Wideband Compact Antennas for Wireless Communication Applications Minh-Chau Huynh

Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical and Computer Engineering Dr. Warren Stutzman, Chair Dr. William Davis Dr. Ahmad Safaai-Jazi Dr. Timothy Pratt Dr. Beate Schmittmann November 22, 2004 Blacksburg, Virginia Keywords: Small antenna, compact antenna, wideband antenna, radiation efficiency, radiation effects Copyright 2004, Minh-Chau Huynh

Wideband Compact Antennas for Wireless Communication Applications Minh-Chau Huynh (ABSTRACT) Recent technologies enable wireless communication devices to become physically smaller in size. Antenna size is obviously a major factor that limits miniaturization. In the past few years, new designs of low-profile antennas for handheld wireless devices have been developed. The major drawback of many low-profile antenna designs is their narrow impedance bandwidth. Furthermore, the market trend of personal wireless devices is moving toward a universal system that can be used anywhere and rapid expansion of the wireless communication industry has created a need for connectivity among various wireless devices using short-range wireless links in the Bluetooth operating band to get rid of the cable connections. This requires therefore multiple frequency band operation. In summary, physically small size, wide bandwidth, and high efficiency are the desired characteristics of antennas in mobile systems. This dissertation presents a comprehensive analysis of a new wide-bandwidth compact antenna, called WC J-pole antenna, covering 50 % impedance fractional bandwidth. A set of guidelines is also provided for a bandwidth-optimized design at any frequency. A few design variations of the proposed antenna are also presented for existing commercial wireless applications. Efficiency is perhaps the most important characteristic of small antennas for mobile systems. An extension of the Wheeler cap method to moderate-length and wideband antennas is developed to measure quickly efficiency. The dissertation also provides a review of human operator interaction with handset antennas. Since the proposed antenna is intended to be used in the proximity of human body and in a casing, coupling effects of human body and casing on the antenna

characteristics and radio frequency (RF) energy absorption into the human body are investigated.

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Acknowledgements I would like to thank Dr. Warren Stutzman who has served many years as my advisor and committee chairman for my Masters and Ph.D. degrees. Dr. Stutzman provided invaluable advice and support throughout the work on this dissertation and my career development. I thank him for his encouragement, patience, generous amounts of time and effort, countless edits of my dissertation, and especially for giving me a chance to be part of Virginia Tech Antenna Group. I am indebted to Dr. William Davis for numerous discussions and suggestions on the technical aspect of the work presented in this dissertation. I would like to thank Dr. Koichiro Takamizawa, Dr. Nathan Cummings, and Taeyoung Yang for many suggestions and ideas on the modeling and simulation aspect of my work. I would also like to thank other members of my advisory committee, Dr. Ahmad Safaai-Jazi, Dr. Timothy Pratt, and Dr. Beate Schmittmann for their help on completing my Ph.D. work. I am also thankful to my beloved friend Marion Mangin for her moral support and for showing me that there is more than just work in life. Without her, my days would have been just black and white. Many thanks also go to my labmates and friends Kai Dietze, Gaurav Joshi, and Stani Licul with whom I shared many laughs, jokes, and sandwiches at Substation. Finally, I am grateful to my family for their overwhelming support and encouragement throughout my studies and life. Without my parents and their dedication on their children’s education, I would have not had any opportunity to be where I am right now.

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Contents

1 INTRODUCTION ................................................................................................................................... 1 1.1 1.2 1.3

INTRODUCTION ................................................................................................................................ 1 OVERVIEW OF THE DISSERTATION .................................................................................................. 4 REFERENCES ................................................................................................................................... 4

2 LITERATURE REVIEW ....................................................................................................................... 5 2.1 INTRODUCTION ................................................................................................................................ 5 2.2 ELECTRICALLY SMALL ANTENNAS ................................................................................................. 5 2.3 FUNDAMENTAL LIMITATIONS ON THE RADIATION Q OF SMALL ANTENNAS ................................... 7 2.3.1 Overview of Theoretical Investigations on the Fundamental Limits ...................................... 7 2.3.2 Fundamental Limitations of Electrically Small Antennas .................................................... 11 2.4 TECHNIQUES FOR REDUCING ANTENNA SIZE ................................................................................ 13 2.4.1 Use of Short Circuits and Ground Planes ............................................................................ 13 2.4.2 Optimizing the Antenna Geometry........................................................................................ 15 2.4.3 Antenna Loading with High-Dielectric Materials ................................................................ 16 2.4.4 Use of Antenna Environment ................................................................................................ 17 2.4.5 Effects on Antenna Performance Characteristics................................................................. 18 2.4.6 Examples of Practical Small Antennas for Hand-Held Wireless Communications .............. 21 2.5 TECHNIQUES FOR WIDENING IMPEDANCE BANDWIDTH FOR MSA’S ............................................. 27 2.5.1 Planar Multi-Resonator Configurations............................................................................... 28 2.5.2 Multilayer Configurations .................................................................................................... 35 2.6 SUMMARY ..................................................................................................................................... 40 2.7 REFERENCES ................................................................................................................................. 41

3 ANTENNA EFFICIENCY MEASUREMENTS ................................................................................. 44 3.1 INTRODUCTION .............................................................................................................................. 44 3.2 WHEELER CAP METHOD ON SMALL ANTENNAS............................................................................ 45 3.3 EXTENSION OF WHEELER CAP METHOD TO WIDEBAND ANTENNAS ............................................. 49 3.4 EXPERIMENTAL RESULTS OF EFFICIENCY EVALUATION USING WIDEBAND WHEELER CAP.......... 53 3.4.1 Lossy Monopole on Finite Ground Plane............................................................................. 54 3.4.2 Planar Half-Disk UWB Monopole Antenna ......................................................................... 57 3.4.3 Measurement Sensitivity Tests .............................................................................................. 59 3.5 SUMMARY ..................................................................................................................................... 61 3.6 REFERENCES ................................................................................................................................. 61

4 A REVIEW OF RADIATION EFFECTS ON HUMAN OPERATORS OF HAND-HELD RADIOS ...................................................................................................................................................... 63

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4.1 BIOLOGICAL EFFECTS OF RADIO-FREQUENCY RADIATION ........................................................... 63 4.2 RF RADIATION BASICS ................................................................................................................. 64 4.3 BIOLOGICAL EFFECTS AND HEALTH ISSUES .................................................................................. 65 4.4 RF SAFETY AND REGULATIONS .................................................................................................... 67 4.5 HUMAN OPERATOR INFLUENCE ON HAND-HELD RADIO PERFORMANCE ...................................... 69 4.5.1 Computational Simulation .................................................................................................... 70 4.5.2 Electromagnetic Modeling of the Human Operator ............................................................. 71 4.6 HUMAN OPERATOR EFFECTS ON ANTENNA CHARACTERISTICS .................................................... 74 4.7 POWER ABSORPTION IN THE HEAD AND SAR................................................................................ 78 4.8 SUMMARY ..................................................................................................................................... 80 4.9 REFERENCES ................................................................................................................................. 81

5 A NEW WIDEBAND COMPACT ANTENNA .................................................................................. 85 5.1 INTRODUCTION .............................................................................................................................. 85 5.2 THE ANTENNA STRUCTURE ........................................................................................................... 86 5.3 RESULTS OF PRELIMINARY NUMERICAL SIMULATION AND EXPERIMENTAL INVESTIGATIONS ...... 89 5.4 ANTENNA STRUCTURE SIMILARITIES OF WC J-POLE TO OTHER ANTENNAS ................................ 90 5.5 ANALYSIS OF THE COMPACT PLANER WIDEBAND J-POLE ANTENNA ............................................ 97 5.5.1 Parametric Analysis of the Compact WC J-pole Antenna .................................................... 97 5.5.1.1 5.5.1.2 5.5.1.3 5.5.1.4 5.5.1.5 5.5.1.6

Variation of Feed Probe Height hP ................................................................................................. 100 Variation of Feed Plate Area .......................................................................................................... 102 Variation of Feed Probe Position, dp .............................................................................................. 109 Variation of the Short Plate Height, hs ........................................................................................... 116 Variation of the Lower Plate Length, LA ........................................................................................ 119 Variation of the Top Plate Length, LB ............................................................................................ 122

5.5.2 Summary of the WC J-pole Parameter Variation ............................................................... 125 5.5.3 Design Case for the Largest Impedance Bandwidth........................................................... 128 5.6 SUMMARY ................................................................................................................................... 131 5.7 REFERENCES ............................................................................................................................... 131

6 VARIATIONS OF THE WC J-POLE FOR A FEW COMMERCIAL APPLICATIONS........... 133 6.1 INTRODUCTION ............................................................................................................................ 133 6.2 WIDEBAND COMPACT ANTENNA FOR COVERING APPLICATIONS FROM GPS TO BLUETOOTH BANDS (WCJP #1) .................................................................................................................................. 133 6.3 DUAL-BAND COMPACT ANTENNA FOR PERSONAL WIRELESS COMMUNICATIONS, BLUETOOTH, AND U-NII BANDS (DCLA #1)................................................................................................................ 137 6.4 DUAL-BAND COMPACT ANTENNA FOR 2.45/5.25 GHZ WLAN (DCLA #2)........................... 141 6.5 SUMMARY ................................................................................................................................... 146

7 CONCLUSIONS.................................................................................................................................. 147 7.1 7.2 7.3 7.4

SUMMARY ................................................................................................................................... 147 CONTRIBUTIONS .......................................................................................................................... 148 FUTURE WORK ............................................................................................................................ 149 REFERENCE ................................................................................................................................. 150

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List of Figures Figure 2-1

Figure 2-2 Figure 2-3 Figure 2-4 Figure 2-5 Figure 2-6 Figure 2-7 Figure 2-8 Figure 2-9

Figure 2-10 Figure 2-11 Figure 2-12 Figure 2-13 Figure 2-14 Figure 2-15 Figure 2-16 Figure 2-17 Figure 2-18 Figure 2-19 Figure 2-20

Fundamental limit curves of radiation Q versus ka. The top curve (solid curve) is a plot of (2.3) and is for efficiency, er = 1 , or 100% . The dashed curve is the plot of (2.2) with efficiency er = 1 . .................................................................................................................. 10 Comparison of several practical antenna (Q,ka) values (see Table 2-1) to the fundamental limits curve based on (2.3) with er = 100%............................................................................. 12 An example of size reduction using a ground plane................................................................ 14 Size reduction using short circuit in microstrip patch antenna................................................ 14 The effect of notches and slots in a microstrip-patch antenna for size reduction: (a) regular microstrip patch antenna and (b) microstrip patch antennas with notches and slots............... 15 Meander-line printed antenna: an example of reducing antenna size by increasing its electrical length. ..................................................................................................................................... 16 Size reduction of a monopole by dielectric loading. ............................................................... 17 The smart monobloc integrated-L antenna (SMILA) [17]. ..................................................... 18 Effects of permittivity value on the electric field intensity of a rectangular microstrip-patch antenna with (a) εr=1.07 and (b) εr=10.2 at resonance simulated using FDTD method. The colors representing the normalized electric field go from blue (weak E-field) to green to yellow to red (strong E-field).................................................................................................. 20 Geometry of the dual-frequency PIFA with a branch-line slit; dimensions in the figure are in millimeters [20]....................................................................................................................... 22 Measured and simulated return loss of the PIFA shown in Fig. 2-10 [20]. ............................. 23 Simulated patch surface current at (a) 950 MHz and (b) 1790 MHz for the PIFA shown in Fig. 2-10 [20].................................................................................................................................. 23 (a) The branch line planar monopole in a wrapped structure for GSM/DCS/PCS multi-band mobile phone antenna; (b) the monopole unwrapped into a planar structure [21]. ................. 25 Measured and simulated return loss for the monopole in the wrapped structure shown in Fig. 2-13a [21]................................................................................................................................ 26 Geometry of the dual-band printed inverted-F antenna [22]................................................... 27 Measured input impedance for the antenna shown in Fig. 2-15 with h1 = 10 mm, h2 = 5 mm, l1 = 21 mm, l2 = 10 mm, d = 3 mm, and w = wf =3.05 [22]......................................................... 27 Various gap-coupled multiresonator rectangular MSAs configurations: gap-coupling (a) along the radiating edges of the fed patch, (b) along the non-radiating edges of the fed patch, and (c) along all edges of the fed patch [28]. ...................................................................................... 29 VSWR plots of two coupled resonators having (a) narrow and (b) wide bandwidth: (….) individual resonators and () overall response...................................................................... 30 A single parasitic patch element gap-coupled with one fed patch antenna: (a) geometry, (b) computed input impedance, and (c) VSWR plots of a single fed patch with no parasitic element (---) and with one parasitic patch element () [28]. ................................................. 31 Computed input impedance (a) and VSWR plots (b) of two gap-coupled patches along radiating edge for two feed point locations x of the parasitic MSA of Fig. 2-19a: (---) 0.7 cm and () 1.1 cm [28]................................................................................................................ 32

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Figure 2-21 Computed input impedance (a) and VSWR plots (b) of two gap-coupled patches along radiating edge for three values of L1 of the parasitic MSA of Fig. 2-19a: (---) 2.8 cm, () 2.9 cm, and (— – —) 3.0 cm [28]................................................................................................. 32 Figure 2-22 Computed input impedance (a) and VSWR plots (b) of two gap-coupled patches along radiating edge for three values of s for the parasitic patch antenna of Fig. 2-19a: (---) 0.05 cm, (— – —) 0.1 cm, and () 0.15 cm [28]. ................................................................................ 33 Figure 2-23 Computed radiation patterns of two gap-coupled patches along radiating edge at frequencies (a) 2.9 GHz, (b) 3.0 GHz, and (c) 3.1 GHz: () E-plane, (---) H-plane [28]. ........................ 34 Figure 2-24 Computed radiation patterns of three gap-coupled patches along radiating edges at frequencies of Fig. 2-17a: (a) 2.89 GHz and (b) 3.09 GHz: () E-plane, (---) H-plane [28]. ................... 35 Figure 2-25 Configurations of (a) Electromagnetically-coupled and (b) aperture-coupled MSAs............. 36 Figure 2-26. Geometry of a stacked microstrip patch coaxial-fed on a large ground plane: (a) top and (b) side views................................................................................................................................ 37 Figure 2-27 Impedance and VSWR over the frequency range from 3.3 to 4.1 GHz for the stacked microstrip patch of Fig. 2-26 simulated to examine upper patch size variation for four values of L2: (— – —) 2.0 cm, (---) 2.3 cm, (——) 2.45 cm, and (…) 2.6 cm [28]........................... 38 Figure 2-28 Impedance and VSWR over the frequency range from 3.3 to 3.9 GHz for the stacked microstrip patch of Fig. 2-26 simulated to examine lower patch size variation for three different values of L1: (— – —) 2.50 cm, (——) 2.55 cm, and (---) 2.60 cm [28]. ................ 39 Figure 2-29 Impedance and VSWR over the frequency range from 3.4 to 4.0 GHz for the stacked microstrip patch of Fig. 2-26 simulated to examine the effect of misalignment of the top patch from the lower patch for different offset values: (——) not offset, (…) oy=0.1 cm, (— – —) ox=0.1 cm, and (---) ox=-0.1 cm [27]...................................................................................... 40 Figure 3-1 Circuit model of an antenna resistance showing radiation and loss resistances...................... 46 Figure 3-2 Illustration of the method for determining radiation efficiency of wideband antennas. Instead of inhibiting radiation, a Wideband Wheeler Cap allows the antenna to transmit and receive the reflected signal [9]: (a) Antenna radiating in free space and (b) A “Wideband Wheeler Cap,” sized to be larger than the radianshpere dimension of the traditional Wheeler cap. ..... 50 Figure 3-3 Power budget for a TX-RX pair of the wideband Wheeler cap method if all the transmitted power is available at the receive antenna. ............................................................................... 51 Figure 3-4 Power fraction link budget for an antenna inside the wideband Wheeler cap......................... 52 Figure 3-5 Antenna radiation efficiency measurement setup using a 15-cm radius wideband Wheeler cap probed by an HP 8720 network analyzer. ............................................................................... 53 Figure 3-6 Lossy monopole on a finite ground plane (100 by 140 mm) with a 50-Ohm resistor at the tip of the antenna to insert some dissipation loss in the antenna. ................................................. 54 Figure 3-7 Magnitude of S11 measured using the setup in Fig. 3-5 of the lossy monopole of Fig. 3-6: (a) in free space, and (b) in the spherical cap of 15-cm radius. .................................................... 55 Figure 3-8 Processed |S11| (blue curve) of the monopole in the spherical cap by taking the largest value for the raw data over 100 MHz span, assuming that the impedance characteristics of the antenna does not change drastically within that span.............................................................. 56 Figure 3-9 Total efficiency ηT (radiation efficiency including impedance mismatch) of the lossy monopole of Fig. 3-7 evaluated using (3.13) with experimental data (solid curve) and numerical data (asterisk). ........................................................................................................ 56 Figure 3-10 A planar half-disk UWB monopole antenna designed for operating from 3.1 to 10.6 GHz [10]. 57 Figure 3-11 Magnitude of S11 for the planar half-disk UWB monopole antenna of Fig. 3-10 measured: (a) in free space and (b) inside the spherical cap of 15-cm radius................................................ 58 Figure 3-12 Total efficiency ηT (radiation efficiency including impedance mismatch) of the planar halfdisk UWB monopole antenna of Fig. 3-10 evaluated using (3.13). ........................................ 59 Figure 3-13 Measurement setup for the planar half-disk UWB monopole antenna placed away from the center of the spherical Wheeler cap. ....................................................................................... 60 Figure 3-14 Comparison of the antenna total efficiency for test antenna locations near and away (Fig. 313) from the center of the spherical Wheeler cap. .................................................................. 60 Figure 4-1 The electromagnetic spectrum. ............................................................................................... 65

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Figure 4-2

ANSI/IEEE and ICNIRP/CELENEC maximum permissible exposure in terms of power density for uncontrolled environment in RF frequency range [5]. .......................................... 69 Figure 4-3 Field component value locations of a Yee cell used in FDTD computations. The Ecomponents are in the middle of the edges and the H-components are in the center of the faces [10].......................................................................................................................................... 70 Figure 4-4 Popular human head models used in numerical simulations................................................... 73 Figure 4-5 Computed radiation patterns in φ-plane (θ = 90°) for a monopole oriented along the z-axis and mounted on a metal box. Operation is at 900 MHz and the antenna is 1.5 cm away from a human head modeled as a rectangular box of 20-cm side length and as a sphere of 20-cm diameter with three layers of dielectric material (skin-skull-brain); see Figs 4a and b........... 75 Figure 4-6 Computed return loss values of a monopole on a handset for three cases: no human operator (“monopole only”); only a hand present (“with hand”); and both an operator hand and head present (“with hand and head”). The separation distance between 20-cm diameter spherical head model (skin-skull-brain) and the antenna is 1.5 cm and the hand model (muscle-bone) is located 6.0 cm below the antenna. .......................................................................................... 77 Figure 4-7 Handset model showing the PIFA antenna location and hand model for the results in Fig. 4-8. 77 Figure 4-8 Computed return loss values of the side-mounted PIFA on a handset shown in Fig. 7 without the hand and with the hand for three different hand locations d. ............................................ 78 Figure 5-1 Geometry of the compact wideband J-pole (WC J-pole) antenna designed for operation from fL = 1.77 GHz to fU = 2.45 GHz with center frequency fc = 2.11 GHz. ...................................... 88 Figure 5-2 Geometry of a conventional planar inverted-F antenna (PIFA)................................................. 88 Figure 5-3 Impedance characteristics as function of frequency for the compact WC J-pole of Fig. 5-1 computed using the IE3D simulation code: (a) Smith chart of complex impedance from 1.5 to 3.5 GHz; (b) VSWR referenced to 50 Ohms. The measured VSWR (dashed curve) is also plotted in (b)............................................................................................................................ 89 Figure 5-4 Far-field radiation patterns of the WC J-pole antenna of Fig. 5-1 at 2.2 GHz computed using IE3D for: (a) xz-plane, (b) yz-plane, and (c) xy-plane............................................................ 91 Figure 5-5 Average current distribution of (a) the WC J-pole and (b) a conventional PIFA on a large finite ground plane. The magnitude of the current distribution goes from minimum (blue color) to maximum (red color). The results were obtained from simulation using IE3D. ...... 92 Figure 5-6 Structure of a J-pole antenna with dimensions for operation at 1.8 GHz with a wire radius of 0.635 mm. ............................................................................................................................... 93 Figure 5-7 Return loss of J-pole antenna of Fig. 5-6 matched to 50-Ohm impedance computed using IE3D. ................................................................................................................................................ 93 Figure 5-8 Planar J-Pole antenna compared to a wire J-pole antenna. ........................................................ 94 Figure 5-9 Impedance properties of the planar J-pole antenna of Fig. 5-8 for various antenna widths WS computed using IE3D: (a) resistance, (b) reactance, and (c) magnitude of S11. ...................... 96 Figure 5-10 Illustration of the increase in current path length accomplished by narrowing the short plate (WS) width of the planar J-pole antenna of Fig. 5-8. ............................................................... 96 Figure 5-11 WC J-pole antenna structure with its dimension parameters. .................................................. 99 Figure 5-12 Input impedance (a) and VSWR relative to 50 Ohms (b) of the WC J-pole antenna of Fig. 511 and Table 5-2 computed using IE3D................................................................................ 100 Figure 5-13 Impedance of the WC J-pole antenna shown in Fig. 5-11 and Table 5-2 for various feed probe height hp values: (a) real part of impedance (antenna input resistance), (b) imaginary part of impedance (antenna input reactance), and (c) |S11| of the structure matched to 50-Ohm impedance, The height of the top plate is 5.6 mm. ............................................................... 102 Figure 5-14 Feed plate area variation of the capacitive antenna structure. The square shape feed plate side length LF increases from 2 to 22 mm. ................................................................................... 104 Figure 5-15 Impedance of the antenna structure with square feed plate LF×LF area variation from LF×LF = 2x2 mm2 (case 1) to LF×LF = 10x10 mm2 to LF×LF = 22x22 mm2 (case 3), according to Fig. 514. 105 Figure 5-16 Geometry of the WC J-pole antenna for the study of the feed plate area not shielded by the top plate....................................................................................................................................... 107

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Figure 5-17 Input impedance for the WC J-pole antenna of Fig. 5-16 and Table 5-4 calculated using IE3D: (a) resistance, (b) reactance, and (c) return loss referenced to 50 Ohms............................... 108 Figure 5-18 Geometry of the WC J-pole antenna. dp is the probe feed position away from the short plate. .............................................................................................................................................. 110 Figure 5-19 Input impedance of WC J-pole antenna shown in Fig. 5-18 and Table 5-5 calculated for values of feed probe position dp from 4 mm to 20 mm away from the antenna short plate with all other dimensions fixed. .................................................................................................... 111 Figure 5-20 Geometry of the WC J-pole antenna. dp is the probe feed position away from the short plate and the feed plate is moved along with the probe feed. ........................................................ 113 Figure 5-21 Impedance of the antenna structure shown in Fig. 5-20 and Table 5-6 computed using IE3D for the case when the feed plate is completely shielded by the top plate. ............................. 114 Figure 5-22 Impedance of the antenna structure shown in Fig. 5-20 and Table 5-6 computed using IE3D for the case when a portion of the feed plate is unshielded by the top plate. ........................ 116 Figure 5-23 Geometry of the WC J-pole antenna for the height hs study.................................................. 117 Figure 5-24 Impedance of the antenna structure shown in Fig. 5-23 and Table 5-7 computed for various short plate heights hs. ............................................................................................................ 119 Figure 5-25 Geometry of the WC J-pole antenna for the analysis on the variation of the lower plate length LA. 120 Figure 5-26 Impedance of the antenna structure shown in Fig. 5-25 and Table 5-8 for lower plate length variation LA study.................................................................................................................. 122 Figure 5-27 Geometry of the WC J-pole antenna for the analysis on the variation of the top plate length LB........................................................................................................................................... 123 Figure 5-28 Impedance of the antenna structure shown in Fig. 5-27 and Table 5-9 for top plate length variation LB study.................................................................................................................. 125 Figure 5-29 Dimensions of the WC J-pole antenna for the optimal impedance bandwidth of about 50 %. 128 Figure 5-30 Input Impedance (a) and return loss for a 50-Ohm input impedance match (b) of the WC Jpole antenna of Fig. 5-29 with its impedance bandwidth optimized..................................... 129 Figure 6-1 Wideband compact J-pole antenna (WCJP #1) designed to cover frequency bands from GPS to Bluetooth bands. ............................................................................................................... 135 Figure 6-2 VSWR values computed using IE3D for the WC J-Pole of Fig. 6-1 relative to 50-Ohms. Note an impedance match (VSWR≤2) is achieved for the frequency bands of interest. ............... 136 Figure 6-3 Computed values of maximum gain over the WCJ-Pole operating band.............................. 136 Figure 6-4 Geometry of the dual-band compact antenna and its prototype (DCLA #1)......................... 138 Figure 6-5 Computed and measured |S11| of the dual-band compact antenna of Fig. 6-4. A 10-dB return loss (-10 dB S11) corresponds to a 2:1 VSWR. ..................................................................... 138 Figure 6-6 Computed (solid curves) and measured (dashed curves) radiation patterns of the dual-band compact antenna of Fig. 6-4 in the frequency bands of interest in the yz plane for both Eθ (red curves) and Eφ (blue curves) cuts: (a) 2.2 GHz and (b) 5.2 GHz. ......................................... 139 Figure 6-7 Computed and measured gain of the dual-band compact antenna of Fig. 6-4. ..................... 140 Figure 6-8 Measured radiation efficiency of the dual-band compact antenna of Fig. 6-4 using the wideband Wheeler cap method described in Chapter 3. ....................................................... 140 Figure 6-9 Overall dimensions of the DCLA #2a structure for WLAN used in IE3D simulation.......... 142 Figure 6-10 Computed return loss of the DCLA #2 for WLAN shown in Fig. 6-9.................................. 142 Figure 6-11 Comparison of the measured |S11| values of the DCLA #2a built with a coax cable and the numerical results of DCLA #2a without the coax simulated using IE3D. ............................ 143 Figure 6-12 Dimensions of the DCLA structure for WLAN (DCLA #2b) including the coax cable and tuned to compensate the detuning coupling effects due to the coax. .................................... 144 Figure 6-13 Numerical and measured return loss of the dual-band antenna shown in Figure 6-12 with the coax cable attached. .............................................................................................................. 145 Figure 6-14 Example of antenna placement for the DCLA #2 in laptop computers that are WiFi enabled. 146

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List of Tables Table 1-1 Table 2-1 Table 4-1 Table 4-2

Table 4-3 Table 4-4 Table 5-1 Table 5-2 Table 5-3 Table 5-4 Table 5-5 Table 5-6 Table 5-7 Table 5-8 Table 5-9 Table 5-10 Table 5-11 Table 6-1 Table 7-1

Frequency Bands for a Few Popular Wireless Applications. ................................................ 3 Characteristics of Antennas Used to Examine Bandwidth-Size Relationships..................... 12 Relative Permittivity, Conductivity, and Mass Density of the Tissues in the Hand and Head 0near 900 MHz [11]. ........................................................................................................... 71 Comparison of Computed Antenna Efficiency and Power Absorbed in the Head for Simple (Rectangular and Spherical Shape) and Complex Models Developed from Yale University and from Gent University (cf. Figure 4). Transmit Power is 1 W at 915 MHz; Distance of Monopole Antenna on a Handset from Head is 1.5 cm [15]. .............................................. 72 Effects of the Change in the Separation Between the Antenna and the Head Model from University of Gent, Belgium; 915 MHz, 1W [15]............................................................... 74 Popular Standards Limits on Exposure to the General Public of Electromagnetic Radiation in the RF. ............................................................................................................................. 80 Geometric parameters of the WC J-pole antenna of Fig. 5-1. ............................................... 87 Geometric parameters of the preliminary WC J-pole antenna of Fig. 5-11. ......................... 98 Geometric parameters of the WC J-pole antenna of Fig. 5-14 for the feed plate area study. ........................................................................................................................................... 103 Geometric parameters of the WC J-pole antenna of Fig. 5-16 for the feed plate area study. ........................................................................................................................................... 106 Geometric parameters of the WC J-pole antenna of Fig. 5-18 for feed probe position study. ........................................................................................................................................... 109 Geometric parameters of the WC J-pole antenna of Fig. 5-20 for feed assembly position study. ................................................................................................................................. 112 Geometric parameters of the WC J-pole antenna of Fig. 5-23 for antenna height hs study. ........................................................................................................................................... 117 Geometric parameters of the WC J-pole antenna of Fig. 5-25 for antenna length LA study. ........................................................................................................................................... 120 Geometric parameters of the WC J-pole antenna of Fig. 5-27 for antenna length LB study. ........................................................................................................................................... 123 Summary of the WC J-pole Parameter Variation Analyzed in the Previous Section. The Geometric Parameters are shown in Fig. 5-11................................................................... 127 Electrical dimensions of the optimum bandwidth (50%) WC J-pole in terms of the wavelength λL of the lower frequency fL of the operating band. See Fig. 5-11 for geometry. ........................................................................................................................................... 130 Frequency Bands for a Few Wireless Applications. ........................................................... 135 List of the three versions of the WC J-pole antenna with their characteristics ................... 149

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Chapter 1

Introduction 1.1

Introduction

Wireless is a term used to describe telecommunications in which electromagnetic waves carry the signal over part or the entire communication path [1]. Common examples of wireless equipment in use today include: cell phones, pagers, global positioning systems (GPS), cordless computer peripherals such as wireless keyboards, cordless telephone sets, remote garage-door openers, two-way radios, satellite televisions, wireless local area network (WLAN) and wireless personal area network (WPAN). Wireless technology is rapidly evolving, and is playing an increasing role in the lives of people throughout the world. In addition, ever larger numbers of people are relying on wireless technology, either directly or indirectly. More recent examples of wireless communications include the following technologies: -

Global System for Mobile Communication (GSM): a digital mobile telephone system used in Europe and other parts of the world;

-

General Packet Radio Service (GPRS): a packet-based wireless communication service that provides continuous connection to the internet for mobile phone and computer users;

-

Enhanced Data GSM Environment (EDGE): a faster version of the Global System for Mobile (GSM) wireless service designed to deliver data at rates up 1

to 384 Kbps and enable the delivery of multimedia and other broadband applications to mobile phone and computer users; -

Universal Mobile Telecommunications System (UMTS): a broadband packetbased system offering a integrated set of services to mobile computer and phone users no matter where they are located in the world;

-

Wireless Application Protocol (WAP): a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for internet access.

Wireless can be divided into three categories: fixed, mobile, and portable. Fixed wireless refers to the operation of wireless devices or systems in fixed locations such as homes and offices. Mobile wireless applications refer to devices or systems aboard moving vehicles. Examples include the automotive cell phone and onboard GPS system. Portable wireless applies to the operation of autonomous, battery-powered wireless devices or systems outside the office, home, or vehicle; examples include handheld cell phones and personal communication system (PCS) units. Recent technologies enable wireless communication devices to become physically smaller in size. Antenna size is obviously a major factor that limits miniaturization. Antenna physical size is inversely proportional to its operating frequency. However, reducing the antenna physical size also means reducing its electrical size since the operating frequency of these devices does not change. Electrical size is expressed as a fraction of wavelength, λ. As an example of electrical size, a half-wave dipole antenna is a half-wave antenna operating at 1800 MHz (λ = c/f = 16.6 cm) is 8.3 cm long because its electrical size is 0.5 λ. If a wireless device is required to have a physically small antenna, say half the size or 4.15 cm, and still operate at 1800 MHz, then it requires an antenna with a physical size of 4.15 cm, corresponding to an electrical size of 0.25 λ. Many applications at around 1800 MHz require antennas in the order of 0.25 λ or less. Examples of antennas of quarter-wavelength electrical size that are used include monopole antennas, helical antennas, and PIFAs (planar inverted-F antenna). In the past few years, new designs of low-profile antennas for handheld wireless devices have been developed. The major drawback of many low-profile antenna designs 2

is their narrow impedance bandwidth. Some designs can barely cover the bandwidth requirement and hence, may not be used because there is no margin in the bandwidth for potential detuning effects due, for example, to the presence of a human operator. Furthermore, the market trend of personal wireless devices is moving toward a universal system that can be used anywhere. Rapid expansion of the wireless communication industry has created a need for connectivity among various wireless devices using shortrange wireless links in the Bluetooth operating band to get rid of the cable connections. This requires therefore multiple frequency band operation. A list of a few useful wireless applications and their operating frequencies is shown Table 1-1. Dual-band and tri-band compact antennas have been realized to help the transition of new wireless system generations go smoothly but the current market demand needs wireless systems to operate in more than three bands. In summary, physically small size, wide bandwidth, and high efficiency are the desired characteristics of antennas in mobile systems. Table 1-1 Frequency Bands for a Few Popular Wireless Applications. Wireless Applications

Frequency Band (MHz)

Bandwidth, MHz (%)

GPS

1570.42-1580.42

10 (0.7%)

DCS-1800

1710-1880

170 (10.6%)

PCS-1900

1850-1990

140 (7.3%)

IMT-2000/UMTS

1885-2200

315 (15.5%)

ISM (including WLAN)

2400-2483

83 (3.4%)

Bluetooth

2400-2500

100 (4.1%)

U-NII

5150-5350 / 5725-5825

200 (3.8%) / 100 (1.7%) (12.3 % for both)

This dissertation presents a comprehensive investigation of a new wide bandwidth, low-profile, compact antenna with a primary focus on the analysis of how the antenna operates, as detailed in the next section.

3

1.2

Overview of the Dissertation

This dissertation is organized into three parts. The first two chapters review the previous work and provide necessary background information. A review of small antennas, techniques for reducing antenna size, and methods for widening antenna impedance bandwidth is included in Chapter 2, providing useful information for the analysis of the new antenna. Chapter 2 also includes a section on the fundamental limitations on bandwidth and size of small antennas. The second part of the dissertation, contained in Chapters 3 and 4, discusses small antenna efficiency measurements. Efficiency is perhaps the most important characteristic of small antennas for mobile systems. The Wheeler Cap method for measuring small antenna efficiency is described in Chapter 3 and extension of this method to moderatelength and wideband antennas. Chapter 4 provides a discussion on human operator interaction. Since the proposed antenna is intended to be used in the proximity of human body and in a casing, coupling effects of human body and casing on the antenna characteristics and radio frequency (RF) energy absorption into the human body should be investigated. Chapters 5 and 6 present the main investigation of this dissertation. Analysis the new wide-bandwidth, low-profile, compact antenna is discussed in Chapter 5. Simulation and experimental results for the antenna are performed to support the analysis. Chapter 6 presents a few design variations of the proposed antenna for various wireless commercial applications. Conclusions and future investigations are then presented in Chapter 7.

1.3

References

[1]

IT-Specific Encyclopedia. Available at http://whatis.techtarget.com.

4

Chapter 2

Literature Review 2.1

Introduction

This chapter presents a literature review on electrically small antennas, antennas that are a fraction of a wavelength in maximum extend. With the current trend in technology and consumer electronics that demand smaller wireless devices, small antennas are a hot research topic. However, there is a fundamental limitation on how much antenna size can be reduced. In this chapter, a review of the research on fundamental limits of small antennas is also introduced, explaining how the physical size of antennas is related to impedance bandwidth and radiation efficiency. Several techniques for reducing antenna size are also presented, including the use of a short-circuit, geometry optimization, material loading, and the environment surrounding the antenna. These techniques can reduce antenna size but with a penalty to pay, which is often the reduction of impedance bandwidth. Therefore, a few methods for enhancing impedance bandwidth of small antennas are reviewed.

2.2

Electrically Small Antennas

There are many reasons for using electrically small antennas, including: esthetics, safety, cost, aerodynamics, and physical size constraints due to system considerations. There is 5

no consensus on the definition of ‘electrically small’. An electrically small antenna is one whose size is a small fraction of a wavelength. A maximum dimension of λ/30 is often adopted for electrically small antennas [1]. This criterion was probably first chosen because it permits the use of two approximations: constant current for small loop and a linear current distribution for short monopole or dipole antennas. However, according to H.A. Wheeler [2], an electrically small antenna is one that is smaller than the radiansphere, which is a spherical volume having a radius of λ/2π, or 0.16 λ. The surface of the radiansphere forms the boundary between the near field and the far field. Interior to the sphere (the near filed), the stored energy dominates over the radiating energy. In current practice, it is popular to use a maximum extent criterion of λ/8 for electrically small antennas. The analysis of electrically small antennas was initiated in the mid-1940s by H.A Wheeler [2]. Wheeler used the radiation power factor, PFRAD, to quantify the radiation of an antenna. PFRAD is defined as the ratio of the radiated power to the stored power. This corresponds to the ratio of the antenna resistance R to the antenna reactance X and is proportional to the volume occupied by an antenna. Using a simple lumped circuit, he deduced that this ratio is equivalent to the bandwidth multiplied by the efficiency, in cases where the antenna is matched to the tuned circuit. This early paper was the first attempt to confirm mathematically the intuition we have that the product of (efficiency × bandwidth) is directly related to PFRAD, hence, to the volume occupied by the antenna, since this product is equal to the ratio of the antenna resistance to the antenna reactance. According to Wheeler’s formulation, electrically small antennas can be characterized by their low radiation resistance and large reactance (small PFRAD), and thus, small impedance bandwidth and low efficiency. The worldwide growth of personal wireless communication devices has been tremendous. One of the trends in wireless mobile technology in the last decade has been to dramatically decrease the size and the weight of the handset. With this progress in mobile terminal size reduction, the design of antennas is acquiring even greater importance. Antennas must be small, and yet achieve specified electrical performance, such as wide bandwidth, operation in dual or triple frequency bands, diversity, and so forth. Accordingly, antenna designers have encountered difficulty in designing antennas 6

that can maintain electrical performance characteristics while being reduced in size because, in general, efficiency and bandwidth degrade with size decrease. The next sections discuss how much an antenna can be reduced in size.

2.3

Fundamental Limitations on the Radiation Q of Small Antennas

The radiative properties of electrically small antennas were first investigated by Wheeler [2]. Later, Chu [3] derived an expression for the minimum radiation quality factor Q of an antenna enclosed inside a sphere of a given radius. The quality factor Q is defined as the radian frequency ω times the ratio of the reactive energy stored about the antenna to the radiated power [9]. It is a measure of the reactive energy that necessarily accompanies real output power. In 1960, Harrington [4] extended Chu's theory to include circularly polarized antennas. Later, Collin [5] and Fante [6] derived exact expressions for the radiation Q based on the evanescent energy stored around an antenna. McLean [7] corrected an error in Chu's approximate expression and re-derived the exact expression using non-propagating energy. Most recently, Caswell and Davis [8], and Grimes and Grimes [9] re-derived the fundamental limit on radiation Q using a time-domain approach. Their work is summarized in the following section to show the concept of the fundamental limit applied to practical small antennas.

2.3.1

Overview of Theoretical Investigations on the Fundamental Limits

There have been numerous theoretical investigations of antenna size and performance over the past five decades. Reducing antenna volume generally degrades antenna performance. It is, therefore, important to examine the fundamental limits and parameter tradeoffs involved in size reduction. Certainly at some point, electrical performance specifications will not be satisfied if the allocated volume for the antenna region is reduced too much. In particular, it is well known that size reduction is obtained at the

7

expense of bandwidth and efficiency. However, this size-performance trade-off must be quantified in order to have useful guidelines in the search for more compact antennas. The concept of Chu's work [3] was to place an antenna inside a sphere of radius a that it just encloses the antenna and then to represent the fields outside the sphere as a weighted sum of spherical wave modes. Then, radiation Q can be computed using [3]  2ωWe  P Q =  rad 2ωWm   Prad

We > W m

(2.1) W m > We

where We and Wm are the time-average, non-propagating, stored electric and magnetic energy, respectively, ω denotes radian frequency, and Prad denotes radiated power. Chu assumed that all of the stored energy is outside the sphere enclosing the antenna. This concept leads to the minimum possible radiation Q since any stored energy inside the sphere would increase the radiation Q of the antenna. However, the calculation of this radiation Q is not straightforward because the total time-average stored energy outside the sphere is infinite, just as it is for any propagating wave or combination of propagating waves and non-propagating fields [3]. Chu addressed this by deriving an RLC equivalent ladder network for each spherical waveguide mode and then radiation Q is computed from the stored energy in the inductors and capacitors of the equivalent circuit network. However, this is still a tedious computation if many modes exist. So, Chu approximated the system as an equivalent second-order series RLC network and solved the problem assuming that the antenna only excites the n = 1 mode. The resultant radiation Q expression is

1 + 2(ka ) (ka )3 1 + (ka )2 2

Q=

[

(2.2)

]

where the algebraic error cited by McLean [7] has been accounted for, k is the wave number associated with the electromagnetic field and a if the radius of the sphere enclosing the antenna. Chu also showed that an antenna which excites only the n = 1 mode has the lowest possible radiation Q of any linearly polarized antenna.

8

MacLean [7] re-examined this fundamental limit in order to achieve the highest possible accuracy. McLean presented an exact expression for the minimum radiation Q. Like Chu, he assumed that the antenna radiates only one mode, in this case the n = 1 spherical mode. The fields of this mode are equivalent to the fields radiated by a short dipole antenna. He computed the stored energy due to the total fields and then subtracted the stored energy due to the radiated fields, leaving only the non-propagating stored energy from which the radiation Q is determined. The resulting expression for Q is

Q=

1 1 + 3 (ka ) ka

(2.3)

This relationship between the radiation Q and the antenna size represented by ka is plotted in Fig. 2-1. The family of curves includes the dependence of antenna radiation efficiency, er. Therefore, the plot in Fig 2-1 shows the family of curves from  1 1 er  +  . The top curve is the plot of (2.3) and is for 100% radiation efficiency 3 ka   (ka )

(er=1).

The vertical axis is logarithmic. So as ka increases, Q decreases rapidly,

indicating a strong relationship between antenna size and radiation Q. Figure 2-1 shows also how McLean formulation in (2.3) differs from Chu’s (2.2). For electrically small antennas with ka less the 0.5, the Q values from (2.2) and (2.3) are the same. Bandwidth is an important parameter of interest for antennas and is related to the radiation quality factor Q. From a terminal standpoint the antenna can be viewed as a circuit device. Viewing the antenna as a resonant, parallel RLC circuit, fractional bandwidth is simply the inverse of Q, BW3dB = 1 / Q

(2.3)

BW3dB denotes the 3-dB bandwidth of the antenna and is defined as BW3dB =

fU − f L fC

(2.4)

where f U and f L correspond to the upper and lower 3-dB points and f c is the center frequency, f C = ( f U + f L ) / 2 , of the frequency band of interest. The 3-dB point is where the signal is 3 dB below its peak value I the band from fL to fU. Once bandwidth is determined, Q can be found from (2.3). 9

2

10

McLean Chu

1

Q

10

e r =1 0

10

e r =1 e r =0.5

e r =0.1

-1

10

0

0.2

0.4

0.6

0.8

1 ka

1.2

1.4

e r =0.25 1.6

1.8

2

Figure 2-1 Fundamental limit curves of radiation Q versus ka. The top curve

(solid curve) is a plot of (2.3) and is for efficiency, er = 1 , or 100% . The dashed curve is the plot of (2.2) with efficiency er = 1 . In practice, antenna bandwidth is often defined in terms of VSWR values. Because

Q is defined by the 3 dB points of the equivalent circuit model, Q is not just the inverse of the VSWR bandwidth, as in (2.3). The relationship between Q and VSWR is determined as follows [11]: Q=

VSWR − 1 BWVSWR VSWR

(2.5)

For VSWR = 2, (2.5) reduces to Q=

1

(2.6)

2 BWVSWR = 2

The family of fundamental limit curves in Fig. 2-1 can be explained as follows. A point for an antenna is located on the graph using the Q value for the antenna and its ka 10

value at midband, fc. The enclosing radius, a , of an antenna includes any images of the antenna in a ground plane. Realizable antennas have Q values above the curve corresponding to the efficiency of the antenna, er . That is, points corresponding to realizable antennas must lie above the curves. The curves are also used to find the size limit for a given bandwidth.

2.3.2

Fundamental Limitations of Electrically Small Antennas

The fundamental limits presented in the previous section are based on theory and several assumptions are involved. It is therefore important to compare this theory to actual antennas in order to see if the fundamental limits are also useful for practical antennas. We present results of such a study in this section. The antennas of Table 2-1 used in the comparison were found in the literature or were fabricated at the Virginia Tech Antenna Laboratory. Figure 2-2 shows the bandwidth-size performance values for the practical antennas listed in Table 2-1 along with the fundamental limit curve based on (2.3) for 100 % radiation efficiency. To achieve small size, it is important for a point on the plot to be as close to the desired efficiency curve as possible. A point that falls on the curve indicates that the antenna has achieved maximum bandwidth for its size. The example antennas shown in Fig. 2-2 represent a wide variety of antennas and none exceed the fundamental limits. Thus, we can conclude that the fundamental limits appear to provide realistic limits.

11

Table 2-1 Characteristics of Antennas Used to Examine Bandwidth-Size

Relationships. Center Frequency

Bandwidth

Radius of Enclosing

(fc)

(VSWR