In this paper, a precise design method of high-order bandpass filters (BPFs) with complicated coupling topologies is proposed, and is demonstrated through the design of an 11-pole BPF using TM010 mode dielectric resonators (DRs). A novel Z-shaped coupling structure is proposed which avoids the mixed use of TM010 and TM01δ modes and enables the tuning and assembling of the filter much easier. The coupling topology of the BPF includes three cascade triplets (CTs) of DRs, and both the capacitive and inductive couplings in the CTs are designed independently tunable, which produce consequently three controllable transmission zeros on both sides of the passband of filter. A procedure of mapping the coupling matrix of BPF to its physical dimensions is developed, and an iterative optimization of these physical dimensions is implemented to achieve best performance. The design of the 11-pole BPF is shown highly precise by the excellent agreement between the electromagnetic simulated response of the filter and the desired target specifications.

This paper proposes a dual-polarized filtering antenna with extracted-pole unit (EPU) using LTCC substrate. The EPU realizes the high skirt characteristic of the bandpass filter with transmission zeros (TZs) located near the passband without cross coupling. The filtering antenna with EPU is designed and fabricated in 28GHz band for 5G Band-n257 (26.5-29.5GHz). The measured S11 is less than -10.6dB in Band-n257, and the isolation between two ports for dual polarization is greater than 20.0dB. The measured peak antenna gain is 4.0dBi at 28.8GHz and the gain is larger than 2.5dBi in Band-n257. The frequency characteristics of the measured antenna gain shows the high skirt characteristic out of band, which are in good agreement with electromagnetic (EM)-simulated results.

A novel compact 5-pole bandpass filter (BPF) using two different types of resonators, one is coaxial TEM-mode resonator and the other dielectric triple-mode resonator, is proposed in this paper. The coaxial resonator is a simple single-mode resonator, while the triple-mode dielectric resonator (DR) includes one TM01δ mode and two degenerate HE11 modes. An excellent spurious performance of the BPF is obtained due to the different resonant behaviors of these two types of resonators used in the BPF. The coupling scheme of the 5-pole BPF includes two cascade triplets (CTs) which produce two transmission zeros (TZs) and a sharp skirt of the passband. Behaviors of the resonances, the inter-resonance couplings, as well as their tuning methods are investigated in detail. A procedure of mapping the coupling matrix of the BPF to its physical dimensions is developed, and an optimization of these physical dimensions is implemented to achieve best performance of the filter. The designed BPF is operated at 1.84GHz with a bandwidth of 51MHz. The stopband rejection is better than 20dB up to 9.7GHz (about 5.39×f0) except 7.85GHz. Good agreement between the designed and theoretically synthesized responses of the BPF is reached, verifying well the proposed configuration of the BPF and its design method.

In this paper, we propose a design method for a diplexer using a surface acoustic wave (SAW) filter, a multilayer ceramic filter, chip inductors, and chip capacitors. A controllable transmission zero can be created in the stopband by designing matching circuits based on the out-of-band characteristics of the SAW filter using this method. The proposed method can achieve good attenuation performance and a compact size because it does not use an additional resonator for creating the controllable transmission zero and the matching circuits are composed of only five components. A diplexer is designed for 2.4 GHz wireless systems and a global positioning system receiver using the proposed method. It is compact (8.0 mm × 8.0 mm), and the measurement results indicate good attenuation performance with the controllable transmission zero.

This paper presents a K-band fully reconfigurable waveguide resonator filter with a new negative coupling structure. A pair of transmission zeros as well as the center frequency and bandwidth of the presented filter can be adjusted. The filter adopts the concept of a frequency-tunable coupling resonator in designing the coupling structure, which allows for controlling the coupling coefficient. All coupling values in the filter structure can be tuned by adjusting the resonant frequency of each frequency-tunable coupling resonator. This work also presents a design method in detail for the coupling resonator with a negative coupling coefficient. In addition, the approach for separating the resonant peak produced by the coupling resonator with a negative coupling value from the passband for the purpose of improving the stopband performance is described. For verifying the presented filter structure, a fourth-order waveguide filter has been fabricated and measured. The fabricated filter has the center frequency tuning range from 18.34GHz to 18.75GHz, the bandwidth tuning ratio of 1.94 : 1.

This paper proposes a new and simple microstrip bandpass filter structure for the design of a fully canonical transversal array filter. The filter is constructed by the parallel arrangement of microstrip even- and odd-mode half-wavelength resonators. In this filter, transmission zeros (TZs) are not produced by cross couplings used in conventional filter designs, but by an intrinsic negative coupling of the odd-mode resonators having open ends with respect to the even-mode resonators with shorted ends. Thus, the control of the resonant frequency and the external Q factor of each resonator makes it possible to form both a specified passband and TZs. As an example, a fully canonical bandpass filter with 2-GHz center frequency, 6% bandwidth, and four TZs is synthesized with a coupling-matrix optimization, and its structural parameters are designed. The designed filter achieves a rapid roll-off and low-loss passband response, which can be confirmed numerically and experimentally.

This paper proposes a compact three-mode H-shaped resonator bandpass filter fed by antiparallel coupled input/output lines. To investigate the resonant behavior of the H-shaped resonator, even/odd-mode resonance conditions of the resonator are first derived analytically. The multimode resonances of the H-shaped resonator filter are modeled by a multipath circuit formed with resonance paths. Moreover, a direct source/load coupling path is connected in parallel, of which the value shows a frequency dependency because of the antiparallel coupled feeding lines, thereby generating four transmission zeros (TZs) greater than the number of a theoretical limitation. The H-shaped resonator bandpass filter is synthesized, developed, and tested, showing a third-order passband response with four TZs located near the passband, and a wide stopband property.

This paper proposes a new three-mode resonator, which consists of a parallel-coupled microstrip line resonator embedded with a slotline resonator, and develops a compact low-loss bandpass filter (BPF) with a sharp roll-off response because of four transmission zeros (TZ) located very near the passband. Resonance mechanism and properties of the three modes are first analyzed by using an eigen-mode analysis, and then an equivalent circuit model is established for expressing a novel coupling scheme of the developed BPF. It is made clear from the results of circuit analysis that the four TZs are produced because of multiple paths between the input/output stub lines formed by the three resonant modes and the direct source/load coupling. The validity of the proposed resonator and filter is supported by the comparison between simulated and measured results.

In this paper, we propose a novel feeding structure for a coaxial-excited compact waveguide filter, which is composed of planar resonators called frequency-selective surfaces (FSSs). In our proposed feeding structure, new FSSs located at the input and output ports are directly excited by the coaxial line. By using the FSSs, the transition from the TEM mode to the TE10 mode is realized by the resonance of the FSSs. Therefore, the backshort length from the coaxial probe to the shorted waveguide end can be made much shorter than one-quarter of the guided wavelength. Additionally, the coaxial-excited FSS provides one transmission zero at each stopband. As a design example, a three-stage bandpass filter with 4% bandwidth at the X band is demonstrated. The designed filter has a very compact size of one cavity and has high skirt selectivity with six transmission zeros. The effectiveness of the design is confirmed by the comparison of frequency characteristics obtained by the simulation and measurement.

A novel low insertion-loss and wideband microstrip bandpass filter has been designed and tested. The basic configuration of this novel dual-mode filter is a square ring resonator with direct-connected orthogonal feed lines, and dual-perturbation elements are introduced within the resonator at symmetrical location. The effects of the size of the perturbation element are studied. A new filter having wider bandwidth and transmission zeros are presented. The proposed filter responses are in good agreement with the simulations and experiments.

In this paper, a compact ultra-wideband bandpass filter (UWB-BPF) using pseudo-interdigital stepped-impedance resonators (PIDT-SIRs) is designed and implemented on a commercial printed circuit board (PCB) of RT/Duroid 5880 substrate. The first two resonant modes of the SIR are coupled together and they are applied to create a wide passband. The proposed filter at center frequency f0 of 7.1 GHz has very good measured characteristics including the bandwidth of 3.68-10.46 GHz (3-dB fractional bandwidth of 95%), low insertion loss of -0.50.4 dB, sharp rejection due to two transmission zeros in the passband edge created by the inter-stage coupling. Experimental results of the fabricated filter show a good agreement with the predicted results.

Novel dual-band bandpass filter (BPF) with quasi-elliptic function response by using the meander coupled step-impedance resonators (SIRs) is presented. By tuning the appropriate impedance ratio (K) and physical length of SIRs, the BPF has good dual-band performance at 2.4 and 5.2 GHz with high selectivity, due to the transmission zeros appeared in two passband edges. Measured results of the proposed BPF have a good agreement with the electromagnetic (EM) simulated results.

A novel wideband bandpass filter with improved stopband characteristics is presented in this paper. Dual-mode square ring resonator is used in the proposed filter. New formulas based on the even- and odd-mode analysis are derived to facilitate the design of transmission zeros of the square ring resonator. A short-circuited stub and a piece of aperture-enhanced parallel-coupled lines are introduced to the input and output of the resonator to lower the passband return loss and widen the stopband of the filter significantly. The filter has a 50% fractional bandwidth, is compact in configuration, and shows remarkably improved performance compared with previously reported filters of the same kind. The measured filtering response shows a good agreement with the simulated result.

In this paper a newly designed internally-coupled asymmetric stepped-impedance resonator (SIR) bandpass filter (BPF) is proposed. The asymmetric SIR structure not only can effectively reduce the circuit size but also can provide two flexibly tunable transmission zeros near the lower and upper passband edges. The first transmission zero is due to the series resonance of the quarter-wavelength open stepped-impedance stub, and the second one is produced by anti-parallel coupling between adjacent SIRs. The proposed BPF was fabricated and simulated using the commercial software HFSS, and agreement between the measured and simulated results was observed. A 0.9-dB insertion loss and a shape factor of 3.6 were achieved in the passband, thus indicating that the proposed filter structure is of practical value.

Periodically nonuniform coupled microstrip line (PNC-ML) loaded with transverse slits is characterized using the fullwave method of moments and short-open calibration technique. Guided-wave characteristics of both even- and odd-modes are thoroughly investigated in terms of two extracted per-unit-length transmission parameters, i.e., phase constants and characteristic impedances. As such, frequency-dependent coupling between the lines of the finite-extended PNCML is exposed via two dissimilar impedances. Meanwhile, two phase constants try to be equalized at a certain frequency by properly adjusting the slit depth and periodicity, aiming at realizing the transmission zero. Further, equivalent J-inverter network parameters of this finite-length PNCML are derived to reveal the relationship between the transmission zero and harmonic resonance. By allocating this zero to the frequency twice the fundamental passband, one-stage and two-stage PNCML filters are then designed, fabricated and measured to showcase the advantageous capacity of the proposed technique in harmonic suppression.

This paper focuses on the characteristics of tunable half-wavelength resonators and their applications to bandpass filters (BPFs). First, the resonance characteristics of various tunable half-wavelength resonators are examined for the tunabilities of transmission zeros and the center frequency of the proposed BPFs. We examine four types of tunable half-wavelength resonators, namely, an end-coupling resonator and three types of tap-coupling resonators. Secondly, the proposition and design of two types of BPFs using acquired resonators are carried out. The fabrication and experimental application of the resonators and designed BPFs are also performed based on coplanar waveguide (CPW) technologies. Their calculated and measured results are compared with each other. The results show that tunabilities of the transmission zero and the center frequency of the proposed BPF are obtained as expected.

The paper reports a compact and high performance dual-band bandpass filter (BPF) using two types of dual-mode resonators. The dual mode cross shaped resonator and the three dual mode ring resonators in the designed dual-band BPF are excited to control the first and second passband, respectively. It is shown that the designed and fabricated dual-band BPF has narrow bandwidths and very sharp attenuation rate due to the existence of the transmission zeros. The frequency response of the designed dual-band BPF shows good agreement between the simulations and experiments.

The investigation presents a low cost and low insertion loss X-band dual mode bandpass filter (BPF) based on inexpensive commercial FR4 substrate. The proposed filter at a central frequency f0 of 11.3 GHz has high filter performance filter with a fractional bandwidth of 14%, the insertion loss of -2.7 dB, and two transmission zeros. The designed procedures are presented in this letter and the fabricated filter verifies the proposed designed concept.

Novel microstrip lowpass filters are developed with reduced size and significantly improved stopband characteristics. After introducing quarter-wavelength open stubs, we get one or two transmission zeros in the stopband. By folding the high impedance microstrip lines, we reduce the size of the filter. Three-pole and five-pole lowpass filters are designed, and their measured frequency responses agree well with theoretical predictions.

This paper describes synthesis of a complex RiCR filter with a finite transmission zero except zero frequency. The frequency response of the proposed filter is similar to the conventional elliptic filter. The proposed filter can be composed of fewer elements than the conventional one. A new kernel function is proposed. As an example, a fifth-order RiCR filter is designed. We compare the proposed filter with the conventional complex elliptic filter from the viewpoint of the frequency response and the number of the required elements.