“…The insertion loss (IL) of pass-band and the fractional bandwidth (FBW) of the 10-dB-absorption band (with the reflec-tion coefficient less than −10 dB) are two criteria for evaluating FSR. In recent years, various FSRs have been widely investigated, such as the FSRs with high-frequency passband proposed in [13,14,15,16], and the FSRs with twosided absorption bands reported in [17,18,19,20,21,22,23,24,25,26]. Besides, to meet the stealth requirements of low-frequency antenna systems, several FSRs with lowfrequency pass-band were also presented in [27,28,29,30,31].…”