Due to the heavy jamming band of high frequency, frequency selecting strategies are serious issues for the system designed to achieve its best performance. Pole is independent of the direction and polarization of the incident wave, but the residue corresponding to the pole is related to the direction and polarization of the incident wave. And the value of residue is proportional to the value of the pole. This paper chooses the frequency which can maximize the residue in the high-frequency band as the optimal frequency for accurate extraction. The simulation result of a large number of ship targets shows remarkable rise in average recognition rate by using this method, compared with the average recognition rate of randomly selected frequency.

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In this paper, we propose a miniaturized 2.5-dimensional (2.5D) frequency selective surface (FSS) structure with high angular stability. A novel closed-loop FSS is formed by combining the Jerusalem cross (JC) structure with the conventional rectangular closed loop using vias. This approach further enhances the coupling performance of the FSS and thus achieves miniaturized design. The unit cell size of the proposed FSS is 0.019λ0 × 0.019λ0 at the resonant frequency, and the metal is printed on a dielectric substrate with a thickness of 0.003λ0. The proposed FSS has a resonant frequency of 850 MHz and exhibits band-stop characteristics. It is insensitive to the incident angle with a good operating performance in both the TE and TM wave modes. Therefore, it can be well used as an electromagnetic shield for the GSM 850 band. In order to facilitate the rapid analysis and design of the FSS, the equivalent circuit model is further analyzed and established, and values of the corresponding lumped components are derived. In addition, a prototype FSS is fabricated using printed circuit board technology and is tested in a microwave anechoic chamber. The full-wave analysis simulation, equivalent circuit model simulation, and practical measurement results reflect a high level of consistency.