Contents I. Introduction
Bandpass frequency selective surfaces (FSSs) have been widely used in radomes of various antennas to prevent interference or reduce radar cross section (RCS) [1]. However, out-of-band reflected waves typically enhance scattering in other directions, which might be detected by bistatic radar. In order to overcome this defect, the concept of FSR is proposed, which can transmit in-band signals and absorb out-of-band signals. Generally, FSRs can be divided into three types according to the relative positions of the absorption band and the transmission band: the absorption band is located above, below, and on both sides of the transmission band, to absorb out-of-band electromagnetic(EM) waves. An FSR using resistive film as a lossy layer was first proposed [2], and its passband is located at low frequencies. Similar implementations include inserting lumped resistors between metallic structures [3]. In 2014, a design that can reduce reflections in both high and low frequency bands was proposed, and low insertion loss was achieved by shorting the lumped resistance with a series circuit at the passband [4], and another method was to introduce parallel resonances in the lossy layer [5]. Recently, more and more attention has been focused on wideband, low profile, and reconfigurable properties of FSRs. However, most of the wideband FSRs have only one absorption band behind the passband, instead of having absorption bands on both sides of the passband, and the function is single, which cannot cope with the complex electromagnetic environment. In order to solve these problems, a few wideband FSRs in ATA mode have been proposed, but [6] uses resistive film as the lossy layer, which does not solve the problem of high insertion loss in the passband, and [7] does not have reconfigurability. Furthermore, by loading PIN diodes, the reconfiguration of ATA FSR and absorber can be achieved [8], but these are polarization-sensitive or narrowband. In recent years, microfluidic technology has been exploited to develop active metasurfaces [9], and liquid metals have gained extensive attention due to their self-healing properties, which can adopt arbitrary shapes based on the geometry of microfluidic channels. In [10], a reconfigurable FSS based on liquid metal was proposed. By injecting Eutectic gallium indium(EGaIn) into the top and bottom microchannels, the switching of four states was achieved. A similar metasurface with switchable absorption band was also reported [11]. But liquid metal-based reconfigurable FSR have never been seen.
