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Abstract 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.

Abstract This work outlines the design, implementation, and experimental results of a Class‐F power amplifier (PA) with a novel output matching topology by utilizing substrate integrated waveguide (SIW). The design methodology is first proposed with one highly integrated SIW matching structure. The impedances at fundamental, second harmonic and third harmonic frequencies are analysed to explain good impedance matching for the PA as well as the isolation mechanism of the direct current (DC) and radio frequency (RF). For demonstration, an S‐band 10‐W Class‐F PA is fabricated and tested. The simulation and measurement results agree very well with each other, which verifies the proposed technology of SIW output matching. To the authors’ knowledge, this is the first time to use a highly integrated SIW matching in Class‐F PA, which simplifies the output matching for third‐order harmonics and DC/RF isolation structures.

Abstract 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.

Abstract Design of a low cost Schottky diode based single‐balanced mixer (SBM) in full V‐band with an ultra‐wide intermediate frequency (IF) bandwidth is presented here. By loading multiple quarter‐wavelength short‐ended stubs at both radio frequency (RF) and local oscillator (LO) frequency on the multistage stepped impedance matching networks, a broadband grounding circuit is provided for IF signal and thus output power nulls are eliminated over an ultra‐wide IF band. The hybrid network is constructed by a 3 dB branch coupler and a 90‐degree delay line, which enables the required balanced outputs with equal amplitude and 180‐degree phase difference. A prototype of the SBM is designed, fabricated and tested. Measured results demonstrate that, for RF and LO frequency over entire V‐band, the mixer has a conversion loss from 6.8 to 11.4 dB with an ultra‐wide IF range of 0.01–22 GHz.

Abstract 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.

Abstract A design method of compact wideband balun bandpass filter (BPF) with high selectivity based on corrugated slot is proposed here. To realize wideband balun response in compact size, metal strips are deployed parallel to the half‐mode corrugated slot (HMCS). Besides, the coupling of corrugated slot to the metal stripline provides the conversion of slot to microstrip structure at the balanced ports. By placing a pair of resonators at the balanced ports, a wideband balun BPF response with high selectivity can be realized. In addition, a U‐shaped resonator is also loaded at the end of the corrugated slot to suppress harmonics. Afterwards, a compact wideband balun BPF centred at 2.42 GHz with 3‐dB FBW of 45% is designed, fabricated, and measured. The effective size is 0.13λg × 0.39λg (λg is the guided wavelength of the centre frequency) and the measured results indicate that the magnitude imbalance is less than 0.45 dB and the phase imbalance is less than 4.8° within the operation frequency band.

Abstract 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.

Abstract 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.

Abstract In this letter, a novel wideband bandstop filter with controllable equal‐ripple response is newly presented. The proposed topology consists of two cascaded transmission lines and three open‐circuited stubs, where, one two‐section OS is shunted between two transmission lines, two three‐section OSs are shunted at two output ports, respectively. By using the proposed design approach, both passband and stopband provide controllable equal‐ripple responses independently, where there are five transmission zeros and six reflection zeros in a single period of frequency domain. For demonstration, an experimental BSF centered at 2 GHz with 20‐dB FBW 129.5% and 30‐dB stopband rejection is designed and fabricated. The simulations and measurements agree with the design theory very well.

Abstract 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.

Abstract Cross‐coupled microwave filter with flexible topology is an essential component in a modern communication system. Among the various filter networks, the use of an extended box section can effectively reduce the difficulty of filter design, but the synthesis approach for this popular structure is not well developed. This paper presents the synthesis method based on the folded coupling matrix to derive the extended box sections. At the same time, an eighth‐degree filter with extended sixth‐degree box sections is synthesized to demonstrate the correctness of the approach. Finally, the new approach's feature to derive advanced circuits such as peculiar quintets is discussed, indicating that it has certain universality.

Abstract Here, a novel design of absorber loading dual‐layer frequency selective surfaces (FSSs) in the Salisbury screen is proposed. The design of frequency selective surfaces is mainly composed of cross‐dipole and dual‐loop metal patch. Simulation results show that this absorbing structure can achieve ultra‐wideband absorption from C band to K band and the fractional bandwidth reaches 117.1% below –10 dB. The thickness of the proposed absorbing structure is 0.078 λL and the unit cell size is 0.262 λL, where λL is the wavelength of the lowest absorbing frequency. Moreover, it shows excellent polarization stability in TE and TM modes, and it has great angular stability. Finally, the prototype of this structure is fabricated, and the measured results are in good agreement with the simulation results.

Abstract A low‐loss Ka‐band transition from waveguide to substrate integrated waveguide (SIW) is presented based on ridge waveguide stepped‐impedance transformer. This novel transition can operate across a full Ka‐band (26.5 to 40 GHz) with an insertion loss lower than 0.59 dB and return loss over 14.2 dB. Furthermore, compared to existing transitions the introduction of ridges reduces the metal usage, resulting in weight reduction and offering an encouraging application for weight‐sensitive satellite communications (SatComs).

Abstract 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.

Abstract This letter presents a wideband quasi‐Yagi antenna with high selective filtering responses and good radiation performance. The proposed antenna is formed by a microstrip‐slotline‐microstrip transition structure as the filtering network, a driver dipole, and a parasitic strip element. Attributing to the integrated wideband filtering balun network, a wideband performance in the antenna with stable gain can be achieved. Moreover, two radiation nulls are introduced near both sides of the gain passband by utilizing the source‐load coupling in the filtering network to acquire a sharp skirt selectivity. To verify the design concept, a prototype is fabricated and measured. Results exhibit an impedance bandwidth of 63.5% from 3.0 to 5.79 GHz with a stable gain average of 5 dBi over the whole operation band. Besides, two radiation nulls occur at 2.6 and 6.85 GHz, respectively. The simulated results are in good agreement with the measured counterparts.

Abstract Here, a ‘truncated C’‐shaped planar coupled line resonator for UWB retransmission‐based chipless RFID has been presented. The resonator is small compared to other existing resonators in the retransmission‐based chipless RFID family. Two different coded multi‐resonator prototype tags have been designed and validated with newly designed planar microstrip fingertip‐shaped antennas. A good agreement is found between simulation and measurement responses. From the system validation, different coding combinations have been extracted when all resonators are present (111111111) and when three resonators are absent (110110101). It is found that with this resonator, 9 bits can be imposed instead of the existing planar rectangular resonator that gives only 1 bit with a larger size. Hence, as much as 86.7% of the occupied area is reduced by the resonator presented in this work. This work is anticipated to motivate the researchers in developing UWB passive RFID tags with higher bit density and smaller sizes for item tagging and sensing applications.

Abstract 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.

Abstract This letter reports an accurate and rigorous design method for a distributed‐element reflectionless bandpass filter capable of producing a reflectionless response at two ports simultaneously. The design method relies on novel transmission‐line reflectionless filter prototypes. This work presents a stub‐based reflectionless filter prototype and its design formulas in terms of a target transmission response. It also provides a stub‐less coupled‐line reflectionless filter prototype equivalent to the stub‐based prototype in case where stubs are not feasible. A second‐order bandpass filter has been designed and fabricated to verify the design method. Reasonable agreement between the measured performance and the simulated one has been obtained, confirming that the proposed design is valid.

Abstract Meander‐line and multi‐layer antennas have been used extensively to design compact UHF radio‐frequency identification (RFID) tags; however the overall size reduction of meander‐line antennas is limited by the amount of parasitic inductance that can be introduced by each meander‐line segment and multi‐layer antennas can be too costly. In this paper, a new compact antenna topology for passive UHF RFID tags based on zeroth order resonant (ZOR) design techniques is presented. The antenna consists of lossy coplanar conductors and inter‐connected inter‐digital capacitor (IDC) unit‐cells with a ZOR frequency near the operating frequency, which is a key component in the design process because the unit‐cells chosen for the design are inductive at the operating frequency. This makes the unit‐cells very useful for antenna miniaturization. The new design in this work has several benefits; namely the coplanar layout can be printed on a single layer, matching inductive loops that reduce antenna efficiency are not required and ZOR analysis can be used for the design. Finally, for validation, a prototype antenna was designed with a unit‐cell ZOR resonant frequency of 900 MHz, fabricated and characterized. A maximum read‐range of 7.6 m was determined.

Abstract This letter describes a design method for a six‐port rat race circuit to obtain quadrature differential waveforms from a single end. The proposed six‐port rat‐race coupler succeeded in low insertion loss and wide bandwidth by using a 1/2 wavelength transmission line and an open stub. Over the frequency range from 0.89 to 1.05 GHz, a prototype 0.96‐GHz‐band six‐port rat‐race coupler was confirmed to have a transmission loss of 7.03 ± 0.45 dB, a reflection loss and isolation of 15 dB or more, and a phase error of within ± 3.3 ○.

Abstract Butler matrices represent a popular class of feeding networks for antenna arrays. Large dimensions and the lack of flexibility in terms of achievable output phase difference make conventional Butler structures of limited use for modern communication devices. In this work, a compact planar 4 × 4 matrix with non‐standard relative phase shifts of –30, 150, –120, and 60° has been proposed. The structure is designed to operate at the centre frequency of 2.45 GHz. Small dimensions of 31.3 × 22.9 mm make it useful for Internet of Things applications. The structure operates from 2.35 to 2.55 GHz, which covers the industrial, scientific and medical bandwidth. At the centre frequency, the measured amplitude and phase imbalance are 1.65 dB and ±4.3°, respectively. The proposed circuit has been compared to the state‐of‐the‐art structures from the literature.

Abstract 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.

Abstract 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.

Abstract A broadband directional coupler based on ferrite cores is proposed for high frequency (HF)/Very high frequency (VHF)/Ultra High Frequency (UHF) communication systems. The transformer‐type coupler is composed of two double‐aperture ferrite cores and 1‐mm‐diameter strands. The ferrite cores with different relative permeabilities are screened to achieve an ultra‐wide bandwidth (5 –1000 MHz). The coupler demonstrates excellent electric performances. The voltage standing wave ratio of each port is less than 1.2, a stable −20 dB coupling level is maintained over the operating bandwidth and the insertion loss is less than 0.8 dB. Such couplers have compact sizes and are of very low cost.

Abstract In this correspondence, high‐precision computer numerical control milling is utilised to demonstrate a doubly loaded iris, cross‐coupled waveguide bandpass filter for inline operation within the W‐band. This sixth‐order filter is designed in a stacked H‐plane configuration and utilises both E‐plane and H‐plane doubly loaded irises to maintain the flow of a TE102‐mode electromagnetic field. In this configuration, a high rejection level is maintained outside of the passband while a good unloaded quality factor is obtained through the support of larger resonator dimensions. The filter is designed for approximately 2% fractional bandwidth centred at 97.5 GHz and has been fabricated as three brass components. Measurements of the filter agree very well with the simulated results and demonstrate a spurious‐free design over the full W‐band.

Abstract A compact four‐order fully‐canonical dual‐band bandpass filter (DBBPF) with independently controllable bandwidths is reported in this letter. The filter comprises of a pair of identical electrically end‐coupled dual‐mode open stub‐loaded resonators (OSLRs) and two different pairs of magnetically coupled λ/4 short‐circuit resonators (SCRs) that are embedded within the OSLRs for compactness. Specifically, the fundamental odd‐ and even‐mode of the OSLRs are coupled with the upper and lower pairs of λ/4 SCRs to generate the first and second passbands, respectively. In addition, the input and output ports feed the two OSLRs and provide a magnetic source‐load coupling for the filter. Owing to the special layout arrangement, the proposed filter can achieve compact size and fully‐canonical frequency response with independently controllable bandwidths, simultaneously. A four‐pole DBBPF protype centred at 2 and 3.5 GHz is designed and implemented. The measurements and the simulated ones are in good consistency.

Abstract In this letter, a novel frequency‐reconfigurable substrate integrated waveguide (SIW) bandpass filter (BPF) based on gravity field is proposed. By inverting the proposed filter, the liquid metal would flow into or out of the non‐metallic via‐holes under the influence of gravity, so that the filter centre frequency can be switched from 1.76 GHz (4.54%) in state 1 to 2.02 GHz (5.94%) in state 2, and the insertion losses are maintained at 2.6 dB and 2.4 dB, respectively.