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A design method for highly‐selective bandpass filters based on novel dual‐mode fan‐shaped patch resonator (FSPR) is presented in this article. Different from traditional dual‐mode patch filters, two higher‐order modes TM020 and TM110 are utilised to generate a passband, and the two resonant frequencies can be controlled well by its radius and angle. A dual‐mode filter with two transmission zeros (TZs) is implemented by using one FSPR, and locations of TZs can be controlled well. To further improve the selectivity of FSPR filters, four‐pole filters based on two identical dual‐mode FSPRs with co‐directional and reverse‐directional cascade are simulated, fabricated and measured. Furthermore, three‐ and four‐pole filters based on dual‐mode FSPR and single‐mode uniform impedance resonators are proposed, which are also simulated, fabricated, and measured. The later proposed filters not only achieve high selectivity and wide stopband, but also have simple structure and small size.

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A new rasorber with a switchable transmission/reflection band between two wide absorption bands is proposed based on an ultrathin low-profile frequency selective surface. To miniaturise the periodicity in the lossy layer, convoluted elements and lumped capacitors are applied, while in the bandpass layer, non-resonate elements are implemented with PIN diodes embedded. Moreover, a square loop loaded with resistors is introduced to the bottom of the lossy layer, which consequently extends the bandwidth of the high-frequency absorption band without an increase in thickness. The simulation indicates that the proposed design presents a switchable transmission/reflection band in two-sided absorption bands with a periodicity of 0.089λ L (wavelength at the lowest operating frequency) and thickness of 0.066λ L . Angular stability and polarisation insensitivity are also obtained due to the low profile and symmetry. A prototype was fabricated and measured, according to which the switchable transmission band covers 3.67-3.93 GHz; the absorption bands cover 2.37-3.23 GHz and 4.09-6.63 GHz in the OFF state of the diodes, and 2.23-3.02 GHz and 4.01-6.53 GHz in the ON state. As reasonable agreements between the measurement and simulation results are observed, the performance of the design is verified. K E Y W O R D Sactive filters, diodes, frequency selective surfaces, radar absorbing materials Min Tang and Qikun Liu contributed equally to this work and should be considered as co-first authors.

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