MODERN AND future wireless systems are placing

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1 Wideband Planar Monopole Antennas With Dual Band-Notched Characteristics Wang-Sang Lee, Dong-Zo Kim, Ki-Jin Kim, and Jong-Won Yu, Member, IEEE Abstract Wideband planar monopole antennas with dual bandnotched characteristics are presented. The proposed antenna consists of a wideband planar monopole antenna and the multiple -, -, and inverted L-shaped slots, producing band-notched characteristics. In order to generate dual band-notched characteristic, we propose nine types of planar monopole antennas, which have two or three ( or inverted L)-shaped slots in the radiator. This technique is suitable for creating ultra-wideband antenna with narrow frequency notches or for creating multiband antennas. Index Terms Frequency-notched antenna, multiband antenna, planar monopole antenna. I. INTRODUCTION MODERN AND future wireless systems are placing greater demands on antenna designs. Many systems now operate in two or more frequency bands, requiring dual- or triple-band operation of fundamentally narrowband antennas. A variety of techniques have been used to create multiband antennas. A typical technique is to create a single antenna that is responsive to multiple bands of interest. Such an antenna is a composite structure of narrowband resonant sections that can couple to the corresponding narrow bands of interest. Although narrowband resonant structures may individually be quite responsive to their particular narrowband resonant frequencies of interest, when combined together to form a composite multiband antenna, the performance of narrowband resonance components will inevitably suffer. In particular, mutual coupling is introduced between narrowband resonant components. This coupling can lead to spurious and undesired modes of operation, as well as limit the performance of the desired modes. Such an antenna performs the complex process case by case about the required frequency band. Another technique for a multiband antenna design is to create a wideband antenna with frequency notch filters [1]. Such a frequency notched wideband antenna is shown in Fig. 1. A typical system comprises a wideband antenna element, connected via a transmission line to a frequency filter shown in Fig. 1(a). A wideband antenna element has a frequency response sensitive across a wideband range of frequencies from to. A frequency notch filter passes a wideband range of frequencies from to with the exception of those frequencies Manuscript received September 29, 2005; revised December 23, 2005. This work was supported in part by the Brain Korea 21 Project and by the Samsung Electro-Mechanics Company Ltd. The authors are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea (e-mail: wslee718@kaist.ac.kr; kdzgnh@kaist.ac.kr; sergeant@kaist.ac.kr; drjwyu@ee. kaist.ac.kr). Digital Object Identifier 10.1109/TMTT.2006.874895 Fig. 1. (a) RF front end with wideband antenna and two notch filters. (b) Proposed dual band-notched antenna. in the vicinity of and. The resulting spectral response of a frequency notched wideband antenna system is sensitive to a wideband range of frequencies from to with the exception of those frequencies in the vicinity of and. Such a dual band-notched antenna may be created following the method shown in Fig. 1(b). Narrowband resonant structures are incorporated in a wideband antenna element so as to create a frequency notched wideband antenna element. It has been recently demonstrated that by etching a proper slot (such as a -shaped slot [2], [3], a -shaped slot [4], or a bent slot [5]) or inserting proper slits (such as a pair of narrow slits [6]) in the interior of the radiating element, a single notched or rejected band within a wide operating bandwidth can be obtained. This band-reject operation is achieved when the length of the embedded slot is approximately one-half wave length of the desired notch frequency. In this case, destructive interference for the excited surface currents in the antenna will occur, which causes the antenna to be nonresponsive at that frequency. This paper studies the characteristics of wideband planar monopole antennas with dual band-notched characteristics. A variety of implementations are possible, eight of which are shown in Fig. 2. One is a wideband planar monopole with three -shaped slots proposed in [7] and [8] and the other seven are derived from two or three ( or inverted L)-shaped slots. II. ANTENNA DESIGN The planar monopole has recently been investigated as an antenna with wideband properties [9] [13]. These antennas exhibit good impedance matching and high efficiency. 0018-9480/$20.00 2006 IEEE

2 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES Fig. 4. Simulated return loss versus frequency of single band-notched planar monopole antenna. Fig. 2. Geometry of the proposed dual band-notched planar monopole antennas. (a) Ant. A. (b) Ant. B. (c) Ant. C. (d) Ant. D. (e) Ant. E. (f) Ant. F. (g) Ant. G. (h) Ant. H. Fig. 3. Geometry of the proposed single band-notched planar monopole antennas with: (a) [, (b) \, (c) [[, (d) \\, and (e) inverted-l shaped slot. Fig. 2 shows the proposed dual band-notched antennas, which consists of a wideband planar monopole antenna and multiple -( -or inverted-l) shaped slots. Prototype antennas were fabricated and mounted on a circular finite ground plane with a radius of 75 mm. A 50- subminiature A (SMA) connector, centrally mounted from the back of the ground plane, was used to excite the antenna. A copper planar element of thickness 0.2 mm, size 20 27 mm, and beveling angle 12 is vertically mounted with a spacing of 1 mm (G) over the circular ground plane. To implement dual band-notched antennas, the -, -, and inverted-l shaped slots are arranged to be symmetric to the center line of the planar element. The - (or -, inverted-l) shaped slots are etched using the parameters: 1) for the width of the th slot; 2) for the half length of the th slot; 3) for the coordinates of the upper/lower vertex of the th slot; and 4) for the distance between the two arms of the th slot. Fig. 5. Measured and simulated return loss versus frequency of Ant. F.: (a) without slot (reference antenna), (b) with \-shaped slot, (c) with [-shaped slots, and (d) with \[-shaped slots. By adjusting the slot parameters, the lower and upper edge frequencies of the notched band within the antenna s operating bandwidth can be controlled. The effects of varying the slot lengths on the notched frequency band will be discussed in more detail in Section III. It should also be noted that the high- notched operation can be achieved by adjusting the slot parameters. The detailed effects of the distances on the band-notched operation will be analyzed in Section III. III. SIMULATION AND EXPERIMENT RESULTS Fig. 3 shows the proposed single band-notched planar monopole antennas. It is found that, simply by embedding -, -, -, -, and inverted-l shaped slot in the planar monopole, single band-notched characteristic for the wideband planar monopole antenna can be achieved as shown in Fig. 4. The -, -, -, -, and inverted-l shaped slot is placed symmetrically with respect to the center line of the planar monopole.

LEE et al.: WIDEBAND PLANAR MONOPOLE ANTENNAS WITH DUAL BAND-NOTCHED CHARACTERISTICS 3 Fig. 6. Simulated return loss for Ant. F as a function of (t; X ;Z ;W ). Fig. 8. Conceptual equivalent-circuit model for Ant. F with [- and \-shaped slots in (a), at the passband in (b), at the first notch frequency in (c), at the second notch frequency in (d). Fig. 7. Normalized surface current of Ant. F. (a) Surface current at the passband, 2.4 GHz. (b) Surface current at the first notch frequency, 2.96 GHz. (c) Surface current at the second notch frequency, 4.81 GHz. Fig. 5 shows the measured and simulated return loss for the proposed antenna F (Ant. F. of Fig. 2). The antenna dimensional parameters are (mm) and (mm), respectively. For the reference antenna, as shown in Fig. 5(a), the impedance bandwidth defined by 10-dB return loss is approximately 3.7 GHz (from 2.0 to 5.7 GHz). From the measured return loss, we can observe that the upper -shaped slot makes one notch band at 2.96 GHz and the other -shaped slot make the other notch band at 4.77 GHz. Note that the effect of mutual coupling between notch bands is little. It is also noted that good agreement between the measured data and simulated results, which are obtained using a time-domain finite integration technique (CST Microwave Studio) is observed. It was found that for the -or -shaped band-notching feature, the center frequency of the notch band, i.e.,, can be predicted Fig. 9. Measured and simulated return loss versus frequency. (a) Ant. A. (b) Ant. B. (c) Ant. C. (d) Ant. D. (e) Ant. E. (f) Ant. F. (g) Ant. G. (h) Ant. H. accurately using the equation where is the speed of the light. (1)

4 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES TABLE I MEASUREMENTS VERSUS THEORETICAL PREDICTION FOR DUAL BAND-NOTCHED ANTENNAS Fig. 10. Normalized surface current of Ant. H. (a) Surface current at the passband, 2.4 GHz. (b) Surface current at the first notch frequency, 3.02 GHz. (c) Surface current at the second notch frequency, 4.75 GHz. Fig. 12. Return losses with various \; [ and inverted-l shaped slot lengths L and L. Fig. 11. Conceptual equivalent-circuit model for Ant. H with two inverted-l shaped slots in (a), at the passband frequency in (b), at the first notch frequency in (c), and at the second notch frequency in (d). Fig. 6 shows the simulated return loss for the proposed antenna F as a function of. We can also see that the notch bandwidth and frequency can be controlled by the slot position and the distance between the two arms. The etched feature becomes resonant at the frequency where the length is the quarter-wavelength. Fig. 7 shows the normalized surface current distribution at each band for Ant. F. In Fig. 7(a), there are more current distributions near the feeding point at the passband frequency. In Fig. 7(a) and (c), we can see more and stronger current distributions near the edge of a (or )-shaped slot at the notch band frequency. As shown in Fig. 7(b), at the notch frequency at 2.96 GHz, current is concentrated around the bottom edge of the -shaped slot and is oppositely directed between the interior and exterior of the slot. This causes the antenna to operate in a transmission-line-like mode, which transforms the

LEE et al.: WIDEBAND PLANAR MONOPOLE ANTENNAS WITH DUAL BAND-NOTCHED CHARACTERISTICS 5 Fig. 13. Measured radiation patterns: (a) at 2.4-GHz passband frequency, (b) at 5.2-GHz passband frequency, (c) at first notch band frequency, and (d) at second notch band frequency. nearly high impedance (open circuit) at the top of the slot to nearly zero impedance (short circuit) at the antenna feeding. This zero impedance at the feeding point leads to the desired high attenuation and impedance mismatching near the notch frequency. Also, as shown in Fig. 7(c), at the notch frequency at 4.81 GHz, current is concentrated around the top edge of the -shaped slot and is oppositely directed between the interior and exterior of the slot. This causes the antenna to operate in a transmission-line-like mode, which transforms the nearly zero impedance at the top of the slot to nearly high impedance at the antenna feeding. This, in turn, leads to the desired high attenuation near the notch frequency. Fig. 8 shows the conceptual equivalent-circuit model for Ant. F, which have a series stub, a shunt stub, and antenna resistance. The stubs are a short-circuit stub with mm and an open-circuit stub with mm. When is equal to in Fig. 8(c), GHz, the input impedance at the feeding point is zero (short circuit). Also, when is equal to in Fig. 8(d), GHz, the input impedance at the feeding point is high (open circuit) due to the quarter-wave transformer. In these cases, destructive interference for the excited surface currents in the antenna will occur, which causes the antenna to be nonresponsive at those frequencies. Fig. 9 shows the measured and simulated return losses for the proposed antennas. From the measured return loss, we can observe that dual band-notched characteristics are created by etched ( or inverted L)-shaped slots. The slot creates the second notch frequency and the slot creates the first notch frequency. The inverted-l shaped slot, as shown in Fig. 9(g) and (h), creates a frequency notch where the corner length [see Fig. 9(g) and (h)] and the notch length add up and form a resonant structure When the path is a half-wavelength at a particular frequency, a destructive interference takes place, causing the antenna to be nonresponsive at that frequency. Measurements are compared to theoretical predictions in Table I. (2)

6 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES wideband antenna, a tri-band antenna or wideband antenna with two notch filters can be obtained. The ability to integrate with filter in the antenna can significantly relax the requirements imposed upon the filtering electronics within the wireless device, such as ultra-wideband (UWB) systems and software-defined radio (SDR) systems. REFERENCES Fig. 14. Measured transmission loss (S ) between the reference antenna and Ant. E and F. Fig. 10 shows the normalized surface current distribution at each band for Ant. H. Fig. 11 shows the conceptual equivalentcircuit model for Ant. H, which have two shunt stubs. When is equal to a half-wavelength at 3.04 GHz in Fig. 11(c), the input impedance at the feeding point is zero (short circuit). Also, when is equal to a half-wavelength at 4.75 GHz in Fig. 11(d), the input impedance at the feeding point is zero (short circuit). Fig. 12 shows the simulated results for the proposed antenna with various values of and. It can be concluded that the notch bands for the proposed antenna are indeed controlled by the length of the -, -, and inverted-l shaped slots. The radiation characteristics of proposed antennas E and F were also studied. The radiation patterns at the passband frequency are about the same as those of the reference antenna, i.e., the antenna without a slot. In the case of the notch band frequency, as shown in Fig. 13(c) and (d), it is noted that the antenna radiation gain reduction due to the slots is more than approximately 10 15 db in the direction of maximum gain. Fig. 14 shows the measured transmission loss of proposed antennas E and F. A sharp decrease of antenna gain in the notch bands at 2.9 3.0 and 4.6 4.8 GHz is shown. For other passband frequencies outside the notch bands, the antenna gains with a notch filter are similar to those without it. IV. CONCLUSION In this paper, wideband planar monopole antennas with dual band-notched characteristics have been proposed. We showed that by etching the proper slots (such as inverted-l, and two inverted-l) in the interior of the [1] H. G. Schantz, G. Wolenec, and E. M. Myszka, Frequency notched UWB antennas, in Proc. IEEE Ultra Wideband Syst. Technol. Conf., Reston, VA, Nov. 2003, pp. 214 218. [2] S. W. Su, K. L. Wong, and C. L. Tang, Band-notched ultra-wideband planar monopole antenna, Microw. Opt. Technol. Lett., vol. 44, pp. 217 219, 2005. [3] A. Kerkhoff and H. Ling, A parametric study of band-notched UWB planar monopole antenna, in IEEE AP-S Symp., Monterey, CA, 2004, pp. 830 833. [4] W. S. Lee, K. J. Kim, D. Z. Kim, and J. W. Yu, Compact frequencynotched wideband planar monopole antenna with a L-shaped ground plane, Microw. Opt. Technol. Lett., vol. 46, pp. 563 566, Sep. 2005. [5] Y. Kim and D. H. Kwon, CPW-fed planar ultra wideband antenna having a frequency band notch function, Electron. Lett., vol. 40, no. 7, pp. 403 405, Apr. 2004. [6] H. Yoon, H. Kim, K. Chang, Y. J. Yoon, and Y. H. Kim, A study on the UWB antenna with band-rejection characteristic, in IEEE AP-S Symp., Monterey, CA, 2004, pp. 1784 1787. [7] W. S. Lee, D. Z. Kim, K. J. Kim, and J. W. Yu, Multiple frequency notched planar monopole antenna for multi-band wireless system, in Proc. 35th Eur. Microw. Conf., Paris, France, Oct. 2005, pp. 1935 1937. [8] W. S. Lee, W. G. Lim, and J. W. Yu, Multiple band-notched planar monopole antenna for multiband wireless system, IEEE Microw. Wireless Compon. Lett., vol. 15, no. 9, pp. 576 578, 2005. [9] N. P. Agrawall, G. Kumar, and K. P. Ray, Wide-band planar monopole antennas, IEEE Trans. Antennas Propag., vol. 46, no. 2, pp. 294 295, Feb. 1998. [10] E. Lee, P. S. Hall, and P. Gardner, Compact wideband planar monopole antenna, Electron. Lett., vol. 35, no. 25, pp. 2157 2158, Dec. 1999. [11] M. J. Ammann, Square planar monopole antenna, in Proc. IEE Nat. Antennas Propag. Conf, York, U.K., 1999, pp. 37 40. [12], Control of the impedance bandwidth of wideband planar monopole antennas using a beveling technique, Microw. Opt. Technol. Lett., vol. 30, no. 4, pp. 229 232, 2001. [13] S. W. Su, K. L. Wong, and C. L. Tang, Ultra-wideband square planar monopole antenna for IEEE 802.16a operation in the 2 11 GHz band, Microw. Opt. Technol. Lett., vol. 42, no. 6, pp. 463 466, 2004. Wang-Sang Lee received the B.S. degree in electrical engineering from Soong-Sil University, Seoul, Korea, in 2004, and the M.S. degree in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejon, Korea, in 2006. He is currently with KAIST. His research interests include the UWB antennas with filter and RF systems. Dong-Zo Kim received the B.S. degree in electrical engineering from Han-Yang University, Seoul, Korea, in 2004, and the M.S. degree in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejon, Korea, in 2006. He is currently with KAIST. His research interests include active antennas and two-port network systems.

LEE et al.: WIDEBAND PLANAR MONOPOLE ANTENNAS WITH DUAL BAND-NOTCHED CHARACTERISTICS 7 Ki-Jin Kim received the B.S. degree in electrical engineering from the Kook-Min University, Seoul, Korea, in 2004 and the M.S. degree in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejon, Korea, in 2006. He is currently with KAIST. His research interests include beam control and mutual coupling rejection of multiantennas. Jong-Won Yu (M 05) was born in Yeosu, Korea, in 1970. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejon, Korea, in 1992, 1994, and 1998, respectively. In 1998, he joined the Code Division Multiple Access (CDMA) Division, System Large Scale Integration (LSI) Business, Samsung Electronics Company Ltd., where he was a Senior Engineer with the Field Application Group. In 2002, he joined the RF Team, Telson U.S.A. Company, where he was a Senior Engineer with the RF Hardware (H/W) Group involved with research on CDMA/personal communication system (PCS)/advanced mobile phone service (AMPS)/global positioning system (GPS) transceivers. He is currently an Associate Professor with KAIST. His current research areas include RF/microwave transceivers, antennas, wave propagation analysis, and electromagnetic interference (EMI)/electromagnetic compatibility (EMC).