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1158 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 8, AUGUST 2002 Communications A Broad-Band Planar Quasi-Yagi Antenna Noriaki Kaneda, W. R.

1 1158 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 8, AUGUST 2002 Communications A Broad-Band Planar Quasi-Yagi Antenna Noriaki Kaneda, W. R. Deal, Yongxi Qian, Rod Waterhouse, and Tatsuo Itoh Abstract In this paper, a novel broadband planar antenna based on the classic Yagi Uda dipole array is presented. This quasi-yagi antenna achieves a measured 48% bandwidth for VSWR 2, better than 12 db front-to-back ratio, smaller than 15 db cross polarization, 3 5 db absolute gain and a nominal efficiency of 93% across the operating bandwidth. Finite-difference time-domain simulation is used for optimization of the antenna and the results agree very well with the measurement. Additionally, a gain-enhanced design is presented, where higher gain has been achieved at the cost of reduced bandwidth. These quasi-yagi antennas are realized on a high dielectric constant substrate and are completely compatible with microstrip circuitry and solid-state devices. The antenna should find wide applications in wireless communication systems, power combining, phased arrays, and active arrays, as well as millimeter-wave imaging arrays. Index Terms Broad-Band antennas, microstrip antennas, Yagi Uda arrays. I. INTRODUCTION The Yagi Uda antenna, first published in an English language journal in 1928 [1], has been used extensively as an end-fire antenna. However, only limited success has been achieved at adapting this antenna to microwave/millimeter wave operation. Several interesting approaches for this are a microstrip Yagi array based on the microstrip patch antenna [2], and a coplanar-stripline fed printed Yagi Uda antenna with the reflector element printed on the back of a thick, low-permittivity slab at 60 GHz [3]. In this paper, we report a new type of planar Yagi Uda antenna that is well suited to microwave and millimeter wave frequencies. We have recently proposed and demonstrated a novel uniplanar quasi-yagi antenna that has both the compactness of resonant-type antennas and broadband characteristics of traveling-wave radiators [4] [7]. The Yagi Uda dipole array type of antenna is realized on a high dielectric constant substrate with a microstrip feed. Unlike the traditional Yagi dipole design, we employ the truncated microstrip ground plane [8] as the reflecting element, thus eliminating the need for a reflector dipole. This results in a very compact design (< 0 =2 by 0=2), which is totally compatible with any microstrip-based MMIC circuitry. Following the first experimental demonstration in [4], an X-band prototype with 17% bandwidth, 6.5 db gain, 18 db front-to-back ratio and 015 db cross-polarization level has been designed and tested successfully [5]. In this paper, we present further information on the design and performances of broadband quasi-yagi antenna. We have achieved extremely broad bandwidth (measured 48% for VSWR < 2), good radiation profile (front-to-back ratio >12 db, cross-polarization Manuscript received August 30, 1999; revised February 14, N. Kaneda is with Lucent Technologies, Holmdel, NJ USA ( kaneda@lucent.com). W. R. Deal is with Lucix Corporations, Camarillo, CA USA. Y. Qian is with the Microsemi Corporation, Los Angeles, CA USA. R. Waterhouse is with the RMIT University, Melbourne, Australia. T. Itoh is with the Electrical Engineering Department, University of California, Los Angeles, CA USA. Publisher Item Identifier /TAP Fig. 1. A schematic of the quasi-yagi antenna. < 012 db), acceptable absolute gain (3 5 db) and high efficiency (93%). Furthermore, the mutual coupling between neighboring elements of a quasi-yagi antenna array is found to be very low, with a measured level < 018 db for either stacked or side-by-side two-element structure with 0=2 separation. Such a compact array with low mutual coupling characteristics should find wide applications in modern communications and radar systems as well as millimeter-wave imaging arrays. II. QUASI-YAGI ANTENNA CONCEPT Fig. 1 shows the schematic of the uniplanar quasi-yagi antenna. The antenna is constructed on a single piece of relatively high permittivity substrate (0.635 mm thick Duroid " r =10:2for an X-band prototype) with metallization on both sides. The top metallization consists of a microstrip feed, a broadband microstrip-to-coplanar stripline (CPS) balun and two dipole-elements, one of which is the driver element fed by CPS, and the second dipole being the parasitic director. The broad-band microstrip-to-cps transition was previously reported in [9]. The metallization on the bottom plane is a truncated microstrip ground, which serves as the reflector element for the antenna. The parasitic director element on the top plane simultaneously directs the antenna propagation toward the endfire direction, and acts as an impedance matching element. As with the classic Yagi Uda antenna, proper design requires careful optimization of the driver, director, and reflector parameters, which include element spacing, length, and width. By choosing the antenna X/02$ IEEE

2 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 8, AUGUST Fig. 2. FDTD simulation and measured input return loss characteristic of the prototype quasi-yagi antenna. parameters properly, the quasi-yagi antenna demonstrates broadband (40 50% for VSWR < 2) characteristics with modest gains (4 db) or narrower bandwidth (10 20% for VSWR < 2) with higher gains (6.5 db). The current design features one director element. Incorporating additional elements has the potential for increasing gain or bandwidth. However, this also increases the number of design parameters as well as the complexity of design optimization, and has not been investigated extensively. III. BROADBAND QUASI-YAGI ANTENNA The dimensions of the X-band quasi-yagi were optimized using in-house finite-difference time-domain (FDTD) code to achieve broadband performance. The antenna is realized on mm thick Duroid with " r = 10:2. The antenna s dimensions are (unit: mm): W1 = W3 = W4 = W5 = W dri = W dir = 0:6, W2 = 1:2, W 6 = S 5 = S 6 =0:3, L 1 =3:3, L 2 = L 5 =1:5, L 3 =4:8, L 4 =1:8, S ref =3:9, S dir =3:0, S sub =1:5, L dri =8:7, and L dir =3:3. The length of the antenna s director element is shorter than the conventional Yagi Uda antenna design and contributes to the broadband characteristics of the antenna. The total area of the substrate is approximately 0=2 by 0=2 at the center frequency. The FDTD simulation and measured results of the return loss of the antenna are shown in Fig. 2. The simulated and measured bandwidths (VSWR < 2) are 43% and 48%, respectively. To the authors knowledge, no uniplanar antenna has ever achieved such a wide instantaneous bandwidth with such a compact design. The radiation patterns are measured across the VSWR < 2:1 bandwidth. Fig. 3 shows the copolar and cross-polar patterns at 9.5 GHz. Across the entire bandwidth the front-to-back ratio was better than 12 db and the cross-polarization level is better than 012 db. These measured results are in close agreement with the simulated results, which are not shown here for the sake of brevity. As can be seen, the broadband quasi-yagi antenna has a broad, single-beam pattern in both the E- and H-plane cuts. The gain was measured as db across the entire 10 db return-loss bandwidth. This is in good agreement with the simulated results of db. Finally, the radiation efficiency of the antenna has been measured using the reflection method [10]. The antenna was placed in a waveguide-sliding short structure and the input return loss was measured at 20 positions. This data and the input return loss for the antenna radiating in free space were incorporated into a MATLAB program to extract the radiation efficiency. The radiation efficiency of the overall Fig. 3. Measured E- and H-planes co- and cross-polarization radiation patterns of the antenna at 9.5 GHz. structure is quite high, nominally 93% across the operating band. The error in measurement is estimated from the ripple to be roughly 63%. IV. MUTUAL COUPLING CHARACTERISTICS The mutual coupling between elements within an array environment is particularly important in the design of small arrays of printed antennas. The lower the coupling, the less computational intensive numerical analysis is required to analyze the structure and therefore the easier to design the array. The quasi-yagi antenna demonstrates very low-mutual coupling when placed in an array environment. Two two-element configurations were examined, namely, when the second element is positioned in the vertical plane (or stacked) and when it is positioned in the horizontal plane (or coplanar) to the first. The spacing between antennas is 15 mm, which corresponds to 0=2 at 10 GHz. The substrate for the coplanar arrays is truncated at both sides, with a total width of 30 mm. The mutual coupling is determined by the measured direct transmission coefficient S21 of the arrays. The coupling level is measured quite low across the entire operating bandwidth with a maximum measured mutual coupling of db at 12 GHz. Additionally, the coupling level is extremely flat over the operating bandwidth and only creeps up at the edges of the operating bandwidth where the antenna performance degrades. V. GAIN-ENHANCED DESIGN The design of quasi-yagi antenna can be optimized so that the antenna demonstrates higher gain at the expense of reduced bandwidth. As with the broadband version of the quasi-yagi antenna, the antenna is realized on mm thick Duroid with " r =10:2. The precise dimensions of the antenna are (unit: mm): W 1 = W 3 = W 4 = W 5 = W6 = 0:58, W2 = 1:12, W6 = 0:25, W dri = W dir = 1:17, S5 = S6 = 0:25, L1 = 3:2, L2 = 2:5, L3 = 5:3, L4 = 2:0, L 5 =0:76, S ref =7:7, S dir =3:25, S sub =3:0, L dri =9:4, and L dir =5:5. In this design, the distance between the driver element and the reflector element is still much larger and that the strip width is also considerably wider than in the previous design. The input return loss of the antenna is measured and the bandwidth for VSWR < 2 is

3 1160 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 50, NO. 8, AUGUST 2002 as millimeter-wave imaging arrays. The broad pattern, low-mutual coupling and wide instantaneous bandwidth allow this antenna to be incorporated into multi-frequency phased arrays with very large scanning capability. Fig. 4. Measured E- and H-planes co- and cross-polarization radiation patterns of the gain-enhanced antenna at 9.55 GHz. 270 is the endfire radiation direction in this measurement. 11%, which is much wider than the regular patch antenna realized on the similar substrate. The gain of the antenna varies from 5 db to 7 db over the operating bandwidth. In comparison with the broadband design, this design has four times narrower bandwidth but approximately 2 db higher gain. By adding more directors, we expect that the gain can be further increased. To confirm the change in the absolute gain value, the radiation pattern is also plotted in Fig. 4. The beam-width of the radiation pattern is narrower than that of the broadband design while keeping a front-to-back ratio of 15 db and a maximum cross-polarization level of 015 db. The change in the radiation pattern is relatively small for the entire operating bandwidth [4]. VI. CONCLUSION In this paper, a very compact and simple planar antenna based on the modification of the classic Yagi Uda antenna has been presented. The antenna achieves extremely wide frequency bandwidth and good radiation characteristics in terms of beam pattern, front-to-back ratio, cross polarization and low mutual coupling. The antenna experimentally demonstrated a bandwidth of 48% for a VSWR < 2, better than 12 db front-to-back ratio, a gain between 3 5 db, and a nominal radiation efficiency of 93%. Additionally, mutual coupling between antennas in a horizontal and vertical configuration were measured to be better than 020 db in the entire operating band. Finally, a higher gain version of the quasi-yagi antenna has been presented. In this case, measured gain varies between 5 7 db across the operating bandwidth, where the increased gain has been achieved at the cost of reduced bandwidth. Adding additional directors has the potential of increasing the gain ever further. The excellent radiation properties of this antenna make it ideal as either a stand-alone antenna with a broad pattern or as an array element. We believe that this antenna should find wide applications in wireless communication systems, power combining and phased arrays, as well REFERENCES [1] H. Yagi, Beam transmission of the ultra short waves, Proc. IRE, vol. 16, pp , June [2] J. Huang and A. C. Densmore, Microstrip Yagi array antenna for mobile satellite vehicle application, IEEE Trans. Antennas Propagat., vol. 39, pp , July [3] K. Uehara, K. Miyashita, K. I. Matsume, K. H. Hatakeyama, and K. Mizuno, Lens-coupled imaging arrays for the millimeter and submillimeter-wave regions, IEEE Trans. Microwave Theory Technol., vol. 40, pp , May [4] N. Kaneda, Y. Qian, and T. Itoh, A novel Yagi Uda dipole array fed by a microstrip-to-cps transition, in Proc Asia Pacific Microwave Conf. Dig., Yokohama, Japan, Dec. 1998, pp [5] Y. Qian, W. R. Deal, N. Kaneda, and T. Itoh, A microstrip-fed quasi-yagi antenna with broadband characteristics, Electron. Lett., vol. 34, no. 23, pp , Nov [6], A uniplanar quasi-yagi antenna with wide bandwidth and low mutual coupling characteristics, in Proc IEEE Antennas Propagat. Int. Symp. Dig., vol. 2, Orlando, FL, July 1999, pp [7] N. Kaneda, Y. Qian, and T. Itoh, A broadband microstrip-to-waveguide transition using quasi-yagi antenna, in Proc IEEE Microwave Theory Tech. Int. Microwave Symp. Dig., vol. 4, Anaheim, CA, June 1999, pp [8] W. Wiesbeck, Dipol- und Yagi-antennen fur die mikrowellen-streifenleitungstechnik, Microwellen J., Mar [9] Y. Qian and T. Itoh, A broadband uniplanar microstrip-to-cps transition, 1997 Asia Pacific Microwave Conf. Dig., pp , Dec [10] R. H. Johnston and J. G. McRory, An improved small antenna radiationefficiency measurement method, IEEE Antennas Propagat. Mag., vol. 40, pp. 40 8, Oct Miniature Microstrip Antenna With a Partially Filled High-Permittivity Substrate Byungje Lee and Frances J. Harackiewicz Abstract A new technique to reduce the overall dimension of a microstrip antenna using a partially filled high-permittivity substrate is proposed. The miniaturized microstrip antenna for a repeater system in a mobile communication cellular band ( MHz) is designed with the proposed technique and manufactured with light weight and small size. Comparison between simulations, based on HP HFSS software and measurements are provided. Index Terms Patch antenna, repeater system. I. INTRODUCTION Microstrip patches are currently being used for many applications. However, the size of a conventional microstrip patch antenna is still Manuscript received November 17, 1999; revised October 29, This work was supported in part by the Ministry of Information & Communication of Korea ( Support Project of University Information Technology Research Center supervised by IITA). B. Lee is with the RFIC Research and Education Center & Mission Technology Research Center, Kwangwoon University, Seoul , Korea ( bjlee@daisy.kwangwoon.ac.kr). F. J. Harackiewicz is with the Department of Electrical and Computer Engineering, College of Engineering, Southern Illinois University at Carbondale, Carbondale, IL USA. Publisher Item Identifier /TAP X/02$ IEEE

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