GPS satellites. It is not a mandatory feature for a phone. However, it gradually has become a standard functionality for most middle to high tier phones.
• DCS is the abbreviation of Digital Cellular Service. It is the name of the 1800 MHz band.
• PCS is the abbreviation of Personal Communications Service. It is the name of the 1900 MHz band.
• UMTS is the abbreviation of Universal Mobile Telecommunications System [28]. UMTS is one of the third-generation (3G) mobile telecommunications technologies. The most common form of UMTS is W-CDMA. The Chinese version 3G system, TD-SCDMA, also belongs to the UMTS family. The main competitor of UMTS is CDMA2000, which is another 3G standard. As most countries allocate the 2100 MHz band to 3G systems, UMTS is used as the 2100 MHz band's alternative name. In fact, UMTS has been deployed in different bands, such as 850 MHz and 900 MHz bands.
• WLAN is the abbreviation of wireless local area network. WLAN actually involves several standards, such as 802.11b, 802.11g, 802.11a, and so on. WLAN is also known as Wi-Fi [29], which is the abbreviation of Wireless Fidelity. From the antenna point of view, the 802.11b/g uses the 2.4 GHz band and the 802.11a uses the 5GHz band, which is omitted in Figure 1.11.
• Bluetooth [30] is a different standard from WLAN. It is an open wireless protocol for exchanging data over short distances. However, Bluetooth shares the same 2.4 GHz frequency band with WLAN.

Besides the above-mentioned bands, some other technologies, such as FM radio, analog TV, and digital TV, are also used in cellular phones. Their band allocations are omitted in Figure 1.11. Another point worth mentioning is that most technologies and band allocations used in Japan are different from the rest of the world. As the Japanese market is the most difficult one to penetrate, related information to that market is also omitted.

In this chapter, only the basic terminologies are introduced. For more in-depth information about different cellular communication technologies, Wikipedia [31] is a good place to start.

References

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