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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 3055 Compact Six-Sector Antenna Employing Three Intersecting Dual-Beam Microstrip Yagi Uda Arrays With Common Director Naoki Honma, Member, IEEE, Tomohiro Seki, Member, IEEE, Kenjiro Nishikawa, Member, IEEE, Koichi Tsunekawa, Member, IEEE, and Kunio Sawaya, Senior Member, IEEE Abstract A novel compact planar six-sector antenna suitable for wireless terminal is presented. Our new antenna design yields low-profile and extremely compact multisector antennas since it allows microstrip Yagi Uda array antennas to share director elements. Two microstrip Yagi Uda array antennas are combined to form a unit linear array that has feed elements at both ends, only one of which is excited at any one time, and the other is terminated. A numerical analysis shows that applying a resistive load to the feed port of the terminated element is effective in reducing the undesired radiation. The radiation pattern of a six-sector antenna, which employs three intersecting unit linear arrays, is investigated, and the optimum termination condition is identified. From the results of measurements, it is found that our antenna achieves high gain, at least 10 dbi, even though the undesired radiation is suppressed. It is shown that the radiation pattern suitable for a six-sector antenna can be obtained even with a 1.83 wavelength diameter substrate, i.e., the area is 75% smaller than that of a regular antenna with six individual single-beam microstrip Yagi Uda arrays in a radial configuration. Index Terms Microstrip antennas, mobile antennas, multisector antenna, small antennas, Yagi Uda arrays. I. INTRODUCTION STUDIES on high-speed indoor wireless access systems, which use the microwave band or the millimeter band, are being actively pursued [1], [2]. These systems have to reduce the effect of multipath fading in order to maintain high data transmission speeds. Narrow-beam antennas and adaptive arrays are solutions that can overcome this problem and the effects of these antennas in the wireless communication systems have been studied [3] [6]. Narrow-beam antennas are the easiest way to improve the data transmission speed because a fixed beam antenna can be used and they achieve relatively high gain. A multisector antenna comprising several narrow-beam antennas with different beam directions is one way of providing wireless connections with high data transmission speed because the terminals or the access points can switch the beam direction so as to direct the beam to the desired signal [7] [11]. Mobile terminals usually utilize only one sector of the multisector antennas at any one moment because only one connection to one access point is necessary. This demands the use of sector switching mechanisms. A sector antenna with too few sectors Manuscript received September 17, 2005; revised April 7, 2006. The authors are with NTT Network Innovation Labs, NTT Corp., Kanagawa 234-0847, Japan (e-mail: honma@m.ieice.org). Digital Object Identifier 10.1109/TAP.2006.883980 provides only relatively low gain enhancement effect, which may be canceled by the loss of the sector switching mechanism in the feed circuit. On the other hand, a sector antenna with many sectors requires a complicated multiport sector switching mechanism. Therefore, terminals are likely to use four to six sectors [7], [10], [11]. Moreover, the multisector antennas for mobile terminals must be small and low-profile since the downsizing of the recent mobile terminal demands enhanced portability. A low profile multisector antenna can be fabricated using radially arranged microstrip Yagi Uda arrays [12]. The microstrip Yagi Uda array comprises an exciting microstrip antenna and several parasitic microstrip antennas arranged on the same substrate surface, and it offers high gain [13]. Furthermore, this antenna can be fabricated by using etching techniques which that supports mass production. However, this antenna usually requires a large substrate area because each sector requires an individual array antenna. The other problem of the microstrip Yagi Uda array is the difficulty in obtaining sufficient front-toback ratio (F/B). The high level of undesired radiation degrades the data transmission speed [5]. In [14], we described a dual-beam microstrip Yagi Uda array that has a linear parasitic element array and two exciting elements at the ends of the array. The idea of this paper is that the parasitic elements are shared between two opposite facing microstrip Yagi Uda arrays, and the size of the array antenna can be reduced to almost half that of two individual microstrip Yagi Uda arrays. When one of the two exciting elements is driven, this antenna works as a microstrip Yagi Uda array, and the beam extends away from the driven element. Switching between the two exciting elements of this single linear array yields two beams in opposite directions. Moreover, low F/B levels are available if the optimum termination condition is realized at the quiescent element when the other element is excited. This antenna can achieve both strong size reductions and undesired radiation suppression, however, only two beams share the parasitic elements, and there is room for further size reductions when we apply this technique to six-sector antenna. In this paper, a new compact planar six-sector antenna, based on a microstrip Yagi Uda array with common director elements, is proposed that suits mobile terminals. To obtain six sector beams, three dual-beam microstrip Yagi Uda arrays, shown in [14], are employed. In order to achieve a drastic size reduction, the parasitic elements are to be shared not only by two beams but by all six beams. This is made possible by placing a hexagonal parasitic element at a center of the linear array, and the 0018-926X/$20.00 2006 IEEE

3056 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 three unit linear arrays intersect at this hexagonal element. The optimized termination condition at the feed ports of the quiescent exciting elements is employed to improve the effect of the coupling among the intersecting linear arrays. These techniques of element sharing and feed port termination yield an extremely compact six-sector antenna. Section II briefly reviews the basic idea of the dual-beam microstrip Yagi Uda array antenna, in which the two exciting elements share parasitic elements. Section III describes the geometry and mechanism of the proposed six-sector antenna, in which the parasitic elements are shared among six beams. Section IV shows numerical analyses of the proposed six-sector antenna. The impact of the intersecting linear arrays on the radiation pattern of a six-sector antenna under various termination conditions is investigated. In Section V, the experiments conducted on a fabricated six-sector antenna are described to confirm the validity of the proposed design. The antennas discussed in this paper are designed to operate around the 5 GHz band, which is used in recent wireless local-area networks (LANs) and is being considered for next-generation mobile communications. The 25 GHz band is also being considered for advanced wireless LANs because more frequency channels will be needed to offer high-speed data transmission to many more users [11], [15]. Though the proposed antenna can be used at much higher frequencies, such as 25 GHz, this paper discusses only 5 GHz band operation since the intention here is to confirm the basic performance of our antenna with little manufacturing error. II. REVIEW OF DUAL-BEAM MICROSTRIP YAGI UDA ARRAY WITH COMMON PARASITIC ELEMENTS A. Basic Geometry and Mechanism In order to explain the six-sector antenna, the property of the unit linear array, which is the unit component of this antenna, is reviewed. We mainly examine the influence of the termination condition of the quiescent element and the number of parasitic elements. Fig. 1(a) and (b) shows the geometries of a conventional microstrip Yagi Uda array and a dual-beam microstrip Yagi Uda array, respectively. The microstrip antenna elements are formed on a dielectric substrate, which has ground metallization on the back side; the microstrip antenna elements are rectangular patches. Each array antenna has parasitic elements aligned on the -axis. The exciting element with the feed port at the end of the array is slightly larger than the parasitic elements. In the figure, and are the long and short side lengths of the exciting element, respectively, and are the long and short side lengths of each parasitic element, respectively, is the gap between elements, is the thickness of the substrate, and are the long and short side lengths of the substrate, and and are defined as the relative dielectric constant and the loss tangent of the substrate, respectively. The operating frequency is the 5 GHz band, and the dimensions and material properties are set to,,,,,,, and, where is wavelength in vacuum. The short side length of the rectangular substrate is set to, and, which is the Fig. 1. Unit linear array configurations: (a) top view of conventional microstrip Yagi Uda array and (b) top and side views of dual-beam microstrip Yagi Uda array. long side length, is given so that the distance between the edge of the substrate and the center of the end element in the linear array is 4. The dual-beam microstrip Yagi Uda array has two exciting elements, one at each end of the linear array. Each exciting element has a feed port, but only one port is driven at any one time. That is, when one port is excited, the other port is terminated. In these antennas, the beam extends away from the exciting element over the parasitic elements, which work as a director. Therefore, the antenna shown in Fig. 1(a) can work as a single-beam Yagi Uda array, and the antenna shown in Fig. 1(b) can work as a dual-beam Yagi Uda array by switching the feed ports. B. Numerical Analysis The impact on antenna performance of the port termination condition and the number of parasitic elements was numerically analyzed by the method of moments. 1 In this numerical analysis, only the dielectric layer of the substrate is assumed to be an infinite plane in order to simplify the analysis. The positions of the feed ports were optimized to minimize the reflection of the incident power in both models. The impedance of the terminated port #2 is set to the characteristic impedance of the feed line, i.e.,. To evaluate the radiation performance as a planar sector antenna, we defined the F/B and the conical radiation pattern. In this paper, the F/B is defined by the ratio of the directivity of the direction of maximum radiation to that of the direction of the maximum lobe in the range of 60 from the opposite direction. However, since the directions of both the main lobe and the back lobe are tilted above the ground plane [12], [13], the F/B obtained in the general way is underestimated. For this reason, 1 http://www.zeland.com.

HONMA et al.: COMPACT SIX-SECTOR ANTENNA EMPLOYING THREE INTERSECTING 3057 Fig. 2. New definition of F/B. (From [14, p. 1365].) Fig. 4. Radiation patterns of unit array in xz-plane. Fig. 3. Definition of conical-plane beam width. we created a new definition of F/B, suitable for the configuration in Fig. 1. Fig. 2 shows our definition of F/B. The two cones are symmetrical and share the same apex. The main lobe is lies inside one cone. The F/B is defined as the ratio of the maximum level in one cone to that in the direction within the other cone. The definition of the conical plane radiation pattern is shown in Fig. 3. A conical plane including the direction of the peak radiation is assumed. The conical plane radiation pattern can be defined by the pattern obtained from cutting the three-dimensional pattern with an ideal conical plane. By means of using the conical plane radiation pattern, the conical-plane beam width can be defined as the range of the angle that holds the gain decrease from the maximum gain to within 3 db. The radiation patterns were calculated while varying the termination condition so as to find the their effect on the F/B improvement. Fig. 4 shows the radiation pattern of a linear array with three parasitic elements in the -plane when the termination condition of port #2 is short, open, and. It can be seen that the back lobe has high level when is short and open, and that the back lobe is reduced only when. This is because the reflection of the traveling wave at the terminated port is suppressed by the termination load. In order to find the optimum termination condition, the relationship between the radiation pattern and the impedance was calculated. Fig. 5 shows the actual gain, F/B, directivity, and termination loss versus the termination load, where the number of Fig. 5. Radiation properties versus load resistance: (a) actual gain and F/B and (b) directivity and loss caused by termination load. parasitic elements is three. It can be seen that F/B is maximum when is around. The improvement in F/B compared to the cases of short and open termination is more than 8 db. It is also found that the directivity and termination loss are maximum when is around. By considering the slight gain difference over the range of the termination load condition shown in this graph, this can be explained by the load impedance of, which is almost the same as the impedance of the feed line, not attenuating the main-lobe radiation but the undesired radiation caused by the reverse traveling wave, which comes up at the end of the line array. From the calculations described above, it is found that applying a termination load to the quiescent feed port is effective in enhancing F/B without incurring any gain reduction.

3058 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Fig. 6. Beam width in conical plane versus number of director elements. (From [14, p. 1366].) Fig. 8. Radiation patterns in conical plane ( =45 ): (a) pattern of six-sector antenna with 50 termination at all ports and (b) pattern of single linear array in Fig. 1(b) (m =3). Fig. 7. Geometry of planar six-sector antenna. In order to design the sector antennas, we clarified the beam width of the unit linear array. Numerical results of the conical-plane beam width versus the number of parasitic elements are shown in Fig. 6, where port #2 is terminated by a resistor of. It is found that the beamwidth of the proposed linear array is narrower than that of the conventional array. This means the element terminated by works as a director element too. It is also shown that the linear array with three parasitic elements has almost 60 beam width, making it suitable for a six-sector antenna. III. GEOMETRY AND MECHANISM OF SIX-SECTOR MICROSTRIP YAGI UDA ANTENNA WITH COMMON PARASITIC ELEMENTS Fig. 7 shows the configuration of the six-sector microstrip Yagi Uda antenna on a circular substrate. The substrate has ground metallization on the backside. Three unit linear arrays intersect at the center at an angle of 60. Therefore, the shape of the central parasitic element, which is shared by all three linear arrays, is hexagonal. All of the parasitic elements are electrically smaller than the exciting elements. In the figure, is the width of the central hexagonal parasitic element and. The diameter of the substrate is when the distance between the edge of the substrate and the center of the exciting elements is set to 4. The other dimensions and material parameters shown in Fig. 7 are the same as those described in Section II. The substrate area is only 25% of that of a conventional antenna with six individual single beam microstrip Yagi Uda arrays in a radial configuration. It is also much smaller than the antennas that use the technique described in [9] and [10] The key idea of this antenna is the radiation pattern forming provided by controlling the port termination condition at all quiescent elements. For example, consider what happens when element #1 is excited and all other elements are terminated and thus quiescent. Element #1 is strongly coupled to its adjacent parasitic element, the central common parasitic element, and finally to element #4. Therefore, this linear array also works as a microstrip Yagi Uda antenna, whose beam direction is from #1 to #4. However, this linear array is strongly coupled to the two other intersecting linear arrays, which causes undesired radiation. This undesired radiation can be suppressed by changing the termination of the quiescent elements, such as #2, #3, #5, and #6. This issue is discussed below. IV. NUMERICAL ANALYSIS OF SIX-SECTOR ANTENNA A. Effect of Coupling Between Intersecting Linear Arrays In the six-sector antenna shown in Fig. 7, there is strong coupling among the three linear arrays, which impacts the radiation pattern, because all arrays share the central parasitic element. The calculated conical radiation pattern of the antenna in Fig. 7 is shown in Fig. 8. In this calculation, all of the terminated ports are connected to loads of, where the infinite ground plane and substrate are assumed for simplification of the calculation. It can be seen that the beam width is much wider than that of the single linear array. The actual gain and F/B of the six-sector antenna are lower by 1 and 4 db than those of the single linear array, respectively. This means that the two other linear arrays resonate and generate undesired radiation, which degrades the radiation pattern. Table I shows the scattering parameter of six ports. It can be seen that the transmitted power from #1 to #4, and that from #1 to #2 or #6, have almost the same level, which indicates that the power fed to port #1 is distributed

HONMA et al.: COMPACT SIX-SECTOR ANTENNA EMPLOYING THREE INTERSECTING 3059 TABLE I CALCULATED S-PARAMETERS IN A SIX-SECTOR ANTENNA Fig. 10. Actual gain, F/B, and conical-plane beam width versus line length l. Fig. 9. Circuit configuration with finite lines and SP3T switches. not only to the excited linear array but also to the intersecting linear arrays at high levels. Moreover, it is found that the total power consumed by the termination loads is at least 20% of the fed power. Therefore, the gain and F/B are strongly affected by the presence of the intersecting linear arrays. B. Radiation Pattern Improvement Using Reactive Termination Load In order to improve the radiation pattern, other termination conditions were examined because the power consumed at the feed ports can be utilized by changing the termination conditions. Termination with a pure reactive load fully reflects the incident power at the feed ports without any power consumption, and the reflection phase can be controlled by changing the reactance of the load. Arbitrary reactance values can be easily obtained by attaching line stubs of various lengths. Fig. 9 shows the model of the six-sector antenna with lines of length. Each line has a single pole three throw (SP3T) switch at its end and the switch connects the end of the line to either a signal source, resistive termination, or open termination. When one port is excited, the port at the opposite end of the array is connected to the termination load to reduce the back lobe, and all other ports are set to open. In order to obtain a symmetric conical radiation pattern with control of the termination conditions, all lines should have the same length. When the end of the line is open, the line works as an open stub, and its impedance observed at the port is expressed by (1) Fig. 11. js j versus frequency in six-sector antenna: (a) measurement and (b) calculation. Fig. 12. Loss caused by termination versus frequency in six-sector antenna: (a) measurement and (b) calculation. where is the characteristic impedance of the line, which is equal to the impedance of the signal source, and is the phase constant in the line. Fig. 10 shows the calculated actual gain and the beam width as functions of line length. For simplification, the substrate and ground metallization are assumed to be infinite planes. It can be seen that the actual gain and the beam width exhibit periodicity. This is because the reactance of the open line at the port also changes periodically as can be seen from (1). It is also noted that F/B and gain are maximum around, and the beam width is almost 60, which is suitable for a six-sector antenna. We find that making the line length an integral multiple of a half-wavelength gives the optimum radiation pattern in this six-sector antenna geometry.

3060 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Fig. 13. Radiation patterns in vertical plane ( =0 ): (a) measurement of six-sector antenna, (b) calculation of six-sector antenna, and (c) calculation of single liner array. Fig. 14. Radiation patterns in conical plane ( =45 ): (a) measurement of six-sector antenna, (b) calculation of six-sector antenna, and (c) calculation of single liner array. V. MEASUREMENTS Based on the design described above, a six-sector antenna with 1.83 diameter substrate was fabricated and its characteristics were measured. Fig. 11 shows the measured, where ports #2, #3, #5, and #6 were open, port #4 was connected to a load of, and. is the center frequency (5 GHz). The measured values generally agree with the calculated results. It can be seen that is less than 15 db in the 6.5% bandwidth region. Resonance occurs at about due to the resonance of the rectangular parasitic elements. Fig. 12 shows the loss caused by termination. It can be seen that the measured values agree with the calculated results, too. The termination loss is less than 1 db, and this corresponds to the result shown in Fig. 5. Figs. 13 and 14 show, respectively, the vertical radiation pattern in -plane and the conical radiation pattern at, which is equal to the main-lobe direction. and are defined as the co-polarization and cross-polarization components, respectively. The radiation pattern of the single linear array is also calculated so as to investigate the effect of the intersecting arrays on the radiation performance. All calculations for these models assumed an identical finite ground plane. components are not shown in Fig. 13(b) and (c) because the no components appeared in the calculation. It can be seen that the measured and calculated results are in good agreement, which confirms the validity of the numerical design. It can be also seen that the maximum cross-polarization level is 10 db lower than the copolarization level. Originally, the microstrip antenna s cross-polarization component in the conical plane is not low when the substrate is placed parallel to the -plane. Only in the -plane, a very low level of component is observed when the current direction on the microstrip antenna is parallel to -axis. Since the microstrip Yagi Uda array narrows the beam toward the -plane in this case, our antenna achieves a radiation pattern with relatively low cross-polarization level. Furthermore, comparing the radiation pattern of our antenna to that of a single linear array, it is found that no increase in the cross-polarization component caused by the intersecting linear arrays is observed. The reason why our antenna s cross-polarization is low even with the intersecting linear arrays is that our antenna s geometry is symmetrical, which this helps to cancel the radiation of the cross-polarization components from the intersecting arrays. It is found that a conical-plane beam width of 73 and F/B of 17 db can be obtained, and that the proposed six-sector antenna has a radiation pattern suitable for a sector antenna. The measured and calculated frequency dependencies of the F/B and actual gain are shown in Fig. 15. It is found that the measured and calculated values, are in good agreement, and it

HONMA et al.: COMPACT SIX-SECTOR ANTENNA EMPLOYING THREE INTERSECTING 3061 length lines that fully reflect the incident power. We have found the optimum termination condition at the quiescent ports that maximizes F/B and actual gain. Based on our numerical design, a six-sector antenna was fabricated. Measurements found that it offers high F/B value, 17 db, and high actual gain, at least 10 dbi, and that its radiation pattern is suitable for a six-sector antenna. These results indicate that the proposed six-sector antenna offers both high radiation performance and a 75% reduction in substrate area compared to the equivalent conventional antenna with six individual microstrip Yagi Uda arrays in a radial configuration. ACKNOWLEDGMENT The authors thank Associate Professor Dr. Q. Chen of Tohoku University for his helpful advice on numerical analysis. The authors also thank Dr. K. Cho, Dr. T. Maruyama, and F. Kira of NTT DoCoMo for their constant encouragement and advice. Fig. 15. Frequency dependence of antenna characteristics: (a) F/B and (b) actual gain. is seen that F/B exceeds 17 db and that the gain exceeds 10 dbi around the center frequency. The actual gain of the six-sector antenna is 1 db higher than that of the single linear array shown in Fig. 5. This is because of the difference of the ground plane shape. In Fig. 14, the measured beam width is slightly wider than the calculated value even though the measured and calculated gain values show good agreement. In our analysis, only infinitely planar substrates can be treated; this substrate causes surface wave loss, which cannot happen in the measurement. The reason why the calculated and measured gain values are in good agreement is considered to be because the directivity enhancement provided by the narrow beam and the radiation efficiency degradation happen to cancel each other out in the calculation. Meanwhile, an intense F/B deterioration is seen when, and it is also seen that the frequency range for 1 db gain attenuation is. That is, the actual available bandwidth of this antenna is about 5.5%, i.e.,. VI. CONCLUSION A novel compact planar six-sector antenna that employs an array of dual-beam microstrip Yagi Uda antennas with common directors has been proposed for use in mobile terminals. A numerical analysis of the unit dual-beam array showed that the terminating the quiescent ports with the appropriate load enhances the F/B value by at least 8 db. The radiation pattern of a six-sector antenna configured by combining three dual-beam unit arrays has been investigated. To improve the effect of undesirable coupling between unit arrays, we connect the quiescent ports of the other two unit arrays to finite REFERENCES [1] Y. Takimoto, Recent activities on millimeter wave indoor LAN system development in Japan, in Dig. IEEE MTT-S Int. Symp., Jun. 1995, pp. 405 408. [2] N. Morinaga and A. Hashimoto, Technical trend of multimedia mobile and broadband wireless access systems, Trans. IEICE, vol. E82-B, no. 12, pp. 1897 1905, Dec. 1999. [3] R. W. Chang, Synthesis of band-limited orthogonal signals for multichannel data transmission, Bell Syst. Tech. J., vol. 45, pp. 1775 1796, Dec. 1966. [4] R. W. Chang and R. A. Gibby, A theoretical study of performance of an orthogonal multiplexing data transmission scheme, IEEE Trans. Commun., vol. COM-16, pp. 529 540, Aug. 1968. [5] K. Uehara, T. Seki, and K. Kagoshima, New indoor high-speed radio communication system, in IEEE Veh. Technol. Conf., Jul. 1995, vol. 2, pp. 996 1000. [6] Y. Takatori, K. Cho, K. Nishimori, and T. Hori, Adaptive array employing eigenvector beam of maximum eigenvalue and fractionallyspaced TDL with real tap, IEICE Trans. Commun., vol. E83-B, no. 11, pp. 1678 1687, Aug. 2000. [7] J. E. Mitzlaff, Radio propagation and anti-multi-path techniques in the WIN environment, IEEE Network Mag., vol. 5, pp. 21 26, Nov. 1991. [8] T. Seki and T. Hori, Cylindrical multi-sector antenna with self-selecting switching circuit, IEICE Trans. Commun., vol. E84-B, no. 9, pp. 2407 2412, Sep. 2001. [9] T. Maruyama, K. Uehara, T. Hori, and K. Kagoshima, Rigorous analysis of transient radiation mechanism of small multi-sector monopole Yagi Uda array antenna using FDTD method, Int. J. Numer. Model., vol. 12, no. 4, pp. 341 351, 1999. [10] M. Yamamoto, K. Ishizaki, M. Muramoto, K. Sasaki, and K. Itoh, A planar-type five-sector antenna with printed slot array for millimeterwave and microwave wireless LANs, in IEICE 2000 Int. Symp. Antenna Propag., Aug. 2000, vol. 1, pp. 206 208. [11] H. Uno, Y. Saito, G. Ohta, H. Haruki, Y. Koyanagi, and K. Egawa, A planar sector antenna suitable for small WLAN card terminal, in 14th IEEE Personal, Indoor Mobile Radio Commun. (PIMRC 2003), Sep. 2003, vol. 3, pp. 2176 2179. [12] D. Gray, J. W. Lu, and D. V. Thiel, Electronically steerable Yagi Uda microstrip patch antenna array, IEEE Trans. Antennas Propag., vol. 46, pp. 605 608, May 1998. [13] J. Huang, Planar microstrip Yagi array antenna, in IEEE Antennas Propag. Soc. Symp. Dig., Jun. 1989, pp. 894 897. [14] N. Honma, T. Seki, K. Tsunekawa, F. Kira, and K. Cho, Compact multi-sector antenna employing microstrip Yagi-Uda array antenna with common director elements, (in Japanese) IEICE Trans. Commun., vol. J87-B, no. 9, pp. 1363 1371, Sep. 2004. [15] I. Toyoda, F. Nuno, Y. Shimizu, and M. Umehira, Proposal of 5/25-GHz dual band OFDM-based wireless LAN for high-capacity broadband communications, in 16th Annu. IEEE Int. Symp. Personal Indoor Mobile Radio Commun. (PIMRC 2005), I02-05, Sep. 2005.

3062 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Naoki Honma (M 00) was born in Sendai, Japan, in 1973. He received the B.E., M.E., and Ph.D degrees in electrical engineering from Tohoku University, Sendai, in 1996, 1998, and 2005, respectively. In 1998, he joined NTT Radio Communication Systems Laboratories, Nippon Telegraph and Telephone Corporation (NTT), Japan. He is now with NTT Network Innovation Laboratories. His current research interest is planar antennas for high-speed wireless communication systems. Dr. Honma is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan. He received the Young Engineers Award from the IEICE in 2003, the APMC Best Paper Award in 2003 and the Communications Society Best Paper Award in 2006. Tomohiro Seki (M 94) was born in Tokyo, Japan, in 1967. He received the B.E., M.E., and Dr.Eng. degrees in electrical engineering from Tokyo University of Science, Tokyo, in 1991, 1993, and 2006, respectively. In 1993, he joined Nippon Telegraph and Telephone Corporation (NTT), Japan, and has been engaged in research on planar antennas and active integrated antennas for millimeter-wave and microwave bands. He is currently interested in system-on-package technologies for millimeterwave communication systems. He is a Research Engineer with the Radio System Technologies Research Group, NTT Network Innovation Laboratories. Dr. Seki is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan. He received the 1999 Young Engineer Award from the IEICE. Koichi Tsunekawa (M 89) received the B.S., M.S., and Ph.D. degrees in science engineering from Tsukuba University, Ibaraki, Japan, in 1981, 1983, and 1992 respectively. He is a Professor in the Department of Computer Science, Chubu University, Aichi, Japan. In 1983, he joined the NTT Electrical Communications Laboratories, Nippon Telegraph and Telephone (NTT) Corporation, Tokyo, Japan. Since 1984, he has been engaged in the research and development of portable telephone antennas in land mobile communication systems. From 1993 to 2003, he was with NTT DoCoMo Inc. He worked on radio propagation research, intelligent antenna systems for wireless communications, and developing IMT-2000 antenna systems. He was with the Wireless Systems Innovation Laboratory, NTT, from 2003 to 2006. He studied antenna systems for MIMO transmission and millimeter wave broadband access. Currently, his research interests are ubiquitous wireless communication system with computer technology. Kunio Sawaya (M 77 SM 02) was born in Sendai, Japan, in 1949. He received the B.E., M.E., and Ph.D. degrees from Tohoku University, Sendai, in 1971, 1973, and 1976, respectively. He is presently a Professor of Electrical and Communication Engineering, Tohoku University. His areas of interests are antennas in plasma, antennas for mobile communications, theory of scattering and diffraction, antennas for plasma heating, antennas for magnetic resonance imaging, and visualization of electromagnetic field. Dr. Sawaya received the Young Scientists Award in 1981 and Paper Award in 1988 from the Institute of Electronics, Information and Communication Engineers of Japan. He is Chair of the Tohoku Branch of the Institute of Image Information and Television Engineers of Japan. Kenjiro Nishikawa (A 93 M 00) was born in Nara, Japan, in 1965. He received the B.E., M.E. degrees in welding engineering and the Dr. Eng. degree in communication engineering, all from Osaka University, Suita, Japan, in 1989, 1991 and 2004, respectively. In 1991, he joined the NTT Radio Communication Systems Laboratories (now NTT Network Innovation Laboratories), Yokosuka, Japan where he has been engaged in research and development on 3-D and uniplanar MMIC s on Si and GaAs, and their applications. He is currently interested in millimeterwave transceivers, system-in-package technologies, active integrated antennas and high-speed communication systems. Dr. Nishikawa is a Technical Program Committee member of the IEEE Compound Semiconductor IC Symposium and the IEEE Radio and Wireless Symposium. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan. He received the 1996 Young Engineer Award presented by the IEICE.