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In this paper, we combine the design of band-pass frequency selective surfaces (FSSs) with polarization converters to realize a broadband frequency-selective polarization converter (FSPC) with lowbackward scattering, which consists of the top polarization conversion layer backed by a multi-layer bandpass FSS. It is numerically demonstrated that the 1 dB transmission window can be obtained from 8.5 GHz to 11 GHz with a 25.6% fractional bandwidth (FBW), and the bandwidth of reflection below -10 dB is up to 92% from 5.6 GHz to 15.13 GHz. Moreover, the proposed device can achieve two polarization conversion bands (5.66-6.9 GHz and 12.8-15.2GHz) with the polarization conversion ratio over 90%. Besides, by arranging the proposed structure in a checkerboard-like distribution, the backward scattering energy can be reduced in a wide frequency band ranging from 4 to 16 GHz. Both simulation and experimental results are in good agreements, which demonstrates our design strategy. Compared with the conventional polarization conversion designs, the proposed design presents an extra frequency-selective performance and hence can be applied to various practical situations, for instance, working as radomes to transmit the in-band signals with high-efficiency while keeping low-backward scattering for the out-of-band waves.

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In this letter, a novel ultra-compact spoof surface plasmon polaritons (SSPPs) low-pass filter which shows a high-efficiency transmission and an excellent out-of-band suppression is investigated. It consists of a pair of double-layer SSPPs, the top layer using interdigital strip and meander-line, and the bottom layer is metal gratings of fish-bone shape. The vast advantages of the proposed SSPPs structure in lower asymptotic frequency mode compared with traditional SSPPs structure with the same size enables the proposed lowpass filter to be more miniaturized. The dimensions of the miniaturized structure are 0.75λ 0 × 0.1λ 0 (λ 0 is the wavelength corresponding to the central operating frequency). The high-efficient mode conversion is achieved by gradient meander-line lengths. Moreover, we also investigate the transmission performance of the proposed structure when it is conformal. To validate the design concept, the prototype of a straight and bend low-pass filter has been manufactured and measured. The experimental results of the fabricated sample agree with the simulation results well, in which the insertion loss is less than 0.5 dB, the reflection coefficient less than −15 dB, and excellent out-of-band rejection of the attenuation is greater than 30 -dB, which proves the design validity. The proposed ultra-compact SSPPs structure may have great potential applications in miniaturized integrated circuits in the microwave and terahertz (THz) frequencies.