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Source: ANTENNA ENGINEERING HANDBOOK
36-2 CHAPTER THIRTY-SIX
36.1 IMPACT ON ANTENNA DESIGN
Antenna engineering design has many interesting and surprising features, not least of which is its ability to reinvent itself if the demand arises. Such has been the impact of mobile com-munications on antenna design, resulting in unbelievably compact and densely packaged dipole-like handset antennas housed entirely within the mobile phone case itself.
The dipole and monopole were some of the first antennas developed and, together with fundamental electromagnetic theory, have been well established over the past century. Yet new versions of these generic radiators continue to be analyzed, simulated, developed, and even researched today as is evident from the research journals worldwide. Quite simply, prod-uct demand is the driving force for this seemingly endless creation of different physical real-izations of what is essentially an antenna capable of giving a dipole-like radiation pattern.
While the vectorial radiation field of an antenna is uniquely defined by the antenna itself, this is not so for typical engineering radiation patterns specified as the modulus of the radiation field whereby phase information is not retained. Furthermore, this specification is often made even less restrictive by only stating limiting values of sidelobes and other pattern features, between which the measured pattern must lie. Bandwidth specifications are similarly pinned down with only a simple limiting level on the S11 response, hence little information is demanded about the shape of the bandpass characteristic itself. The nonuniqueness of such specifications thus leaves plenty of scope to recast the antenna in a very different form.
The dipole antenna family, and in fact most other types of antennas, can be reinvented in a different physical form to satisfy the demands of new equipment products. An outstanding example is the microstrip patch antenna, made possible with printed antenna technology. When compared with conventional monopoles and loop antennas, the printed version can be expected to have a somewhat lower radiation efficiency but it has opened the door to many innovative new products requiring a very low profile dipole-like radiator that is compatible with cost-cutting manufacturing processes.
The previous example not only illustrates how new and very different physical realiza-tions can be developed with similar radiation pattern characteristics but also highlights the need for a system’s design approach whereby the performance of individual components is not the main criterion but rather the satisfactory performance of the entire system. The deployment of a system approach has gradually become more evident in mobile handset antenna design over some two decades.
The initial thinking was to mount a conventional antenna on the mobile phone case itself, and early examples were physically large and included balanced fed dipoles, mono-poles, and loops. The astronomical demand for mobile phones together with the pull of user preference has resulted in the present-day integral handset antenna. Seldom does one ask of the performance of the antennas in isolation where it is understood that the matching and radiation efficiency data may be less impressive than that of conventional antennas. What matters is how it all functions together as a system.
36.2 CELLULAR HANDSET ANTENNA DESIGN ISSUES
Without doubt, integral handset antenna design is very complicated and presents the designer with perhaps the most formidable antenna design criteria demanded by a product. There are numerous design issues for cellular operation that are summarized next, but the integration as a system further complicates matters, and many, if not most, of the design
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MOBILE HANDSET ANTENNAS 36-3
issues are interrelated, thus there is a high degree of iteration in the design work. The requirements for a global roaming satcom handset antenna have different constraints and are considered separately in Section 36.9.
Antenna Electrical Size
What constitutes an electrically small antenna is well established and seminal contribu-tions include Wheeler,1 Chu,2 and Collin and Rothschild.3 The radian sphere4 of radius a is defined as
a = λπ2
(36-1)
where l is the free-space wavelength. If an antenna’s physical structure lies within this sphere, then electrically small antenna characteristics will be evident. The effect of shape and volume of the antenna structure has also been considered by Wheeler.1
For a lossless antenna, the effects are (i) a high Q factor and hence small bandwidth, (ii) a small radiation resistance resulting in a low radiation efficiency, and (iii) likely greater antenna sensitivity to mechanical and electrical tolerances. When the antenna has intrinsic losses, the bandwidth is less narrow.
It is widely acknowledged5 that for diminishing a
Q ∼ 1 / (ka)3 (36-2)
where k = 2p /l. From an antenna engineering standpoint, the interest6,7 over many years has centered on realizing a small antenna in practice that lies on or exceeds the perfor-mance limits represented by the Q formula. This topic has been vigorously revisited recently7,8 with the upsurge of activity in small antenna design for both mobile and wire-less applications.
For handset antennas, it is the lower bands that enforce antenna electrical size con-straints. For instance, at 800 MHz, a = 5.97 cm and the dimensions of a typical handset are 1 × 4 × 10 cm, so if the antenna has dimensions of 1 × 2 × 4 cm, it lies within the radiansphere. At 1800 MHz, a = 2.64 cm, so the situation is less critical. An unknown factor is, of course, the extent to which a given handset antenna couples to the handset circuit components and battery, etc., because this will in effect increase the electrical size of the antenna.
Ground Plane Electrical Size Effects
When a monopole is mounted on a horizontal ground plane (GP) of finite extent, the dipole-like radiation patterns can be corrupted with a variety of pattern effects depending on the electrical size of the GP. An excellent theoretical illustration with measurements has been given9 showing the increase in pattern distortion as a function of reduced GP electrical size. The mobile handset GP is enforced mainly by the metallized assembly consisting of the circuit board and its components, which as mentioned above is typically 4 × 10 cm. Any plastic outer case will have little effect.
Neglecting hand effects, such a GP could act as an electromagnetic counterpoise to a 10-cm monopole mounted in the same plane as the GP at 750 MHz. Operation at a lower frequency is possible using a monopole version of the sleeve dipole10 to isolate the handset
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Mobile Handset Antennas
36-4 CHAPTER THIRTY-SIX
case, but the resulting antenna has greater height and is not acceptable today where the antenna element is usually mounted on the GP itself. The GP extent in one direction is only 4 cm and some degree of radiation pattern perturbation is likely to be experienced at the lower mobile bands. Consequently, it is common practice in research papers and manufac-turers’ data sheets to show measured and/or simulated handset antenna radiation patterns with the antenna embedded on the GP.
There continues to be much interest in creating a stand-alone chip type antenna11 that is immune to the proximity of the GP and its components. Another way of addressing the latter requirements concerns a balanced feed arrangement,12 but since this concept requires two identical antennas in phase opposition, its application13 so far has been confined to the higher mobile bands due to size limitations.
Time Varying Pattern Effects
Cellular handset antennas have to function in a rich multipath propagation environment which allows the use of antennas with polarization properties that are not tightly specified. Hand and head movement, together with the angular movement of the handset, creates further time varying effects which, together with multipath effects, are mainly beyond the designer’s control. At the lower mobile bands below 1 GHz, the antenna’s constrained elec-trical size commits it to smoother dipole-like patterns with few nulls and thus time varying effects can be better tolerated than at progressively higher frequencies where additional pattern nulls appear. The situation is further assisted by the self-correcting adaptivity of speech communication.
Increasing Band Coverage
It is generally thought that phone users have an appetite for mobile phones having addi-tional functions that can obtain wider news and sales information and provide more games and a host of other facilities like GPS and wireless links. Financial returns from such a multimedia operation are likely to be very significant but at the same time the cost of the mobile handset must be constrained.
Not surprisingly, handset manufacturers aim to achieve several bands from one central antenna not only to maintain cost levels but also because there is little available space to do otherwise. A multifunction antenna is now commonly the stated requirement, and antenna designers have responded with a variety of configurations that demonstrate design concepts and feasibility. Just how far this concept can be extended remains to be seen, but antennas with three bands have been demonstrated.14 It is evident that the multiband operation may require some trading of other performance parameters such as radiation efficiency and antenna input match levels. This will limit the number of bands that can eventually be obtained from a single antenna.
Radiation Efficiency
Whatever type of antenna is chosen to embed in the handset, the designer will strive to minimize the power lost to dissipation in the antenna itself. A measure of the losses is the antenna radiation efficiency ha defined by
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MOBILE HANDSET ANTENNAS 36-5
where the radiated power is Prad and the power dissipated in the antenna is Ploss. In practice the antenna is likely to need a matching network, and losses in the latter are expressed as a matching network efficiency hm where
hm = ha Qm / (Qm + Qa ) (36-4)
where Qm = matching network Q factor and Qa = antenna Q factor. The combined effi-ciency of the antenna and its matching network is then ha hm.15 Further losses are of course incurred when the antenna is embedded in the handset, due to coupling to handset compo-nents, and finally when operating with the hand and head present, which account for much of the additional power loss.
36.3 HELICAL WIRE ANTENNAS AND VARIANTS
The remarkable property of wire antennas, or indeed any form of linear metal conductors, is that the resonant frequency is mainly determined by the electrical length of the current path. As such, a wire monopole (or dipole) can be compacted into an electrically smaller size. The antenna bandwidth, however, is more affected and increases for compacted structures occupying a greater volume.
A common example where the wire monopole is partially wound into a coil at some location on the wire16 is shown in Figure 36-1a. For handset applications, a spiral coil of wire alone constitutes the antenna (see Figure 36-1b) and for small diameters is known as the normal mode helical antenna (NMHA). The device gives dipole radiation patterns when placed on an electrically large GP but has circularly polarized radiation characteristics when the spiral diameter exceeds about one wavelength. The bandwidth, radiation efficiency, and radiation resistance at resonance are significantly reduced but the latter can be restored by tapping into the spiral, as shown in Figure 36-1c, at the expense of a more complicated construction. There are a multitude of variations, and in Figure 36-1d a second spiral coil is wound in antiphase directly on top of the first spiral coil, creating a large bandwidth but very poor radiation efficiency. This is referred to as the double wound antenna.17 Figures 36-1e and f show zigzag-like versions and other designs that can be configured in fractal form or generated by genetic algorithms. Some performance details for spiral antenna devices are compared to those of a wire monopole in Table 36-1.
A compact manufactured NMHA is illustrated in Figure 36-2 where the conducting path is printed into a groove in the cylindrical plastic former to avoid the increased production cost of winding on a wire.
Antenna Type
Resonance (MHz)
Bandwidth (MHz)
Loss (dB)
Efficiency (%)
Normalized Size
Resistance (Ω)
Monopole 150 ∼20 ∼0 ∼100 1 25
NMHA 142 ∼1.5 1.04 81 0.22 5
Double wound 150 ∼9 7.8 16 0.18 21
TABLE 36-1 Performance of Spiral-type Antennas Compared with Monopole Antenna
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MOBILE HANDSET ANTENNAS 36-7
The zigzag antenna configurations have an outstanding manufacturing advantage over the spiral devices because they can be printed on thin dielectric films and then rolled into a cylindrical or other three-dimensional shape. Such assemblies have less size reduction than the spiral types but have attracted use both mounted on a handset case, as shown in Figure 36-3, and embedded within the handset interior as a planar structure. Two printed films designed for different resonant frequencies can also be connected in parallel to obtain a dual-band antenna, and this is more difficult to achieve with spiral conductors.
36.4 EVOLUTION OF THE PIFA AND ITS VARIANTS
The planar inverted F antenna (PIFA) has come into prominence due to its common use in mobile phone handsets. In fact, the PIFA is often regarded as a generic antenna in its own right, but from physical fundamentals it is seen to be a derivative of both wire antennas and printed patch antennas. For instance, a low profile inverted L antenna (ILA) has a low radiation resistance, but by tapping the feed connection along the wire, it can be increased, which is the inverted F antenna (IFA), as shown in Figure 36-4a. This antenna is useful for low profile applications where only a thin wire can be used. Where space is available, the L-shaped wire can be replaced by a planar conductor of width L1, as shown in Figure 36-4b. The planar conductor increases the radiation resistance and offers a two-dimensional choice of feed position to facilitate matching. The end shorting plate of height H need not occupy the entire plate width, as shown in Figure 36-4b.
FIGURE 36-3 (a) Printed zigzag antennas printed on dielec-tric films11 (Courtesy of Allgon); (b) The structure rolled into a cylindrical shape mounted on a plastic former with and without the cover in place
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36-8 CHAPTER THIRTY-SIX
Examination of Figure 36-4 also reveals that the PIFA can be regarded as a quarter-wave patch antenna18 with an air-spaced substrate. The PIFA has both vertical and horizontal radia-tion surfaces, and the radiation patterns can be expected to contain significant cross-polarization, which is not a problem for cellular mobile phones since they have to operate in a high multipath environment. Being purely a simple bent metal antenna with no lossy substrate, the PIFA has good radiation efficiency. However, it is sensitive to its environment, and when embedded in a handset with the presence of the hand and head, the radiation efficiency reduces drastically.19 The radiation patterns are also significantly perturbed by the hand and head movement and appreciable power is dissipated in the head. However, the simplicity of the PIFA and the low manufacturing cost remain out-standing advantages that are attractive to an industry where cost-cutting is a paramount consideration.
The demand for multiband operation at no additional antenna cost has inspired the creation of the multiband PIFA; a dual-band example is shown in Figure 36-5. The sim-plicity of this antenna is remarkable and there would appear to be an unlimited number of ways of configuring the current paths on the top plate of the PIFA or indeed to other metal-plate antennas. This concept has been extended to the tri-band version shown in Figure 36-6.
FIGURE 36-4 (a) Inverted F wire antenna and (b) typical PIFA
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MOBILE HANDSET ANTENNAS 36-9
As already mentioned, radiation patterns having a high cross-polarization content can be expected, and examples of radiation patterns for the tri-band antenna of Figure 36-6 are shown in Figure 36-7.
Similar performance can be obtained with the meander-line concept, which allows a printed zigzag conductor to be printed on a thin planar dielectric film prior to folding into the rectangular 6 × 6.5 × 25 mm3 box shape illustrated in Figure 36-8. This tri-band antenna is very compact and covers the GSM (900 MHz), DCS (1800 MHz), and PCS (1900 MHz) bands.
The large degree of design freedom offered by these simple metal antenna struc-tures has enabled manufacturers to generate their own in-house low-cost designs. Further compacting of these antennas can be achieved by loading the low-frequency resonant zones of the structure with a high-permittivity dielectric slab, albeit at the expense of a somewhat lower radiation efficiency and bandwidth for the low band and the extra cost of the ceramic material.
(a) (b)
FIGURE 36-7 (a) Radiation patterns of the tri-band antenna at 900 MHz (after G.-Y. Lee and K.-L. Wong21
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MOBILE HANDSET ANTENNAS 36-11
36.5 CERAMIC CHIP AND RESONATOR ANTENNAS
A major manufacturing problem with PIFAs and other air-spaced antennas embedded in the handset is that they are not stand-alone electronic components. These antennas couple to the handset components, and a previously proven antenna cannot be simply embedded into a new handset configuration without further design adjustments. The ceramic chip antenna concept goes some way toward alleviating this coupling problem and also reduces the antenna size at the expense of additional material costs. The fields are intensified within the ceramic material, and unwanted characteristics are the narrowing of the bandwidth together with some lowering of the radiation efficiency even though the intrinsic material loss (tande ) is low. Numerous chip antenna designs have been described whereby a high-permittivity ceramic material is associated with a meander-line23 or helical configurations.24
The conducting lines can be readily printed onto the outer surface of the ceramic material. An example of a helical construction is illustrated in Figure 36-9a and its deployment as an antenna embedded in a handset is shown in Figure 36-9b together with radiation patterns.
The typical ceramic chip antenna is likely to be too small to permit resonant modes within its interior. In contrast, the dielectric resonator antenna (DRA)25 relies on a mode being excited, which can be achieved by a small probe or coupled slot. Although the concept of a DRA was introduced by Richtmyer26 in 1939, the practical versions were not investigated until the 1980s, no doubt due to the new availability of high-permittivity, low-loss, temperature-stable ceramic materials at that time. The feasibility of embedding a DRA within a handset has been investigated27 but single mode operation has the characteristic narrow bandwidth which
(a)
(b)FIGURE 36-9 (a) Constructional view of helical ceramic chip antenna11 (Courtesy of Murata Manufacturing Co.) and (b) the chip antenna embedded in a handset and the radiation patterns
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36-12 CHAPTER THIRTY-SIX
limits its application. In another development28 a quadrifilar-type antenna structure is plated onto a high-permittivity dielectric cyl-inder. A balun structure at the base isolates the antenna from the GP, and there is a radiation null at broadside, thus reducing radia-tion exposure to the head. The bandwidth as anticipated is very narrow. The antenna is illustrated in Figure 36-10.
The demand for multiband handset antennas has prompted designers to develop DRAs that have more than one mode excited, thus providing bandwidth windows that correspond to the required cellular bands. This is not difficult if the resonator is not restricted in size, but for handset installation the DRA thick-ness must be < 10 mm, while the handset width and limited space allocation place severe constraints on the other dimensions. Such a multimode DRA has been described29 and the mode generation is governed by the excitation mechanism, which is commonly a coupling strip plated on the dielectric surface. Additional con-ductors can be plated on the dielectric surfaces to provide addi-tional bands, and the nature and location of the conductors are mainly determined by experiment and/or simulation.
The electrical compactness of DRAs, at least for the higher mobile bands, allows provision of balanced pairs12 to reduce currents in the handset GP and hence reduce hand and other envi-ronmental effects. An example is given in Figure 36-11, which
FIGURE 36-11 (a) Balanced ceramic antenna on GP30 (Courtesy of Antenova Ltd.); (b) The 900-MHz bandwidth characteristic; (c) The 1800- and 1900-MHz bandwidth characteristics
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MOBILE HANDSET ANTENNAS 36-13
covers the 900-, 1800-, and 1900-MHz bands.30 The 900-MHz band remains unbalanced while the two higher bands are balanced. It is also claimed that there is less radiation expo-sure to the head.
The use of ferrite material to reduce the electrical size of wire monopoles and other antennas has received some attention31 over the past 60 or so years but has remained a specialized technique for a few applications. More recently32 its application to DRAs has emphasized the enhancements in antenna performance that can be achieved, these being a significant increase in bandwidth and the radiation efficiency. Best performance is obtained when there is approximate equality between the real parts of the material’s complex permittivity and permeability. Theory and simulations33 have established the performance benefits of applying this technique to handset antennas. An example34 is given in Figure 36-12a, which shows a ferrite-loaded monopole antenna covering the four bands 1800 MHz (GSM), 1900 MHz (GSM), 2100 MHz (UMTS), and 2450 MHz (Bluetooth). The conducting rings facilitate the adjustment of the band shape. The quad-band bandpass characteristics are shown in Figure 36-12b together with a previous tri-band characteristic which did not cover the UMTS band.
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36-14 CHAPTER THIRTY-SIX
A summary of the simulated performances for various material properties is given in Table 36-2.
The complex permeability and permittivity of the material are defined as
mr = m ′ + jm ″ ; tandm = m ″/m ′ (36-5)
er = e ′ + je ″ ; tande = e ″/e ′
Simulation using a commercially available ferrite material designed for screening pur-poses with m ′ = 2, tandm = 0.06, e ′ = 24.1, and tan de = 0.0001 established that a quad-band performance was obtainable. The low radiation efficiency may be improved with the devel-opment of a ferrite material specifically for antenna rather than screening applications.
36.6 SAR MEASUREMENT AND MINIMIZATION
This section examines the effects on the user’s health when using a mobile communications device. Methods for reducing the radiation absorbed in the head are also presented.
Radiation Exposure
The public has been concerned for many years about the electromagnetic radiation effects from microwave ovens, overhead power lines, and household electrical installations even though no established evidence of harm has emerged. Non-ionizing electromagnetic waves in the microwave and lower frequency bands create thermal effects in tissues when the water molecules and dissolved ions are made to vibrate. The water content is therefore an indication of the level of absorption. Nonthermal effects are due to the interaction of the applied field with the molecules where it is considered that the latter align themselves along the electric field to minimize potential energy.35 Elegant theories and mathematical models have been published, for instance, “perturbation of DNA due to soliton waves,” “effects on body organs and glands,” and “acceleration and retardation of seed growth at specific millimeter wave frequencies.” There remains a large amount of literature globally on this topic, but the results are often ambiguous and no clear evidence appears to have emerged so far.36
tande = 0.03 tandm = 0.03
tande = 0.0001 tandm = 0.06
e ′ 6 6.38
m ′ 6.6 6.18
Monopole length (mm) 4.5 4.5
Coverage at 1800, 1900, 2100, and 2450 MHz at −6 dB level
Achieved Achieved
Efficiency (%) with the head present at 1800, 1900, 2100, and 2450 MHz, respectively
35.05, 32.6, 15.15, and 22.34 38.5, 34.9, 19.2, and 22
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MOBILE HANDSET ANTENNAS 36-15
It is not surprising, therefore, that the astronomical growth of mobile phone usage has been accompanied by much public concern, particularly as the handset is held close to the head and earlier analog handsets could dissipate a watt of power in the brain. Despite extensive worldwide research and strong support from governments for over a decade, no evidence of a heath risk has been uncovered so far. Protests about the sitting of base sta-tion masts and power lines are continuing, but at the present time the situation with mobile phones remains calm and stable, with most users aware that minimizing handset use is desirable, particularly for children. Text messaging has escalated for children and eases the exposure problem.
Manufacturers for their part have reduced exposure levels with the introduction of digi-tal systems and continue to be responsibly mindful of the situation. On the research front, the electromagnetic exposure to the head and other body regions continues to be of major interest, such as the development of low SAR handsets (as described later in this section), the effect of metal spectacles,37 body-worn jewelry,38 metal spectacles on the mucous mem-branes,39 the exposure to passengers in automobiles when using mobile phones,40 to men-tion but a few. Below we give background details of some current work in this field.
Radio Frequency Dosimetry
With the concern about mobile phone exposure to the human body and particularly the head, it was evident that a regulatory process was urgently needed. The radiation exposure to any part of the human body is now assessed by the Specific Absorption Rate (SAR), which is defined by
SAR = (s / 2r ) | E |2 W/kg (36-6)
where s = tissue conductivity, r = tissue density, and | E |2 = the square of the electric field intensity in the tissue. Background details are given in Fujimoto and James11 (Chapter 6, Section 6.5, and Chapter 7). Radio frequency dosimetry in the human body is very complex due to many factors that affect the absorption rate in tissues. Safety guidelines recommend that the 1g averaged peak SAR should not exceed 1.6 W/kg and the whole body averaged peak SAR should be less than 0.08 W/kg. The 1g averaged peak SAR is preferred because it represents local variations more accurately. Each mobile phone user has a usage profile ranging from those who use their mobile phone occasionally to those in a busy occupation necessitating many hours of continuous phone use each day. An idea of how SAR needs to be interpreted is given in Fujimoto and James11 (Chapter 6, Section 6.5). To determine the likely SAR for a mobile phone, it must be measured in as near a natural scenario as pos-sible together with a model of the human head composed of representative artificial tissue. Table 36-3 summarizes the electrical properties of some common tissues.
Purpose-built equipment is available to assist in standardization, but computer model-ing has played a major role in rapidly evaluating different human body and handset situa-tions; these aspects are detailed below.
Measurement of SAR
Although computer modeling has proliferated, the direct practical measurement of SAR in a phantom head and body is regarded as an important complementary tool. Handset manufactures have now built up their bespoke practical measurement equipment over sev-eral years with phantom models comprised of tissue simulating fluid with the appropriate electrical parameters as listed in Table 36-3. Elaborate three-dimensional electrically small
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36-16 CHAPTER THIRTY-SIX
probes are positioned within the phantom fluid by sophisticated control systems, enabling the SAR to be rapidly assessed within a few minutes. The homogeneous tissue fluid enables a bulk SAR assessment to be performed but cannot represent the layering of different tis-sues and the presence of organs and glands. It does, however, allow the rapid comparison of different handset antennas and the effects of different handset orientations and separation distance from the body region. Most of the measurements are concerned with the absorp-tion in the head, with or without a phantom hand model, and the variation in SAR for the same handset can be up to 100 percent depending on the measurement condi-tions.41 Consumer groups have published league tables of SAR head absorption for commercially available mobile phones but its influence on customer purchasing preference is not very apparent, maybe because of the strong fashion element associated with phones, particularly with younger people.
A photograph of a measurement sys-tem is given in Figure 36-13, showing the phantom in the forefront undergoing probe measurements in its right ear region. Details of the probes and control appara-tus are given in Fujimoto and James11 (pp. 338–340).
There is also some demand from uni-versities and research establishments for ready-made commercially available SAR measurement equipment. One such exam-ple is the SPEAG Dosimetric Assessment System (DASY 4), which provides a six-axes robotic arm scanning the inside of a phantom head with facial features that are filled with tissue simulating liquid.42
Frequency (MHz) Relative Permittivity e ′ Conductivity s (S/m)
300 45.3 0.87
450 43,5 0.87
835 41.5 0.90
900 41.5 0.97
1450 40.5 1.20
1800 40.0 1.40
1900 40.0 1.40
2000 40.0 1.40
2450 39.2 1.80
3000 38.5 2.40
TABLE 36-3 Electrical Parameters of Human Body Tissues
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MOBILE HANDSET ANTENNAS 36-17
The system is computer controlled, enabling measurement of SAR to be performed in a short time. A photograph of the equipment is shown in Figure 36-14.
Techniques for Minimizing SAR
A variety of earlier attempts sought to reduce the handset antenna radiation to the hand and head regions. A pull-up metal mesh shield around the handset case was manufactured but, as might have been expected, it interfered and reduced the handset transmitted and received power. This idea has very recently been revisited with success using a metamaterial shield43
and a view of the latter in position on the handset case is shown in Figure 36-15.A way of reducing SAR in the head has been described44 that is based on the presence
of a ferrite material screen. The research was simulated using FDTD with a detailed head model. It is claimed that provided the handset GP currents are not perturbed, then the SAR in the head is reduced without a reduction in radiation efficiency. An early attempt to reduce the head SAR by creating a radiation pattern null in the direction of the head involved a slot/patch combination.45 A layered spherical head constituted the simulation model. The antenna was bulky and unlikely to be applicable to present-day handset requirements, par-ticularly with the demand for single multiband antenna operation.
Computer modeling is now a foremost tool associated with the minimization of SAR, particularly for the head region. The models have ranged from simple homogeneous cube blocks, spheres, and layered spheres to complex, more-realistic head models exhib-iting the heterogeneous regions of the head and brain. These anatomical models are very detailed and thus escalate computer processing requirements. Some models can be freely downloaded on the Web. Two different anatomical models46,47 are illustrated in Figures 36-16a and b.
FIGURE 36-14 DASY 4 SAR measurement system by SPEAG
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Much research has been carried out on the comparison of different head models and it has been deduced that even though a heterogeneous head model will affect the SAR distribution within the head, it has little effect on the maximum integrated SAR, which is determined mainly by the antenna itself and its distance from the phantom.48–50 It is con-cluded that the maximum averaged SAR is lower in most cases in a heterogeneous head model compared to a homogeneous head model and that a spherical head can be used to give a reasonable, slightly pessimistic assessment.50–52
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Currently, the IEEE Standards Coordinating Committee 34, Subcommittee 2, Work Group 2 is proposing new FDTD computational techniques for determining the SAR in the human body.53 It is expected that this standard will provide better guidelines for simulations involving heterogeneous heads.54
A layered spherical head has been investigated using the eigenvalue expansion method, and an interesting example of the results is shown in Figures 36-17a and b (Fujimoto and James11 pp. 373–374). The result demonstrates that the absorption in the skin exceeds that in the CSF at higher frequencies.
The majority of head model simulation research appears to mainly involve the use of PIFAs and other handset antennas that couple strongly into their environment, as described earlier in this chapter. The head SAR levels can therefore be near the recommended SAR limits.19 The ferrite-loaded handset antenna described in Section 36.5 represents a signifi-cant advance in techniques for reducing the head SAR, and the reductions in SAR are very significant. Table 36-4 gives details of the SAR in the head for this ferrite antenna. A spherical homogeneous head model was used in this research.
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The excellent low SAR in the head is brought about by the use of an antenna monopole excitation probe, the orientation of the probe, and the use of ferrite material, and, for the balanced version, the deployment as well of two coupled antennas in phase opposition. As far as we are aware, these ferrite handset antennas are probably the only way of effectively reducing the SAR in the head while maintaining good prospects for realistic manufactur-ing potential.
36.7 PROVISION FOR GPS AND BLUETOOTH
The provision for GPS and Bluetooth services in recent years was the commencement of adding value to mobile phones. Handset manufacturers are clearly influenced by the rela-tive ease of embedding additional software processing chips within the handset. Additional bulky sensors are invariably required to input, output, and display the information, and this has not been an insurmountable obstacle for TV reception and camera phones where the display panel and camera can protrude somewhat outside the case. The clamshell handset also eases the space problem. The power of the chips continues to increase significantly while chip prices continue to fall. In keeping with this trend, handset manufacturers are deterred from increasing the number of antennas within the handset, both for cost reasons and because of the acute shortage of space. Therefore, we can expect to have only one multiband handset antenna that will encompass the new bands in addition to the established original bands. The isolation between adjacent bands needs to be small to avoid additional software filtering.
We have already shown in this chapter that some of the multiband antennas are in fact able to include the Bluetooth transceiver band at 2450 MHz. One thing in the designer’s favor is that at this higher frequency, the Bluetooth antenna is less electrically small with a quarter-wave length of 3.06 cm. For GPS receiver provision in the L1 1575.42-MHz band, the antenna size limitation is more critical with a quarter-wave length of 4.76 cm. Both GPS and Bluetooth are low-level signal devices and consequently they involve no SAR issue.
A way of reducing the GPS antenna length has already been demonstrated19 and involves an IFA plated onto a dielectrically loaded PIFA. The dielectric loading condenses the IFA fields and hence the antenna length. This example is illustrated in Figures 36-18a and b, show-ing a view of the dielectric loaded PIFA and the isolation between the PIFA and the IFA.
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Mobile Handset Antennas
MOBILE HANDSET ANTENNAS 36-21
How handset manufacturers continue to provide for GPS and Bluetooth depends very much on their own preferred handset and antenna designs, but the examples shown in this section illustrate the feasibility of adding these two services to mobile phones.
36.8 MEASUREMENT OF HANDSET ANTENNAS
The mobile phone handset is a remarkable electronic product: it is packed with communi-cation circuitry that must fit in the handset case, and it must be affordable by the public at large. The complexity of handsets has demanded new thinking about not only how to mea-sure the circuit functions during the manufacturing process but also its overall performance when assembled. All of this must be accomplished in a cost-effective way. The situation is further complicated by the continually increasing processing speeds and the associ-ated high-frequency EMC difficulties within the handset. Instrument manufacturers have responded in earnest and numerous suites of sophisticated instruments are now available to handset manufacturers to automate their component and subsystem production lines.
Measuring the performance of the finished product is a somewhat less-well-defined task because it has to embrace the relationship of the handset with its environment. This includes not only radio propagation effects in the everyday urban scenario but, as already discussed in this chapter, the absorption of waves by the operator’s body and, in particular, head. To what extent these issues are addressed will depend on the handset manufacturers but the cost of testing will be a major consideration and it has to be assumed that some sampling of the manufacturing output will take place.
The performance of an antenna in cellular operation can be defined and related to a particular route. Multipath reflections from moving vehicles, people, airplanes, doors in buildings, etc., create random fluctuations in the propagating radio wave polarization and signal strength. For measurement purposes, the wave polarization can be resolved into vertical polarization (VP) and horizontal polarization (HP), so upon moving the mobile handset along a particular urban route, the mean powers Pv and Ph, respectively, associated with these polarized components can be measured. The antenna will not be able to capture all the polarized power, and Pr < (Pv + Ph), where Pr is the mean antenna power that would have been received over the same route, in the same time period. The antenna performance is defined55 by its mean effective gain (MEG), where
MEG = Pr / (Pv + Ph) (36-7)
MEG = [ + , ) ( , ) + +0∫ P P P G P P P Pv v h h v h/ ( ) ( / ( )θ
π
θθ φ θ φ GG P d dφ φ
π
θ φ θ φ θ θ φ( .sin, ) ( , )]∫0
2
(36-8)
where (q, f ) are spherical angular coordinates, Gq (q, f) and Gf (q, f) are the q and f com-ponents of the antenna power gain pattern, respectively, and Pq (q, f) and Pf (q, f) are the q and f components of the angular density functions of the incoming plane waves, respectively.
The recent trend toward extreme handset compactness and internal integrated antennas does not allow the handset antenna to be evaluated in isolation from the handset itself. In principle the MEG assessment could be applied to the handset itself, but it is clearly not affordable as a production test. Measurements on handset reception and transmission are also carried out56 in a controlled environment, such as an anechoic chamber, but the process can again be very time-consuming and indeed costly.
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The use of a reverberation chamber to test antennas in Rayleigh fading propagation conditions is well established, but its recent application57 to mobile handset testing appears to offer manufacturers an efficient, low-cost method of assessing handset performance. An anechoic chamber requires highly absorbent walls, but a reverberation chamber is a large, metal-walled cavity capable of supporting many modes, which are then greatly per-turbed by rotating reflectors within the excited chamber. This mode-stirring action is ran-dom enough to create a Rayleigh distributed transfer function between the receiving and transmitting antennas inside the chamber: in this application, these are the radiating handset and the receiving wall probe. The Rayleigh distribution model is a useful approximation of realistic mobile propagation conditions, and a demonstration chamber has been described57
that has many attractive features. For instance, a phantom tissue head model can be placed in close proximity to the handset under measurement to enable the actual operating radia-tion efficiency to be obtained. The chamber can also test the addition of diversity functions and the newly emerging MIMO systems. The installation costs and operating times are low and compatible with handset manufacturing requirements.
36.9 SATCOM HANDSET ANTENNAS
Unlike cellular phones, satcom phone systems have had a very uncertain start due to the inade-quate public takeup, which has led to financial difficulties for manufacturers. The systems have, however, survived in a leaner, more responsive form and a stable market has been developed. Around 1998 the systems under development were Iridium, Globalstar, ICO, and Thuraya. A summary of their salient technical details are listed in Mobile Antenna Systems Handbook11 (p. 582). Iridium, Globalstar, and ICO are based on LEO, LEO, and MEO constellations, respectively, while Thuraya has a GEO satellite. Phased-array antennas directing spot beams to earth are used on all systems. Global roaming is provided by satcom phones, enabling the user to communicate from any place on earth. The user community includes military, business people, politicians, emergency services, those working in remote parts of the world or at sea, and so on, and this service differs greatly from that of the cellular phone system. A point in favor of the handset manufacturers is that the handset cases are less restricted in size than cel-lular phones, cost is not such a limitation, and the handset antenna can protrude well above the handset case. It was evident58 in 1998 that the perceived market may not be adequate although the global roaming capability would attract support from the defense sector.
These satcom systems introduced new problems for handset antenna designers because of the low signal margins and the need for circular polarization.58 The quadrifilar helix antenna (QHA) is exten-sively used, and reduced-diameter ver-sions have been developed, giving some loss of bandwidth capability while ensur-ing compact user convenience. QHAs are fitted with a balun to feed the spiral arms from an unbalanced feeder, and various designs exist. A more conventional QHA is shown in Figure 36-19.
The current Iridium 9505A handset has a size of 158 × 62 × 59 mm and has many added-value features including fax and data; an auxiliary handset antenna is also supplied. The Globalstar SAT600
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Mobile Handset Antennas
MOBILE HANDSET ANTENNAS 36-23
handset has a size of 177 × 58 × 39 mm and offers a dual-mode facility combining cellular GSM900 and the satcom channel. Add-on facilities for the Internet are also available. The Thuraya 7101 handset, shown in Figure 36-20, offers data, fax, and GPS facilities; its size is 146 × 53 × 27 mm.
As already mentioned, the QHA is constructed so that it can be pulled up out of the hand-set’s case for use, thus giving it an elevated position clear of the user’s head. Construction details for the Thuraya handset antenna are given in Figure 36-21, which shows the narrow-diameter QHA with the GSM NMHA mounted on its tip.
FIGURE 36-21 Constructional details of the Thuraya handset antenna11 (Courtesy of Allgon)
FIGURE 36-20 Thuraya 7101 handset11 (Courtesy of Thuraya)
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36-24 CHAPTER THIRTY-SIX
The Thuraya earth-to-space link is 1626.5–1660.5 MHz with a space-to-earth link of 1525.0–1559.0 MHz. The Thuraya passband characteristics are shown in Figure 36-22a, and the antenna elevation radiation pattern is shown in Figure 36-22b.
36.10 FUTURE TRENDS
The mobile communications market will continue to grow fueled by user demand for increased functionality, aesthetics, smaller handsets, longer battery life, and ubiquitous access. Some future trends are presented next.
In the immediate future, handset manufacturers will continue to add value to handsets by way of low-cost processing software. This of course will be dependent on the consumer takeup, which from past experience is dictated by individual requirements, fashion, and quite likely other serendipity factors. When the new services require extended band cov-erage, handset antenna innovation will continue to be much in demand.
The system design approach will continue to be much in evidence in handset antenna design. This takes a holistic view of how all the components function together even if their individual performance may be degraded. When compared with conventional antennas, present-day handset antennas, with their low efficiency and proximity to other components, would have been seen as very poor radiators a few decades ago.
Increasing the data rates of mobile phones will continue to receive research attention as more services are sought. New processing algorithms and techniques are being addressed, including the use of multiple antennas and quantum computing.60
The clamshell type of mobile handset has provided more space on handsets for facilities and displays while allowing the handset to be compacted when not in use, but addi-tional antenna design problems are invoked.61 Other types of handset case designs can be expected as services increase.
The quest for a single multiband handset antenna that will cover all the band requirements for new services is inhibited by well-established fundamental antenna action whereby antenna resonances are necessary to achieve good matching, bandwidth, and radiation efficiency. Resonance is determined by the length of current paths and sets constraints
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Mobile Handset Antennas
MOBILE HANDSET ANTENNAS 36-25
on antenna size even if material loading and other compacting techniques are used, as described in this chapter. A future breakthrough in antenna design cannot be ruled out. Much attention has been focused recently on how materials can be synthesized using metamaterials62 but to date the cell sizes of the latter remain too large for the synthesis of an electrically small antenna.
36.11 SYMBOLS
A radius of minimum sphere
DRA dielectric resonator antenna
GP ground plane
GPS global positioning system
IFA inverted F antenna
K free-space wave number
LEO low earth orbit
MEG mean effective gain
MEO medium earth orbit
NMHA normal mode helical antenna
ha antenna radiation efficiency
hm matching network radiation efficiency
PIFA planar inverted F antenna
Q Q factor
Qa antenna Q factor
Qm matching network Q
QHA quadrifilar helix antenna
SAR specific absorption rate
UMTS universal mobile telephone system
tandm permeability loss tangent
tande permittivity loss tangent
mr = m ′+ jm ″ complex permeability
er = e ′ + je ″ complex permittivity
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