THE recent allocation of frequency band from 3.1 to

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 3075 Compact Ultrawideband Rectangular Aperture Antenna and Band-Notched Designs Yi-Cheng Lin, Member, IEEE, and Kuan-Jung Hung Abstract A simple and compact ultrawideband (UWB) aperture antenna with extended band-notched designs is presented. The antenna consists of a rectangular aperture on a printed circuit board ground plane and a T-shaped exciting stub. The proposed planar coplanar waveguide fed antenna is easy to be integrated with radio-frequency/microwave circuitry for low manufacturing cost. The antenna is successfully designed, implemented, and measured. A compact aperture area of 13 23 mm 2 is obtained with promising performances, including broadband matched impedance, stable radiation patterns, and constant group delay. The correlation between the mode-based field distributions and radiation patterns is discussed. Extended from the proposed antenna, three advanced band-notched (5 6 GHz) designs are also presented as a desirable feature for UWB applications. Index Terms Aperture antennas, band-notched UWB antennas, coplanar waveguide (CPW) fed antennas, planar antennas, printed circuit board (PCB) antennas, slot antennas, ultrawideband (UWB) antennas. I. INTRODUCTION THE recent allocation of frequency band from 3.1 to 10.6 GHz by the Federal Communications Commission (FCC) for ultrawideband (UWB) radio applications has presented an opportunity and challenge for antenna designers. The FCC first approved rules for the commercial use of UWB in February 2002. By April of that year, the FCC gave formal approval for the unlicensed use of the technology between 3.1 and 10.6 GHz [1]. Since then, the feasible design and implementation of UWB system has become a highly competitive topic in both academy and industry communities of telecommunications. In particular, the antenna of ultrawide bandwidth is the key component of the UWB system and has attracted significant research power in the past few years [2]. Challenges of the feasible UWB antenna design include the ultrawideband performances of the impedance matching and radiation stability, the compact appearance of the antenna size, and the low manufacturing cost for consumer electronics applications. Among the planar UWB antenna designs in the recent literature, the slot antenna type [2] [7] is one of the most promising candidates for UWB applications. The advantages of slot antennas include wide bandwidth performance and low cost in Manuscript received September 19, 2005; revised February 6, 2006. This work was supported in part by the National Science Council, Taiwan, R.O.C., under Contracts NSC 94-2219-E-002-007 and NSC 94-2752-E-002-002-PAE. Y.-C. Lin is with the Department of Electrical Engineering, National Taiwan University, Taiwan, R.O.C. (e-mail: yclin@cc.ee.ntu.edu.tw). K.-J. Hung is with the Graduate Institute of Communication Engineering, National Taiwan University, Taiwan, R.O.C.. Color versions of Figs. 1 9 and 11 14 are available online at http://ieeexplore. ieee.org. Digital Object Identifier 10.1109/TAP.2006.883982 the printed circuit board (PCB) process. The bandwidth enhancement is the main focus of these slot antenna designs and can be categorized in two kinds. One is to manipulate the field distribution in the slot with a tapered shape [2] or with a feeding scheme to generate multiple resonances of close bands [3]. The other is to use a widened slot (or aperture, precisely speaking) and a fork-like stub for excitation such that a broad bandwidth can be achieved [4] [6]. The latter approach has significant progress on the bandwidth enhancement and has reached the UWB bandwidth requirement recently [6]. However, the design of using fork-like stub requires relatively large aperture and contains many parameters for the complex geometry. In addition, it is difficult to modify the designed antenna for the band-rejection function, a desirable feature in the UWB system. Over the designated bandwidth of UWB systems, there are existing bands used by wireless local-area network (WLAN) (IEEE802.11a and HIPERLAN/2) operating in the 5.15 5.825 GHz band. It is desirable to design the UWB antenna with a notched band at 5 6 GHz [7] to minimize the potential interferences. In this paper, a coplanar waveguide (CPW)-fed rectangular aperture antenna with a T-shaped exciting stub is proposed. Compared to the fork-like stub, the proposed T-shaped stub has three advantages. 1) The aperture area can be significantly reduced (more than 50% from [6]) without compromising the antenna performances. 2) The antenna can be easily extended to the advanced bandnotched design without retuning the dimensions of the original aperture and exciting stub. 3) The exciting T-stub has a simple geometry with less parameters, releasing the computation load in the optimization process. The proposed antenna is successfully designed, built, and verified. A compact aperture area of 13 by 23 mm is achieved. The antenna performs promising characteristics on the impedance matching, radiation patterns, and group delay over the entire UWB band. In this paper, the proposed antenna is further extended to the band-notched function. The design concept is described and three different band-notched designs provided for illustration. The measured return loss and antenna gain spectrum are included, showing the successful band-rejection capability for all three proposed band-notched designs. II. ANTENNA DESIGN A. Antenna Structure Fig. 1 shows the geometry and configuration of the proposed antenna. The antenna consists of a rectangular aperture etched out from the ground plane of a PCB and a CPW-fed T-shaped 0018-926X/$20.00 2006 IEEE

3076 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Fig. 2. Effects of the stub width W on the return loss. The dimensions of other parameters are LL = 35, WW = 30, Ls = 23, Ws = 13, L=10:8, T=2, S = 3:6, and G = 0:4 (unit: mm). Fig. 1. Geometry and configuration of the proposed antenna. stub for excitation. Since the antenna and feeding structure are implemented on the same plane, only one layer of substrate with single-sided metallization is used, making the manufacturing of the antenna very easy and extremely low cost. The CPW transmission line is designed with 50 and terminated with a shape memory alloy connector for the measurement purpose in this paper. In practice, the CPW line is integrated with radio-frequency/microwave circuitry on the system board. Design of the rectangular aperture is determined by minimizing the aperture area while satisfying the input impedance matched for the entire UWB band, especially for the lower frequencies. In this paper, a compact aperture area of 13 23 mm is achieved, that is, the dimension is less than a quarter-wavelength for the lowest frequency (3.1 GHz). The excitation of the antenna is formed by a simple T-shaped stub of only three parameters: the length L, the width W, and the extrusion depth T, as shown in Fig. 1. B. Parametric Study The commercial simulation tool Ansoft HFSS is employed in this paper to perform the design and optimization process. Since the T-shaped stub is the main factor in the optimization process, its three parameters W, L, and T are selected to perform the sensitivity study first. The effects of parameter W, L, and T on the input impedance are simulated and shown in Figs. 2 4, respectively. Fig. 2 shows that the stub width W mainly influences the impedance at lower frequencies (3 4 GHz). Fig. 3 shows that the stub length L may affect the impedance in both low and middle bands. Compared to the stub width W and length L, the extrusion depth T is relatively sensitive to the input impedance over the entire UWB band, as shown in Fig. 4. Note that the implementation tolerance and gap/line limit of PCB fabrication should be controlled based on the aforementioned parametric study curves. III. MEASUREMENT RESULTS AND DISCUSSION A. Impedance Bandwidth The designed antenna of optimized dimensions is implemented with a low-cost FR4 substrate with dielectric Fig. 3. Effects of the stub length L on the return loss. The dimensions of other parameters are LL = 35, WW = 30, Ls = 23, Ws = 13, W=4, T=2, S=3:6, and G=0:4 (unit: mm). Fig. 4. Effects of the extrusion depth T on the return loss. The dimensions of other parameters are LL = 35, WW = 30, Ls = 23, Ws = 13, L=10:8, W=4, S=3:6, and G=0:4 (unit: mm). constant, loss tangent, and thickness mm. The measurement of return loss is carried out with an HP8722-ES network analyzer. The radiation patterns are measured in a far-field anechoic chamber. Fig. 5 shows the measured return loss of the designed antenna with a comparison with simulation results. A good agreement between simulation and measurement is achieved. Fig. 5 shows that the input impedance is well matched as the 10-dB return loss bandwidth covers the entire UWB band (3.1 10.6 GHz).

LIN AND HUNG: COMPACT UWB RECTANGULAR APERTURE ANTENNA 3077 Fig. 5. Measured and simulated return loss of the proposed antenna with optimal dimensions LL = 35, WW = 30, Ls = 23, Ws = 13, L=10:8, W=4, T=2, S=3:6, and G=0:4 (unit: mm). B. Field Distribution and Radiation Patterns From the spectrum of impedance performance in Fig. 5, it can be seen that there are three resonances around the frequencies at 4, 7, and 10 GHz. These resonances correspond to the different modes of field distribution and play important roles on the explanation of the radiation patterns. The electric field distributions of these resonant modes are then simulated and the correspondent radiation patterns are investigated at 4, 7, and 10 GHz, as shown in Figs. 6 8, respectively. Fig. 6(a) shows the first resonant mode at 4 GHz, where the electric fields are concentrated at the upper center part with polarization mainly in the y-axis. This set of field distribution is locally similar to that of mode in a rectangular waveguide [8], and considered as the fundamental mode of the aperture antenna. The radiation pattern of this mode is like a small dipole oriented in the y-axis leading to a bidirectional pattern in the E-plane (yz-plane) and omnidirectional pattern in the H-plane (xz-plane), as shown in Fig. 6(b) and (c), respectively. Fig. 7(a) shows the second resonant mode at 7 GHz where both x- and y-component fields exist. Note that the x-component fields of the left and right sides of the stub are in the opposite directions that cancel out each other at far fields in the symmetric E-plane. Therefore, the E-plane patterns are almost unchanged and still have good polarization isolation (x-polarized level better than 20 db), as shown in Fig. 7(b). However, the x-component fields generate cross-polarized patterns in the H-plane, as shown in Fig. 7(c). The cross-polarized patterns can be modeled as the radiation from two small x-polarized dipoles of out of phase, leading to the nulls in the x-axis (due to the element factor) and z-axis (due to the array factor) with relative maximums in between. Fig. 8(a) shows the third resonant mode at 10 GHz, where the y-component fields are partially shifted to lower part and concentrated to the left and right sides of the CPW feed. This field distribution contains multiple higher order modes and makes the peak of E-plane patterns shift slightly from the z-axis, as shown in Fig. 8(b). Since the x-component fields are similar to that of the second resonant mode, the cross-polarization patterns of H-plane are of similar shape, as shown in Fig. 8(c). Note that Fig. 6. The first resonant mode at 4 GHz: (a) distribution of electric fields, (b) E-plane (yz-plane) patterns, and (c) H-plane (xz-plane) patterns. the copolarized patterns in the H-plane are no longer omnidirectional due to the array factor. However, the radiation in the z-axis is still kept maximal.

3078 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Fig. 7. The second resonant mode at 7 GHz: (a) distribution of electric fields, (b) E-plane (yz-plane) patterns, and (c) H-plane (xz-plane) patterns. Fig. 8. The third resonant mode at 10 GHz: (a) distribution of electric fields, (b) E-plane (yz-plane) patterns, and (c) H-plane (xz-plane) patterns. The simulation and measurement of radiation patterns show a good agreement from Figs. 6 8, including the copolarization and cross-polarization patterns in both E- and H-planes. The characteristics of radiation patterns of the proposed antenna over the entire UWB spectrum are summarized as follows. 1) The direction of maximum radiation is constantly around the z-axis, normal to the aperture plane. 2) The polarization isolation in the direction of maximum radiation is very good, with cross-polarization level better than 20 db. 3) The copolarized gain might degrade slightly for the second resonant mode due to the cross-polarization decoupling. However, it is increased at higher frequencies as the array factor effect is enhanced.

LIN AND HUNG: COMPACT UWB RECTANGULAR APERTURE ANTENNA 3079 Fig. 9. Measured group delay of the proposed antenna. C. Group Delay Characteristics In the UWB system, the phase of the radiated field should vary linearly with the frequency, that is, a stable group delay response is desirable. In this paper, two identical prototypes of the proposed antenna were built for the purpose of group delay measurement. The network analyzer HP 8722-ES with time-domain gating is employed. Measured group delay of the proposed antenna is presented in Fig. 9. The variation of the group delay over the UWB band is less than 300 ps with the average 1 ns, the traveling time of the propagating waves between a pair of proposed antennas 30 cm apart. IV. BAND-NOTCHED DESIGNS The notch-band function is desirable in the UWB system to reduce the interferences with the IEEE802.11a and HIPERLAN/2 WLAN systems operating in the 5 6 GHz band. In this paper, three kinds of band-notched designs are presented to demonstrate the superior features of the proposed antenna using the T-stub excitation scheme. Fig. 10 shows the geometry and dimensions of these designs. The first design embeds an isolated slit of total length equal to half a wavelength for the frequency at 5.5 GHz inside the T-stub, as shown in Fig. 10(a). The second design employs two open-end slits at the top edge of the T-stub, as shown in Fig. 10(b), where the effective length of each slit is around quarter wavelength for the 5.5 GHz resonance. The third design utilizes two parasitic strips of half a wavelength at 5.5 GHz, as shown in Fig. 10(c). Generally speaking, the design concept of the band-rejection function is to make the input impedance singular (minimum resistance) at the sub-resonant frequency. To implement it, a narrow-band resonant structure is added to the original wide-band antenna area. Based on this concept, the above three designs using the isolated slit, the open-end slits, and the parasitic strips, as illustrated in Fig. 10(a) (c), are accomplished. In addition, the field distributions of these designs at the resonant frequency 5.5 GHz are simulated and shown in Fig. 11(a) (c), respectively. Note that when the band-notched designs are applied to the antenna, there is no retuning work required for the previously determined dimensions of the T-stub and aperture of the antenna. Fig. 10. Geometry of the three band-notched designs using (a) the isolated slit (b) the open-end slits, and (c) the parasitic strips. Performances of measured return loss of the three bandnotched designs are shown in Fig. 12. Compared to the original design, all three band-notched designs successfully block out the 5 6 GHz band and still perform good impedance-matching at other frequencies in the UWB band. Fig. 13 shows the measured gain spectrum of the developed antennas of the original and three band-notched designs. Note that the rigorous copolarization gain instead of the total gain is presented here for the case closer to the setup of the group delay testing, where the antennas are copolarization aligned in an LOS environment. Fig. 13 shows that the antennas of band-notched designs successfully perform the rejection in the 5 6 GHz band and good

3080 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 11, NOVEMBER 2006 Fig. 12. Measured return loss of the three band-notched designs, compared to the original design. Fig. 13. Measured copolarized gain of the proposed antennas: the original and three band-notched designs. Fig. 11. The field distribution of three band-notched designs using (a) the isolated slit, (b) the open-end slits, and (c) the parasitic strips. performances at other frequencies in the UWB band. It is also observed that the antenna gain drops slightly around 7 GHz, resulting from decoupled cross-polarized patterns. However, the gain increases at higher frequencies because of the array factor effect, as mentioned in Section III-B. Fig. 14 shows the photograph of the developed antennas of the original and bandnotched designs. Note that the ground plane dimensions are selected as 30 35 mm for all the developed antennas in this paper. In practice, when integrated with the system board of different ground plane size, the antenna might need a retuning for the optimized dimensions. Fig. 14. Photograph of the developed UWB antennas: the original (upper left) and three band-notched designs. V. CONCLUSION In this paper, a compact UWB rectangular aperture antenna is proposed. The antenna structure is simple and the aperture size

LIN AND HUNG: COMPACT UWB RECTANGULAR APERTURE ANTENNA 3081 compact. The advantages of the T-stub exciting scheme are described. Discussion of the correlation between the field distribution of the antenna aperture and the radiation patterns is given. Broad impedance bandwidth, stable radiation patterns, and constant group delay are obtained. Three types of band-notched structures extended from the original design are provided and verified. The simulation and measurement results of the proposed antenna show a good agreement in terms of the return loss and radiation patterns. ACKNOWLEDGMENT The authors would like to acknowledge C.-F. Liu for the helpful discussion on the band-notched designs of this paper. REFERENCES [1] Federal Communications Commission, First report and order, revision of Part 15 of Commission s rule regarding ultra-wideband transmission system FCC 02-48, Apr. 22, 2002. [2] T. G. Ma and S. K. Jeng, Planar miniature tapered-slot-fed annular slot antennas for ultra-wideband radios, IEEE Trans. Antennas Propag., vol. 53, pp. 1194 1202, Mar. 2005. [3] N. Behdad and K. Sarabandi, A multiresonant single-element wideband slot antenna, IEEE Antennas Wireless Propag. Lett., vol. 3, pp. 5 8, Jan. 2004. [4] J. Y. Sze and K. L. Wong, Bandwidth enhancement of a microstripline-fed printed wide-slot antenna, IEEE Trans. Antennas Propag., vol. 49, pp. 1020 1024, Jul. 2001. [5] R. Chair, A. A. Kishk, and K. F. Lee, Ultra-wideband coplanar waveguide-fed rectangular slot antenna, IEEE Antennas Wireless Propag. Lett., vol. 3, no. 1, pp. 227 229, 2004. [6] G. Sorbello, M. Pavone, and L. Russello, Numerical and experimental study of a rectangular slot antenna for UWB communications, Microwave Optical Technol. Lett., vol. 46, no. 4, pp. 315 319, Aug. 2005. [7] I. J. Yoon et al., Ultra-wideband tapered slot antenna with band cutoff characteristic, Electron. Lett., vol. 41, no. 11, pp. 629 630, May 2005. [8] R. F. Harrington, Time-Harmonic Electromagnetic Fields. New York: McGraw-Hill, 1961. Yi-Cheng Lin (S 92 M 98) received the B.S. degree in nuclear engineering from National Tsing-Hua University, Hsingchu, Taiwan, R.O.C., in 1987, the M.S. degree in electrical engineering from National Taiwan University, Taipei, in 1989, and the Ph.D. degree in electrical engineering from The University of Michigan at Ann Arbor in 1997. From 1997 to 2003, he was with Qualcomm Inc., San Diego, CA, as a Staff Engineer working on antenna design and development for mobile satellites, wireless handsets, and MIMO communication systems. Since 2003, he has been a Faculty Member with the Department of Electrical Engineering and Graduate Institute of Communication Engineering, National Taiwan University. His primary research interests include antenna miniaturization, ultrawideband antennas, and diversity antennas for MIMO systems. Kuan-Jung Hung was born in Kaohsiung, Taiwan, R.O.C., in 1982. He received the B.S. degree in electrical engineering from National Taiwan University of Science and Technology, Taipei, in 2003. He is currently pursuing the M.S. degree in communication engineering at National Taiwan University, Taipei. His research interests include the design and analysis of planar antennas for UWB and WLAN applications.