Radomes, serving as space filters to transmit the in-band signals and reflect the out-of-band signals, have become hotspots to protect the electronic devices such as antennas and sensors in aircraft, radar systems. Among them, frequency selective surfaces (FSSs) are the widely used structures [1,2]. However, the FSSs also will be a significant source of radar cross section (RCS) due to the backward scattering of the out-of-band, which are not suitable for the radomes of stealth platforms. To reduce RCS, several designs have been proposed [3,4]. For example, a 10 dB RCS reduction bandwidth of 25% is achieved by using two layers FSS to form phase cancellation [4]. However, the odd multiple of a quarter-wavelength thickness of the radome slab limits its application. Another 10 dB reduction of RCS at 3.4–18 GHz is achieved using magnetic absorbers, but the randomness of the sub-array configuration adds to the design effort [5].

Metasurface is a two-dimensional artificial structure having some specific properties that cannot be found in nature [6]. They have been extensively studied for RCS reduction due to their advantages of low profile, easy integration, and easy fabrication, such as polarization conversion metasurfaces (PCMs) [[7], [8], [9], [10]], artificial magnetic conductor (AMC) [[11], [12], [13]], coding metasurface [14,15]. Recently, phase gradient metasurfaces (PGMs) have attracted numerous interests due to their convenient manipulation of magnitude and phase of electromagnetic (EM) waves [[16], [17], [18], [19], [20]]. A split cross structure is designed to couple the vertically incident EM waves into surface waves, significantly reducing backward RCS, although it is polarization-insensitive, only few frequencies have RCS reduction [21]. Similarly, a polarization insensitive and simple cross-type structure is designed to achieve low-frequency backward RCS reduction. Based on this design, the average RCS reduction can reach 8 dB, however, the RCS bandwidth of this design is narrow [22]. To broaden the bandwidth, a spiral arrangement is designed, which can have the RCS reduction of more than 10 dB in the band of 12.2–23.4 GHz, the relative bandwidth is 62.9% [23]. The above studies are only in single-layer design, while, some composite methods have been applied to improve RCS reduction bandwidth, such as multilayer structures, using FSS instead of metal bases [24]. A multilayer structure with simple patch was designed to improve RCS reduction bandwidth [25]. Although the unit is simple and the RCS reduction of more than 10 dB in the band of 5.3–18 GHz, the multilayer structure has the problem of high profile. The research in the RCS reduction by using PGMs continuous to attract more attentions, while some problems, such as the narrow bandwidth and complex design still have not been adequately addressed.

In this paper, a new single-layer radome design by combining a broadband PGM and a bandpass FSS with the particle swarm optimization (PSO) algorithm for broadband RCS reduction as well as high-efficiency transmission is proposed. A PGMs-FSS consists of six one-dimensional periodic units is first proposed, then the ground plane is replaced by bandpass FSS, and the final sizes of FSS are optimized by PSO algorithm, so that the over 10 dB RCS reduction is obtained, covering X and Ku bands. Compared to the conventional single-layer FSS or PGMs, the proposed design has a significantly improved RCS reduction performance as well as high-efficiency signal transmission.