BROADBAND ELLIPTICAL DIELECTRIC RESONATOR ANTENNA

using CAD files of commercial telephones and the parallelized FDTD code, IEEE Trans Antennas Propagat 46 (1998), 829 – 833. 4. R.F. Harrington, Field computation by moment method, Macmillian, New York, 1968. 5. A. Taflove and S.C. Hagness, Computational electrodynamics: The finite-difference time-domain method, Artech House, Boston, 2000. 6. G.J. Burke and A.J. Poggio, Numerical electromagnetic code (NEC)– method of moments: A user-oriented computer code for analysis of the electromagnetic response of antennas and other structures, Lawrence Livermore Laboratory, 1981. © 2005 Wiley Periodicals, Inc.

BROADBAND ELLIPTICAL DIELECTRIC RESONATOR ANTENNA

Figure 5 Measured electric-field values (right-side scan and left-side scan) vs. the field-probe position (real model) for a 55-mm linear scan along the y-axis without (SC-1) and with (SC-2) the eyeglasses (here, the cellular is placed horizontally; the abscissa origin corresponds to the lens plane). – – right eye (SC-1), – – left eye (SC-1), –■– right eye (SC-2), – – left eye (SC-2). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

P. V. Bijumon,1 Sreedevi K. Menon,2 M. N. Suma,2 B. Lethakumary,2 M. T. Sebastian,1 and P. Mohanan2 1 Ceramic Technology Division Regional Research Laboratory Trivandrum– 695 019, India 2 Centre for Research in Electromagnetics and Antennas (CREMA) Department of Electronics Cochin University of Science and Technology Kochi– 682 022, India Received 13 July 2005

strong enhancement of the electric-field values in SC-2 as compared with the ones obtained without the eyeglasses (that is, SC-1). The results obtained from the two scans (corresponding to the left and right eye lines, respectively) are reported in Figure 5. For the right eye (the one closer to the antenna), which is supposed to be positioned about 20 mm from the lens plane, we have measured a field value of 78.3 V/m, corresponding to 2.12 times the value observed without the eyeglasses. The left eye, on the contrary, exhibits a value of 48.4 V/m, which is more than 2.86 times above the value in the absence of eyeglasses. These values are expected to be different from those which can be measured in the real scenario, where the user’s head is considered. However, also in this case, we anticipate a field increase in the ocular region, induced by the presence of the eyeglasses with metallic frames. CONCLUSION

The enhancement induced by metallic eyeglasses on the e.m. field emitted by a cellular telephone has been investigated numerically and experimentally in a simplified scenario, where we neglect any e.m. effect arising from the presence of the user’s head tissues. A significant local increase of the electric field has been detected. This can be especially relevant in the eye region, where hazardous situations can be envisaged. ACKNOWLEDGMENT

Part of this work has been sponsored by MIUR. REFERENCES 1. Q. Balzano, O. Garay, and T. Manning, Electromagnetic energy exposure of the users of portable cellular telephones, IEEE Trans Vehic Technol 44 (1993), 390 – 402. 2. J. Toftgard, S.N. Hornsleth, and J.B. Andersen, Effects on portable antennas of the presence of a person, IEEE Trans Antennas Propagat 41 (1993), 739 –746. 3. A.D. Tinniswood, C.M. Furse, and O.P. Gandhi, Computations of SAR distributions for two anatomically based models of the human head

DOI 10.1002/mop

ABSTRACT: A broadband low-loss elliptical dielectric resonator antenna (DRA) excited by using the microstrip-line technique is proposed. The effect of geometrical modifications of the microstrip feed line on the gain, bandwidth, and radiation performances of the antenna has also been investigated. The proposed antenna offers enhanced radiation characteristics compared to conventional microstrip-patch antennas. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 65– 67, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21262 Key words: dielectric resonators; dielectric resonator antennas; bandwidth enhancement; Ca5Nb2TiO12 ceramics 1. INTRODUCTION

Ceramic dielectric resonator antennas (DRAs) have inherent advantages such as high radiation efficiency and large impedance bandwidth due to their lower ohmic loss compared with conventional microstrip-patch antennas. The resonant frequency and operating bandwidth of a DRA can be easily controlled by choosing the dielectric constant and dimensions of the dielectric resonator (DR) material. DRAs can be of any shape and exhibit many resonant modes [1] which have different radiation characteristics. L- and T-shaped microstrip-line excitation has been employed for enhancing the bandwidth of microstrip-patch antennas [2] and the same feeding has been successfully incorporated in cylindrical DRAs [3]. In this paper, a microstrip-line-excited elliptical DRA is fabricated with a DR of ␧ dr ⫽ 48 and investigations are also being carried out to study the effect of feed-geometry variations on the gain, bandwidth, and radiation performances of the antenna. 2. ANTENNA GEOMETRY

The antenna and feed configurations studied are shown in Figure 1. The geometry comprises an elliptical dielectric disk resonator of Ca5Nb2TiO12 material [4] synthesized via the conventional solidstate ceramic route. The DR of relative permittivity ␧ dr ⫽ 48, major axis dimension 2a ⫽ 33 mm, minor axis 2b ⫽ 27 mm, and

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Figure 1 (a) Microstrip excitation of the cylindrical dielectric resonator; (b) top view of the L-strip feed; (c) Top view of the T-strip feed

Figure 3 Variation of measured S 11 with frequency in simple L-feed and T-feed elliptical DRAs

height H ⫽ 11.5 mm is energized via microstrip-line methods. In Figure 1(a), the DR is electromagnetically coupled with a 50⍀ microstrip feed line of width 3 mm and length S 1 ⫽ 50 mm, fabricated on a substrate of dielectric constant ␧ r ⫽ 4.28 and thickness h ⫽ 1.6 mm, and backed by a conducting plane. The geometry of the microstrip line is modified into L- and T-shapes, as shown in Figures 1(b) and 1(c), respectively, in order to study its effect on the antenna characteristics of DR.

S 2 ⫽ 35 mm, and then decreases gradually. For the T-shaped feed geometry, the bandwidth shows a maximum value for S 2 ⫽ 30 mm. Variation of the % bandwidth with feed-segment length for both the modified feeds is shown in Figure 2. For both feed geometries, the antenna has maximum impedance bandwidth when the DR was fixed with its geometric center at the junction of the feed line (S 1 ⫽ 50 mm and S 2 ⫽ 0 mm). The return-loss characteristics of the elliptical DRA excited using L- and T-shaped strip-line mechanisms are plotted in Figure 3. With L-shaped feed excitation, the antenna resonates at 1.9 GHz within an operation band of 230 MHz (1.81–2.04 GHz, 12.11%). In the case of the elliptical DRA energized with the T-shaped feed line, the operation band is 325 MHz (2.075–2.40 GHz, 14.77%) at 2.2 GHz. It is noteworthy that the resonant frequency as well as the % bandwidth increases with branching of the feed-line geometry. Figures 4(a),

3. EXPERIMENTAL RESULTS

The position of the DR on a conventional microstrip feed line was optimized for maximum bandwidth. A very good impedance match was obtained when the DR was placed symmetrically with its major axis perpendicular to the feed line. A bandwidth of 10% (1.76 –1.945 GHz) at 1.85 GHz was obtained, as depicted in Figure 3. Based on the single-strip design, the simple microstrip feed line was branched into L- and T-shaped feed lines. For the L-shaped feed, the feed length S 1 is fixed at 50 mm and the segment length S 2 is varied from 0 to 40 mm. The segment length S 2 was optimized for maximum impedance bandwidth. The % bandwidth of the elliptical DRA excited with the L-shaped microstrip line increases with feed-segment length, reaches a maximum when

Figure 2 Variation of % bandwidth with arm length (S 2 ) in L-feed and T-feed elliptical DRAs

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Figure 4 Measured radiation pattern of the proposed elliptical DRA at the center frequency: (a) simple feed; (b) L-feed; (c) T-feed

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DOI 10.1002/mop

TABLE 1 Experimental Results of Elliptical DRA with Different Feed Geometries Feed Geometry

Resonant Frequency [GHz]

2:1 VSWR Bandwidth [%]

Gain [dBi]

E-Plane

H-Plane

Microstrip line L-Feed T-Feed

1.85 1.90 2.20

10.00 12.11 14.77

8.0 9.2 6.5

115° 110° 120°

90° 85° 100°

HPBW [degrees]

4(b), and 4(c) illustrate the measured field patterns at the center frequency of the simple, L-feed, and T-feed excited elliptical DRAs, respectively. The radiation and reflection characteristics of the elliptical DRA energized with various geometries of microstrip-line feed are summarized in Table 1. From the experimental observations, it is found that the elliptical DRA with simple microstrip feed offers moderate gain and bandwidth. It is also noted that L-feed excitation offers high gain and high bandwidth. The bandwidth is found to be larger for T-feed excitation, but the gain is poor. Independent of the geometry of the microstrip feed line employed here, the gain and bandwidth of the elliptical DRA is reasonably high, as compared with that of conventional patch antennas. 4. CONCLUSION

In this paper, an elliptical dielectric resonator antenna (DRA) energized with microstrip feed lines of different geometries has been investigated. The experimental results show that the resonant frequency as well as the operating bandwidth increases with an increase in the branching of the strip line feed. The good radiation performance, wide bandwidth, and exceptionally high gain make the proposed elliptical DRA a more suitable candidate for mobile communication and Bluetooth applications. ACKNOWLEDGMENTS

The authors acknowledge the Council of Scientific and Industrial Research (CSIR), India, and the Kerala State Council for Science Technology and Environment (KSCSTE), India, for providing financial assistance. REFERENCES 1. R.K. Mongia and P. Bhartia, Dielectric resonator antennas: A review and general design relations for resonant frequency and bandwidth, Int J Microwave and Millimeterwave CAE 4 (1994), 230 –247. 2. B. Lethakumary, S.K. Menon, C.K. Aanandan, K. Vasudevan, and P. Mohanan, L-strip excited wideband rectangular microstrip antenna, Microwave Opt Technol Lett 35 (2004), 235–236. 3. S.K. Menon, B. Lethakumary, P. Mohanan, P.V. Bijumon, and M.T. Sebastian, Wideband cylindrical dielectric resonator antenna excited using an L-strip feed, Microwave Opt Technol Lett 42 (2004), 293–294. 4. P.V. Bijumon, P. Mohanan, and M.T. Sebastian, Synthesis, characterization and properties of Ca5A2TiO12 (A ⫽ Nb, Ta) ceramic dielectric materials for applications in microwave telecommunication systems, Jpn J Appl Phys 41 (2002), 3384 –3385. © 2005 Wiley Periodicals, Inc.

DOI 10.1002/mop

SOURCE-GROUP METHOD TO SPEED UP THE RECONSTRUCTION OF OBJECTS FROM RADAR DATA BY USING THE FBTS METHOD Dongling Qiu,1 Hui Zhou,2 Takashi Takenaka,2 and Toshiyuki Tanaka3 1 College of Chemistry and Chemical Engineering Ocean University of China Qingdao 266003, Shandong, China 2 Department of Electrical and Electronic Engineering Nagasaki University 1-14 Bunkyo-machi Nagasaki 852-8521, Japan 3 Graduate School of Science and Technology Nagasaki University 1-14 Bunkyo-machi Nagasaki 852-8521, Japan Received 7 July 2005 ABSTRACT: We have proposed a time-domain forward-backward time-stepping (FBTS) method for reconstructing 3D structures in highly absorptive media. The reconstruction speed is greatly dependent on the number of transmitters. In this paper, we propose a source-group method to speed up the reconstruction. In the source-group method, multiple transmitters arranged at different positions are excited simultaneously, and receivers collect the wave fields. To compare the reconstruction results, three kinds of reconstructions from 16 conventional single transmitter-multiple receiver data sets, four source-group multiple-receiver data sets, and four conventional single-transmitter multiplereceiver data sets are carried out. Reconstruction by the source-group method is several times faster than (and the reconstructed results are almost the same as) those of the first kind of reconstruction, and they are much better than those of the third kind. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 67–71, 2006; Published onlinePublished online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21263 Key words: borehole radar; forward-backward time-stepping method; inverse scattering; source group method INTRODUCTION

Borehole radar has been applied to various fields, such as the characterization of hydraulically conductive fracture zones [1], geological-discontinuity detection [2], archaeological investigation [3], fracture evaluation and saline-tracer monitoring [4], sulfide-deposit survey [5], strata recognition using direct waves [6], and cavity imaging using both direct waves and later arrivals [7]. There are two general ways to utilize borehole radar data for investigating the subsurface. One is signal processing and imaging, and the other is inverse scattering. The direct waves of borehole radar data are normally used for velocity or attenuation tomography [7, 8], and the later arrivals are used for migration imaging [7]. If the later arrivals are used to obtain information about the media between boreholes, the direct waves should be separated from the recorded data. It is not easy to separate the direct waves completely from the reflections or diffractions originated from anomalous bodies, since these waves are usually superimposed on the direct waves. We have developed a forward-backward time-stepping (FBTS) method, which can use total wave fields, including direct couplings and reflections or diffractions, for reconstructing 3D distributions of permittivity and conductivity [9, 10], and have applied it to the reconstruction of objects from borehole radar data [11]. The reconstruction cost of the FBTS method greatly depends on the number of transmitters. Generally, increasing the number of

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