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A new bandpass filter (BPF) with two switchable operating passbands has been proposed. The BPF topology is formed by switchable resonators and stepped‐impedance resonators. The two operating bands can be switched on and off independently. For verification, a switchable BPF with a third‐order Chebyshev bandpass response was designed and fabricated using microstrip technology. The developed BPF was measured to verify the predicted and simulated results. A close agreement was observed between them. The proposed switchable BPF has good in‐band and out‐of‐band performances. Besides, the attenuation is > 30 dB as the passband is switched off.

The letter presents a triple‐mode short‐circuited circular patch resonator for bandpass filter (BPF) design. The resonator employs the TM010 mode as its dominant mode, the resonant frequency of which is lower than that of the TM110 mode. In addition, four radial slots are etched on the patch to extend the current paths of the TM110 and TM210 modes, shifting down the resonant frequencies of the two modes. The adjacent triple modes can be used to design wideband BPF. Meanwhile, the centre frequency (CF) and bandwidth (BW) can be tuned by the length of the slots. Furthermore, the short‐circuited vias are arranged in a circle providing a tuneable parameter to control the CF and BW as well. According to the symmetry of the geometry, the resonator can be bisected to realise half‐mode resonance. Two cascaded half‐mode semi‐circular patch resonators are applied to design BPF with high selectivity. BPFs using the circular patch resonator and the semi‐circular patch resonator are designed and measured for demonstration.

UWB suspended stripline (SSL) bandpass filter with an adjustable notched band and four transmission zeros (Tzs) is proposed here. To prevent interference signals from the wireless local‐area network (WLAN) entering UWB system, a short‐stub resonant network is introduced to this UWB filter. In addition, utilizing different resonant modes of the two short–stubs, four Tzs are generated to improve the skirt selectivity. Both the centre frequency of the notched band and the positions of Tzs can be controlled by tuning the structural parameters. The UWB SSL bandpass filter is designed, fabricated and tested. The measured results are in good agreement with the simulated results.

This letter reports a highly compact microstrip quad‐band bandpass filter based on two folded λ/4 stepped impedance resonators and three folded dual‐mode short stub‐loaded resonators. The two stepped impedance resonators not only form the first passband but also serve as the feed structure and provide the source‐load coupling path for the three higher passbands created by the short stub‐loaded resonators. Thus, a compact size can be obtained, and three pairs of transmission zeros can be realised near the three higher passbands to improve the frequency selectivity. It has been shown that the centre frequencies and bandwidths of the four passbands can be controlled independently. A microstrip quad‐band bandpass filter centred at 1.1/2.05/3/3.55 GHz is implemented and tested achieving an extremely small size of 0.07λg × 0.14λg, where λg is the guided wavelength at 1.1 GHz. This presents one of the smallest quad‐band bandpass filter ever demonstrated in the literature.

This letter presents the design of a novel dual‐mode dual‐band bandpass filter that utilises the TM210 and TM020 modes of cylindrical cavity resonators for millimetre‐wave operation when fed with standard WR‐10 waveguide ports. In this manner, the two selected modes of the cylindrical cavity resonators are demonstrated as a single propagation path for fixed dual‐passband responses without the need of cavity perturbations or tuning screws. The passbands are designated for centre frequencies at approximately 102.8 and 110.9 GHz and exhibit a four‐pole Chebyshev characteristic in each of the passbands, which are separated by a transmission zero location. Simulated and measured results of the prototype are presented to verify the design.

This paper proposes a five‐way filtering power divider (FPD) based on a back‐to‐back quarter‐mode substrate‐integrated waveguide (QMSIW) and microstrip resonator. The FPD is composed of two layers of the dielectric substrate. Back‐to‐back QMSIWs and microstrip resonators are designed on the dielectric substrate 1. On the dielectric substrate 2, the SIW circular cavity is excited by a coaxial probe to realize the power dividing function and is coupled with QMSIWs through five rectangular slots. The measurement results show that the minimum insertion loss of the FPD is 1.2 dB at 9 GHz, the measured functional bandwidth is 4%, and the stopband attenuation from 10 to 19.8 GHz is better than 35 dB, the stopband attenuation at frequencies below 8.3 GHz is greater than 45 dB.

In this letter, a novel eighth‐mode ridged substrate integrated waveguide (EMRSIW) is proposed by bisecting the RSIW resonator along the virtual magnetic walls. The EMRSIW shows a size of nearly one‐eighth of a conventional RSIW resonator and a significant size reduction compared to the conventional eighth‐mode SIW resonator. A prototype of a third‐order bandpass filter with transmission zeros has been designed and tested.

Two dual‐mode filters implemented by folded substrate integrated waveguide (FSIW) are proposed here. The high order modes (TE103 and TE301) in the FSIW cavity are employed to realize the function of filters. The non‐resonating node TE202 mode is employed in the first filter to realize the transmission zero below the passband. A metal pin and an L‐shaped slot are used in another dual‐mode filter to restrict the parasitic mode, TE202 mode. The proposed FSIW dual‐mode filters with simple structures exhibit compact size and good frequency selectivity. As per knowledge, it is the first time that the quarter mode FSIW (QMFSIW) cavities are utilized to design the multimode filters. The measured results agree well with the simulated ones, which validates the feasibility of the compact dual‐mode filters based on the technology of double‐layered FSIW.

A low‐loss Ka‐band coupled‐resonator bandpass filter based on the printed ridge gap waveguide (PRGW) technology is presented for millimetre‐wave communication applications. The PRGW is employed to allow electromagnetic‐wave propagation in the air gap in order to avoid dielectric loss and improve transmission performance which is suitable for millimetre‐wave applications. The bandpass filter is implemented with two transmission zeros which are easily obtained by adjusting the stub length of two T‐shape stub‐loaded resonators (SLR). From its simulation frequency response, the filter has an insertion loss of 0.8 dB, a return loss less than −20 dB during the passband and high selectivity in cut‐off frequency. The prototype of the proposed filter is fabricated and tested and a good agreement can be observed between the measured and simulated results.

The aim of this paper is to investigate the bending characteristics of a low‐pass RF MEMS (radio frequency microelectromechanical system) filter based on a flexible substrate. The proposed filter is achieved by complementary split ring resonator and MEMS bridge which are fabricated on a flexible LCP (liquid crystal polymer) substrate. The bending characteristics of the filter are revealed by theory, simulation and measurement. The experimental results demonstrate that the return loss of the filter deteriorates and the cut‐off frequency of the filter decreases with the curvature of the flexible substrate increasing. The cut‐off frequency is 38.5 GHz and the return loss is −22.7 dB at 10 GHz with the flat substrate. When the curvature of the substrate increases to 28.57 m–1, the cut‐off frequency decreases to 14.8 GHz, and the return loss deteriorates to −5.7 dB at 10 GHz.