2370 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY2015 A Planar Dual-Band Periodic Leaky-Wave Antenna Based on a Mu-Negative (MNG) Transmission Line Roy B. V. B. Simorangkir and Yongshik Lee Abstract—In this study, a planar periodic leaky-wave antenna (LWA) that provides a dual-band full-space scanning property is presented. The LWA is based on a mu-negative (MNG) transmission line that offers a nonlinear dispersion characteristic with a very simple structure. The dual- band beam scanning property is achieved by utilizing the n = -1 and n = -2 space harmonics to overcome the limitation of the MNG line that provides forward scanning only in the n =0 space harmonic. Microstrip bends are introduced to achieve proper matching and thereby suppress the open stopband effect at both broadside radiation frequencies, to avoid interference between two space harmonics, and to ensure efficient radia- tion. The proposed design method is validated by good agreement between the simulated and experimental results for the dual-band LWA that is designed to provide full-space scanning with the first and second broad- side radiation frequencies at 4.3 and 8 GHz. The demonstrated total scan angle range of 279 ◦ is the widest range reported for the dual-band LWA. Index Terms—Composite right-/left-handed, dual-band, full-space scanning, leaky-wave antenna (LWA), mu-negative (MNG), open stopband. I. I NTRODUCTION Leaky-wave antennas (LWAs) have a unique feature that their beam can be scanned by varying the frequency of operation. Therefore, a relatively high directivity is achieved with great structural advantages of extremely simple feeding network, low profile, and low-cost fab- rication. These advantages make planar LWAs suitable for various applications that require single or multiple beam scanning, such as human tracking radars [1], [2], automotive radars [3], [4], and real-time spectrum analyzers [5]. Current research efforts focused on LWAs include dual-band oper- ation to enable scanning in two different operating bands [6]–[9]. One of the most popular methods for realizing planar dual-band LWAs is to utilize the nonlinear dispersion characteristic of extended composite right-/left-handed (E-CRLH) technology [10], [11], which success- fully overcomes the limitation of a CRLH line that cannot provide full-space scanning in two different bands [6]–[8]. Nevertheless, an E-CRLH unit cell consists of a number of reactive elements, which complicate the design procedure. In addition, the E-CRLH unit cells are more vulnerable to parasitic effects not only from the reactive elements but also from the relatively complex layout and via holes for shunt connections in microstrip forms. Therefore, nonideal prop- erties are observed, such as imperfect matching [6]–[8] and backside radiation [8]. This study presents a dual-band periodic LWA based on a mu- negative (MNG) transmission line. An MNG line is a variation of the CRLH line [12], which also provides a nonlinear dispersion charac- teristic. However, with an MNG line, this essential characteristic for the development of dual-band circuits is achieved with a much simpler Manuscript received August 26, 2014; revised January 30, 2015; accepted March 02, 2015. Date of publication March 09, 2015; date of current ver- sion May 01, 2015. This work was supported in part by the Korea Evaluation Institute of Industrial Technology (KEIT) Research Grant of 2015(10047815) and in part by the National Research Foundation (NRF) of Korea government (MEST) under Grant 2011-0016802. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea (e-mail: yongshik. lee@yonsei.ac.kr). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2015.2410802 structure because no shunt connections are required. Therefore, the design and fabrication processes of LWAs are simplified greatly. Furthermore, without any inductors, an MNG line is especially more suitable for high-frequency applications. In addition, a method is demonstrated, for the first time, that utilizes two higher order space harmonics to achieve the dual-band full-space scanning property. This approach allows tuning the dispersion easily while maintaining the simplicity of an MNG unit cell. The experimental results of an LWA with broadside radiation frequencies at 4.3 and 8 GHz are in great agreement with the simulated results, thus validating the proposed approach. II. DESIGN PRINCIPLE A. Utilization of Higher Order Space Harmonics in an MNG Line Fig. 1 shows the schematics of conventional CRLH and MNG unit cells, where CL and LL are the series capacitance and parallel inductance of the left-handed part, respectively, and ZR and θR are the characteristic impedance and electrical length of the right-handed transmission line section, respectively. Compared with a CRLH line from which the MNG line was derived, the shunt inductor is elimi- nated in the MNG line. This makes the MNG line particularly simpler in terms of the structure while still allowing manipulation of its nonlin- ear dispersion characteristic to achieve dual-band operation. However, the dispersion diagram of the MNG line consists of a right-handed band above the mu-zero frequency f M =1/2π √ LRCL and a rejec- tion band below it, where μ< 0 and ǫ> 0 [12]. This indicates that when an MNG line-based LWA is operated in its n =0 space har- monic, it cannot provide the full-space beam scanning property and radiates only in the forward direction above f M. In this study, the above-mentioned limitation of the MNG-line- based LWA is overcome by operating in its higher order space harmonics. When MNG unit cells are cascaded to develop an LWA, it becomes a periodic structure. According to the Bloch–Floquet theorem [13], an infinite number of space harmonics is generated automatically by periodic modulation, whose phase constant β n of the nth harmonic is given by βn(ω)= ±  β0(ω)+ 2πn p  , n =0, ±1, ±2,... (1) where β0 is the phase constant of the n =0 space harmonic, and p is the period of the structure. Because βn is simply a repetition of β0 with a period of 2π, there are higher order space harmonics that also radiate from the backward to forward direction in different frequency bands. Therefore, by utilizing two higher order space harmonics, n = -1 and n = -2 space harmonics, a dual-band periodic LWA can be developed without increasing the complexity of the unit cell. B. Synthesis of an MNG Unit Cell The design equations of an MNG unit cell for dual-band applica- tions can be derived from its dispersion relation as follows: L R = ZR  (φ 2 1 - φ 2 2 ) (ω 2 1 - ω 2 2 ) (2a) CR = 1 ZR  (φ 2 1 - φ 2 2 ) (ω 2 1 - ω 2 2 ) (2b) CL = CR(ω 2 1 - ω 2 2 ) (φ 2 1 ω 2 2 - φ 2 2 ω 2 1 ) (2c) θR = ω1 √ LRCR (2d) 0018-926X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.