This article presents a simple method to design an ultra-wideband (UWB) bandpass filter (BPF) with a notched band. The structure of the filter is simple and is composed of a single half-wavelength resonator loaded with three sets of short-circuit step impedance stubs. An equivalent circuit model is presented to analyze the resonance characteristics. Compared with the traditional quarter-wavelength uniform impedance stub, the novelty of the short-circuit step impedance stub introduces two design parameters: the impedance ratio (K) and the electrical length ratio (α). Therefore, by adjusting these design parameters, the frequencies of the first two notched bands can be tuned widely, so a wide frequency band with a notched band can easily be achieved. With a K of 0.36 and an α of 0.6, the designed filter achieved an ultra-wideband bandpass response with a notched band. The UWB response had a passband range of 2.5 GHz–10.5 GHz and a notched band around 5.1 GHz with an attenuation of about 45 dB. The insertion loss in the entire passband was less than 1.26 dB, and the return loss was larger than 10 dB on average. The maximum group delay variation in the two passbands was less than 0.3 nS. The measurement results showed good agreement with the simulation results.

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A new compact broadband waveguide twist by double-corner-cut square slots is presented. In particular, the proposed module is made from two substrate layers and three copper cladding layers, which can be used as the waveguide twist and broadband filter. A double-corner-cut square slot is etched on each metal layer with relative rotation. It is found that the optimized module can provide the bandwidths of no less than 6.1% at the 10-dB return-loss level or 4.3% at the 20-dB level with a minimum length of 0.07 waveguide width.

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A three-layer circular polarization selective surface operating in Ku-band is proposed in this communication. We design a thin grid-patch structure to achieve linear-to-circular polarization transformation over a wide frequency band. The periodic structure could be analyzed via equivalent circuit model theory. Numerical simulations have been carried out and the prototype structures have been fabricated and experimentally validated. The axial ratio (AR) of the transmitted field remained below 3 dB over the entire bandwidth, and the 3 dB AR bandwidth is approximately 44.6%. The design has a subwavelength thickness of 6 mm (0.28λc). Besides, this design could be also applied to beam scanning antennas because of its capability in condition of large oblique incidence angle (±50 deg).