# A Dual-Beam Leaky-Wave Antenna Based on Squarely Modulated Reactance Surface
## 1 Introduction  
Due to the advantages of high gain, low profiles and simple feeding network, leaky-wave antennas (LWAs) have attracted a lot of attention since being proposed in the 1940s [1]. According to the operating principles, LWAs can be generally divided into two categories, uniform LWAs and periodic LWAs [2; 3; 4]. Compared with uniform LWAs, periodic LWAs exhibit a large scanning range and flexible design schemes, which make them widely used in millimeter-wave bands [5; 6]. One approach to achieving beam-scanning capability is utilizing the periodical shape changing of transmission line (such as triangle-truncated double-side parallel-strip lines [7] and bends of sharpness [8]). Another demonstrated method is sinusoidally modulating the surface impedance of the LWA [9; 10; 11; 12]. Some previous research about sinusoidally modulated reactance surface (SinMRS) has shown the great advantages in the synthesis of radiation patterns. For instance, a simultaneous tapering of the leaky-wave leakage rate and pointing angle along the leaky radiator was used to synthesize radiation beams [13]. Cosine-tapered designs were performed to reduce sidelobe levels and tune the pointing direction over a wide range [14]. Recently, an array of non-uniform SinMRSs with application to near-field focused leaky-wave radiation in the backward Fresnel zone were proposed [15].  
For LWAs based on SinMRS, at least five kinds of unit cells have to be selected to mimic the sinusoidal distribution of surface impedance [16], which makes the construction of LWAs extremely complex. More recently, SinMRS, constructed with five kinds of tunable unit cells, was utilized to validate beam steering at fixed frequency [17; 18]. In theory, the bias voltage of each kind of unit cell should be independently controlled, which leads to the complex bias network and additional energy loss. To avoid this situation, different kinds of unit cell are designed with various dimensions so thattheir surface impedances can satisfy the requirement under a uniform bias voltage control [17; 18]. However, the distribution of surface impedance during one period is restricted in several fixed patterns, and the phase constant and attenuation constant of the SinMRS cannot be controlled independently. To overcome this, squarely modulated reactance surface (SquMRS), which is composed of only two kinds of unit cells, is proposed in [19]. Since each kind of unit cell can be treated as a \"macro cell\" (a macro cell is loaded with only one tunable component) [20], SquMRS can load much less tunable components, which makes it convenient to realize the independent control of phase constant and attenuation constant with the simpler biasing network.  
In this paper, a novel SquMRS structure is proposed to generate the leaky-wave radiation. Firstly, the propagation properties of surface wave travelling along the SquMRS are discussed by using a numerical method based on the equivalent transmission lines model proposed in [19]. In order to verify the feasibility of SquMRS, two LWAs have been designed based on SquMRS and SinMRS, respectively. Finally, a dual-beam LWA is presented that uses an impedance superimposing approach. The measured results show a good agreement with the simulated ones.