针对频率选择表面单元的面积大、带宽窄、阻带抑制度不高、低入射角度以及阻带窄的问题，提出了一种基于3层对称叉指微带、两层方形耦合环以及4层介质层组成的三阶超宽阻带小型化单元频率选择表面单元结构。将三阶超宽阻带小型化单元频率选择表面单元中的对称叉指结构等效为电容、方形耦合环等效为电感、介质层等效为分布参数的短截线，得到了具有集总参数和分布参数的混合参数等效滤波电路。通过电路理论计算得到该混合参数等效电路具有三阶切比雪夫带通滤波器的频率响应特性以及超宽阻带的频率特性。采用电路理论计算和电磁仿真软件HFSS相结合设计了三阶超宽阻带小型化单元频率选择表面，理论计算的结果与电磁仿真软件HFSS仿真的结果相吻合，且具有三阶带通滤波器和超宽阻带的频率响应特性曲线。设计的超宽阻带小型化频率选择表面单元尺寸为6 mm×6 mm，厚度为7 mm。加工了41×41单元三阶超宽阻带频率选择表面，并测试了其频率特性。测得频率选择表面的中心频率为2.48 GHz，相对带宽为11.2%，带内插入损耗小于0.5 dB，带内回波损耗大于18 dB，阻带超过55 GHz，从2.8 GHz到55 GHz的频率范围阻带内抑制度大于25 dB，在0°到70°的入射角度内所设计的频率选择表面具有较好的频率稳定性。实测和仿真结果吻合较好，表明了设计方法和三阶频率选择表面结构的正确性。

In view of the problems of large area, narrow bandwidth, low stopband rejection and narrow stopband of frequency selective surface unit, this paper proposed a novel third-order ultra wide stopband miniaturized frequency selective surface element based on three symmetrical interdigital microstrip metal layer, square metal layer coupled loop and four dielectric layer. The symmetrical interdigital structures is equivalent to capacitance, the square coupling rings is equivalent to inductor, and the dielectric layer is equivalent to short lines. The hybrid parameter equivalent circuit with lumped and distributed parameters was obtained. According to the circuit theory, the frequency response characteristics of the third-order Chebyshev band pass filter and the frequency characteristics of the ultra wide band stopband were calculated. The third-order frequency selective surface was designed by combining the circuit theory calculation with the electromagnetic simulation software HFSS. The results of the theoretical calculation are in agreement with those of the electromagnetic simulation software HFSS, and has a frequency response characteristic curve of third-order band-pass filter and ultra wide stopband. The size of the ultra wide stopband mi-niaturized frequency selective surface element is 6 mm×6 mm, and the thickness is 7 mm. A 41×41 third-order wide band miniaturized element frequency selective surface was machined and tested. The measured center frequency is 2.48 GHz, the relative bandwidth is 11.2%, the in-band insertion loss is less than 0.5 dB, the return loss is greater than 18 dB, the stopband exceeds 55 GHz, and the rejection in the stopband is greater than 25 dB in the frequency range from 2.8 GHz to 55 GHz. It has good frequency stability in the angle of incidence from 0° to 70°. The experimental results are in good agreement with the simulation results, indicating that the design method and the structure of the three-order frequency selective surface adopted in this paper are correct.