## 1 Introduction  
As a two-dimensional artificial material, metasurface (MS) is composed of sub-wavelength element array [1; 2; 3; 4], which has been widely applied in optoelectronics devices [5; 6; 7; 8; 9; 10], such as sensor, detector, etc. Because of its ability to effective manipulation of electromagnetic (EM) waves, MS can be especially applied in radar stealth field [11; 12; 13; 14]. In radar stealth field, the radar echo feature signal can be changed to reduce the detection probability of objects [15; 16; 17]. The radar cross section (RCS) is an important physical quantity to measure the echo capability of target radar, which attracts much attention in stealth platforms of military applications [18; 19; 20]. By designing MS with different sizes and arrangements, effective RCS reduction can be achieved [21; 22; 23; 24].  
In recent years, as an important branch of MS, phase gradient metasurface (GMS) with low RCS has been paid great interest due to its tremendous potential in military practice [25; 26; 27; 28]. This kind of MS merely reflects the incident waves into the backward space rather than transforming EM energy into heat, which lowers the probability of MS being detected by infrared devices [29; 30; 31; 32]. GMS can introduce the artificial wave vector at in-plane direction to control the propagation direction of transmitted and reflected wave beams [33; 34; 35; 36]. The polarization conversion characteristic is used to achieve stable phase difference in a broadband frequency range, which can be applied in the design of GMS. By randomly arranging the basic units in GMS, the incident wave is irregularly reflected back to free space, the scattering energy at each directional beam is small. This designed GMS can be used to achieve the RCS reduction characteristic [37; 38; 39; 40; 41; 42]. More recently, a GMS based on cruciform structure is proposed [33], which can be applied to the RCS reduction characteristic at low frequency ranges, but the relative bandwidth of designed GMS is narrow. Then, a checkerboard MS based on fishbone-shaped is proposed [41], which can achieve a broadband RCS reduction from 6 to 18 GHz. However, the magnitude of the RCS reduction is about 5 dB, and the design scheme is relatively complex. After this, a new concept of checkboard MS is proposed [42], which can achieve a 10 dB RCS reduction in the frequency range of 9.9-19.4 GHz, but the relative bandwidth is not enough. Further, the polarization-independent MS structure is proposed for RCS reduction [22], which can achieve an ultra-broadband 10 dB RCS reduction characteristic in the range of 17-42 GHz, but the RCS cannot be reduced at the lower frequency, which is still a challenge for practical application. Therefore, it is meaningful to explore the new coding ways of MS, which can manipulate transmitted and reflected EM waves at will to achieve a high relative bandwidth at the lower frequency.  
In this paper, the matrix-type random coding theory and the RCS reduction analysis were presented, which revealed a simple and effective method to achieve wideband RCS reduction. Then, six kinds of MSs based on split-ring resonator (SRR) structure were designed to explore the RCS reduction characteristic. Compared with previous works [21; 22; 23; 24; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42], our design has some advantages: Firstly, new random coding method with novel mechanism; Secondly, the structure of basic unit is simple, which can achieve the characteristics of 180\({}^{\circ}\) cross-polarization phase differences by simply rotating the metal cut-wire structure; Thirdly, our design presents the RCS reduction at lower frequency with high relative bandwidth compared with the works published before. Such a simple and effective design may provide some potential applications in the field of stealth.