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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 14, NO. 6, JUNE 2004 277 Electronically Scanned Composite Right/Left Handed Microstrip Leaky-Wave Antenna Sungjoon Lim, Student Member, IEEE, Christophe Caloz, Member, IEEE, and Tatsuo Itoh, Fellow, IEEE Abstract—A novel electronically scanned periodic microstrip leaky-wave (LW) antenna based on the concept of composite right/left-handed (CRLH) metamaterials is presented. This antenna includes varactors modulating the capacitive loading of the unit cell and therefore the propagation constant of the structure, which results in voltage scanning of the radiated beam. An accurate circuit model is proposed. The antenna is demonstrated experimentally to exhibit continuous scanning in the dominant mode from backward to forward angles. The scanning range is at 35 V to at 0 V, and broadside occurs at 9 V, from at the fixed frequency of 3.23 GHz. 10 +7 5 Index Terms—Composite right/left handed (CRLH) structure, electronical scanning, leaky-wave (LW) antenna, metamaterials. I. INTRODUCTION M ODERN commercial and military communication frequency bands are getting saturated. In this context, despite their unique features of high directivity and broad-range scanning, conventional leaky-wave (LW) antennas [1], [2] are often not suitable because of the excessive frequency band they use for scanning. Recently, we proposed a novel transmission line approach of left-handed (LH) metamaterials in [3] and [4] and introduced a generalized composite right/left handed (CRLH) concept [generalized metamaterial with negative at low frequencies (LH) and positive at high frequencies (RH)] in [5] and [6]. Based on this concept, a backfire-to-endfire LW antenna operating in the dominant mode was developed and shown to exhibit backfire to endfire radiation capabilities in the dominant mode [7], while conventional LW antennas operate in a higher mode and cannot efficiently radiate at broadside. However, this antenna is also scanned by frequency. A LW antenna that can be scanned at a fixed frequency is often preferable to a frequency-scanned one. Much effort was directed toward developing an electronically-scanned LW antenna [8]–[10]. The scanning angle from broadside is given by the well-known equation [11] (1) space wavelength, is the space harmonic , and is the period of the structure. Equation (1) shows that the radiation angle is determined by the guided wavelength or by the period . Horn et al. [8] electronically changed by modulating p–i–n diodes in a dielectric waveguide antenna, but the resulting antenna allows only two discrete radiation angles because the diodes have only two states (biased / unbiased). Maheri et al. [9] also controlled but by using a biasing magnetic field in a corrugated ferrite slab configuration, but a dc magnetic field supply is not practical for most applications. Huang et al. [10] reported a different approach, in which the period of the structure was controlled by using p–i–n diodes switches, but their antenna was also restricted to two discrete states. In this letter, we present a novel type of fixed-frequency electronically-scanned LW antenna. The proposed antenna is based on the CRLH metamaterials concept. The radiated beam of this antenna can be continuously scanned. In addition, the antenna is implemented in microstrip technology, whereas all aforementioned tunable antennas are waveguide antennas. Such a lowprofile characteristic makes the proposed antenna easier to fabricate and preferable for the integrated millimeter wave systems. II. PRINCIPLE OF ELECTRONICALLY-SCANNED LW ANTENNA The proposed antenna is designed based on the CRLH structure which consists of a series capacitor and a shunt inductor plus a parasitic series inductor and a parasitic series capacitor [4], [5]. In the CRLH structure, the propagation constant is dependent on the inductive and capacitive loadings of the line. The radiation angle is given by (1), and can be determined once is known. The underlying idea of our antenna is based on the observation that, even though propagation constant is dependent on frequency, it can also be modulated by tuning the inductances and capacitances in the CRLH structure. In the lossless case of ideal CRLH-TL [6], the equivalent circuit model is shown in Fig. 1(a) and the propagation constant can be obtained by (2) where is propagation constant of the waveguide, is the guided wavelength, space wavenumber, is the free is the free Manuscript received October 27, 2003; revised February 19, 2004. This work was supported by the MURI program “Scalable and Reconfigurable Electromagnetic Meta-materials and Devices,” the Department of Defense (N00014-01-1-0803), and the U.S. Office of Naval Research. The review of this letter was arranged by Associate Editor A. Weisshaar. The authors are with the Electrical Engineering Department, University of California, Los Angeles, CA 90095 USA. Digital Object Identifier 10.1109/LMWC.2004.828008 where (3) (4) where is period of the structure. 1531-1309/04$20.00 © 2004 IEEE 278 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 14, NO. 6, JUNE 2004 Fig. 1. Equivalent circuit models for the unit cell of (a) purely passive CRLH LW antenna and (b) the proposed LW antenna including the varactor bias is the network. C (V) is the tunable capacitance of the varactor and C capacitance of the dc block. Fig. 3. Measured (a) radiation patterns and (b) return losses for the antenna shown in Fig. 2 at different bias voltages. At 3.23 GHz, the receiving angles for the reverse voltages 35, 9, and 0 V are 10 , 0 and +7:5 , respectively. Return losses at 3.23 GHz are 6.263, dB, and 11.402 dB at 35, 9, and 0 V, respectively. 0 0 0 0 (8) (9) III. DESIGN AND EXPERIMENTAL RESULTS Fig. 2. Prototype of the proposed antenna including shunt varactors. In the balanced case, defined by gation constant can be expressed from (2)–(4) as , the propa(5) In the proposed antenna, we tune the reactance provided by with a varactor. For this purpose, a dc block has to be introduced for the bias network, which makes the equivalent circuit mode slightly more complicated, as shown in Fig. 1(b). But this structure can still be effectively CRLH in nature. In this case, the propagation constant and scanning angle are the following function of the bias voltage (6) where (7) Fig. 2 shows the prototype of the proposed electronically, scanned LW antenna built on RT/Duroid5880 ( ). For the bias network, ATC 3.9 pF chip capacitors and Murata 4.7 nH chip inductors are used for dc blocks and dc feeds, respectively. The varactors are the Metelics MSV34069E28X. The reverse bias voltage is varied from 0 V to 35 V. In order for the varactor to operate at the reverse bias, positive voltage is supplied at cathode of the varactors and their anode is connected to the ground. The output port of the antenna is terminated to 50 in order to prevent reflection inducing spurious beams on the other side of the normal to the substrate. Forward, broadside and backward scanning at the fixed frequency of 3.23 GHz are demonstrated in Fig. 3(a), where the , 0 , and are obtained at 0, 9, and 35 V, reangles spectively. The return loss of the antenna is shown in Fig. 3(b). At 3.23 GHz, they are 11.402, 11.203, and 6.263 dB at 0, 9, and 35 V, respectively. The continuous electronic-scanning LIM et al.: ELECTRONICALLY-SCANNED COMPOSITE RIGHT/LEFT HANDED MICROSTRIP LEAKY-WAVE ANTENNA 279 tenna as a function of the number of cells at 0 V. The gain from full-wave simulation was found to increase from 4.8 to 5.7 dB at 0 V as the number of cells is increased from four to 27 cells. IV. CONCLUSION Fig. 4. Scanning angle versus reverse bias voltage relationship. The transition reverse voltage is 9 V in the measurement. A novel fixed-frequency electronically-scanned microstip leaky-wave antenna, based on the concept of CRLH metamaterials, is proposed and demonstrated experimentally. The CRLH equivalent circuit model, including the varactors bias network, is proposed, and the principle of the antenna is explained based on this model. At the fixed frequency 3.23 GHz, the beam of the antenna can be continuously steered from backward to forward angles by tuning the reverse voltage of the varactors. Wider scanning angle ranges can be achieved by introducing a varactor in series in addition to the shunt varactor used in the reported antenna. REFERENCES Fig. 5. Full-wave simulated half-power beamwidth (HPBW) as a function of number of cells at 0 V. capability of the antenna is shown in Fig. 4. The numerical and full-wave simulated results are compared with the measured re) four-cell structure shown in Fig. 3 sults. A short (0.753 was used for the proof of concept. As a consequence of this very short length, the half-power beamwidth (HPBW) is 73 at 0 V. The measured gains are 6.1, 5.8, and 5.6 dB at 0, 9, and 35 V, respectively. As in conventional LW antennas, the HPBW and gain can be improved by simply increasing the length of the line. Fig. 5 shows the full-wave simulated HPBW of the an- [1] W. Menzel, “A new-traveling wave antenna in microstrip,” Arch. Elektron. Uebertrag. Tech., vol. 33, no. 4, pp. 137–140, Apr. 1979. [2] A. A. Oliner and K. S. Lee, “The nature of the leakage from higher modes on microstrip line,” in IEEE MTT-S. Dig., Baltimore, MD, June 1986, pp. 57–60. [3] C. Caloz and T. Itoh, “Application of the transmission line theory of lefthanded (LH) materials to the realization of a microstrip LH transmission line,” in Proc. IEEE-APS Conf., vol. 1, San Antonio, TX, June 2002, pp. 412–415. , “Transmission line approach of left-handed (LH) structures and [4] microstrip realization of a low-loss broadband LH filter,” IEEE Trans. Antennas Propagat., vol. 52, pp. 1159–1166, May 2004. [5] , “Novel microwave devices and structures based on the transmission line approach of meta-materials,” in IEEE MTT-S. Dig., vol. 1, Philadelphia, PA, June 2003, pp. 195–198. [6] C. Caloz, A. Sanada, and T. Itoh, “A novel composite right/left-handed coupled-line directional coupler with arbitrary coupling level and broad bandwidth,” IEEE Trans. Microwave Theory Tech., vol. 52, pp. 980–992, Mar. 2004. [7] L. Lei, C. Caloz, and T. Itoh, “Dominant mode leaky-wave antenna with backfire-to-endfire scanning capability,” Electron. Lett., vol. 38, pp. 1414–1416, Nov. 2002. [8] R. E. Horn, H. Jacobs, E. Freibergs, and K. L. Klohn, “Electronic modulated beam steerable silicon waveguide array antenna,” IEEE Trans. Microwave Theory Tech., vol. MTT-28, pp. 647–653, June 1980. [9] H. Maheri, M. Tsutsumi, and N. Kumagi, “Experimental studies of magnetically scannable leaky-wave antennas having a corrugated ferrite slab/dielectric layer structure,” IEEE Trans. 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