This paper presents a reflectionless ultra-wideband filtering power divider based on an unequal-width three-coupled-line section and an absorptive-isolation network. The proposed design realizes ultra-wideband filtering response, reflectionless operation, and improved output-port isolation. A prototype centered at f0 = 2.5 GHz is designed, fabricated, and measured. The measured results show a 3-dB fractional bandwidth of 101%, a 10-dB reflectionless bandwidth of 259%, a minimum insertion loss of 0.15 dB, isolation better than 18 dB, and stopband suppression better than 20 dB up to 2.5f0.

The traditional implementation of the balanced reflectionless filtering devices is mainly based on the complementary-diplexer architecture, which employs the absorption branches that do not have a uniform topology. It not only carries the risk of impedance mismatch but also suffers from the limitation of design flexibility, making it difficult to extend its application to various types of design requirements. In order to address this challenge, a novel balanced reflectionless filtering topology is proposed in this paper. Specifically, by employing the dual-path signal interference technique, it can ensure excellent differential-mode (DM) transmission responses while achieving reflectionless filtering characteristics for DM out-of-band reflected signals. The use of a balanced-to-unbalanced power divider/combiner with absorptive resistors can provide effective suppression and absorption of common-mode noise. To verify the viability of the proposed topology, its applications to an adjustable filtering attenuator and a circularly polarized filtering antenna are designed as multifunctional balanced reflectionless filtering devices. The simulated and measured results of the fabricated microstrip prototypes are in good agreement.

ABSTRACT In this paper, a novel unbalanced‐to‐balanced switchable quadruplexer with high off‐state suppression is proposed. Four channels are realized through four pairs of step impedance resonators and uniform impedance resonators with different center frequencies in the quadruplexer. Furthermore, the flexible and controllable channel switching function is achieved through loading PIN diodes and their bias circuits at the open‐circuited ends of the resonators. To verify the feasibility of the proposed design, a quadruplexer with center frequencies of 1.227, 1.575, 1.9, and 2.3 GHz, respectively, is designed and fabricated. Measurement results indicate that the proposed quadruplexer achieves four independently controllable channels with an off‐state suppression of up to 51 dB. It is expected to be applicable to combined applications of global navigation satellite systems and mobile communication systems.

This paper presents a novel planar balanced-to-balanced diplexer with high isolation and harmonic suppression. Both the lower and higher channels of the diplexer consist of the two-stepped impedance resonators and one uniform impedance resonator. To enhance the inter-channel isolation of the diplexer, the cross-coupling between the resonators is used to introduce extra out-of-band transmission zeros. By utilizing stepped impedance resonators with different impedance ratios, the harmonics of the three resonators, except the fundamental harmonics, are different from each other so as to suppress the odd- and even-mode harmonics effectively and ultimately to achieve the purpose of broadening the stopband and enhancing the common-mode (CM) suppression. To verify the feasibility of this design, a microstrip prototype with dual-channel center frequencies of 2.01 GHz and 3.54 GHz was manufactured. Experimental results demonstrate that the in-band differential-mode (DM) isolation is higher than 41.6 dB and 41.5 dB for the lower and higher channels, respectively. CM suppression is greater than 20 dB in the 0-13.7 GHz range for the lower channel and 20 dB in the 0-9.0 GHz range for the higher channel. The stopband suppression level achieves 20 dB up to 18.5 GHz.

In this paper, a balanced nonreciprocal linear-phase bandpass filter (BPF) based on time-modulated resonators is reported for the first time. A linear-phase response is achieved by integrating group delay equalization circuits adjacent to both the balanced input and output ports of a balanced nonreciprocal BPF, implemented on a microstrip circuit. Compared to conventional passive BPFs, the balanced nonreciprocal BPFs based on time-modulated resonators exhibits different group delay characteristics. This work introduces an innovative group delay equalization circuit, specifically designed for nonreciprocal BPFs based on time-modulated resonators. By utilizing resonators with different resonant frequencies under common-mode and differential-mode excitation, the balanced circuit provides effective common-mode suppression, which enhances immunity to electromagnetic interference and crosstalk compared to single-ended circuits. A microstrip prototype was fabricated for the proof-of-concept demonstration, and the measurements show good agreement with simulations.

A novel filtering power divider (FPD) with high isolation and wideband reflectionless characteristics is proposed. The wideband filtering characteristics of the FPD come from the joint contribution of the coupled lines and the stepped impedance resonator, where the coupled lines provide the passband characteristic, and the stepped impedance resonator generates and tunes the transmission zeros outside the passband to enhance out-of-band suppression and improve frequency selectivity. The absorption-isolation network and absorptive stubs improve isolation between output ports while achieving input reflectionless characteristics. Through theoretical analyses and experimental verification, a prototype microstrip line was developed, with simulation results showing good agreement with measured data. Measured results indicate that the proposed FPD-with a center frequency of 2.47 GHz-delivers excellent isolation: better than 30 dB within the passband and superior to 22 dB over the entire 0 to 7 GHz frequency range. The input reflectionless bandwidth is up to 277% from 0 to 6.87 GHz.

A wideband 3 & times; 4 beamforming network (BFN) is presented, integrating a 3 & times; 4 Blass-like matrix with Chebyshev amplitude distribution for low sidelobe levels (SLLs) and a 4 & times; 4 phase-compensation network to ensure frequency-invariant beam pointing. Phase shifters with adjustable phase-slope are introduced to flatten the phase response of the Blass-like matrix, and further to realize quasi-true-time-delay characteristics for the overall BFN. A prototype operating at 5.8 GHz was fabricated and measured, showing an output amplitude ratio of 0.332:1:1:0.332, an amplitude imbalance of 1.5 dB, and a linear phase error of 10 degrees over a 31% bandwidth. The BFN enables a multibeam antenna array to achieve low SLLs of -15 dB and stable beam pointing from 4.9 GHz to 6.7 GHz, validating its effectiveness for wideband multibeam applications.

This letter proposes a broadband filtering patch antenna based on quad-mode resonance, exhibiting high out-of-band rejection level and enhanced selectivity. The antenna employs a stacked structure of the driven patch and parasitic patch to achieve dual-mode resonance within the band. By loading C-shaped slots and flag-shaped strips on the driven patch, two additional modes are also generated within the band, achieving quad-mode resonance and obtaining a broadband response. Meanwhile, the two loaded structures introduce two near-band radiation nulls based on far-field radiation cancellation, achieving high out-of-band rejection level and enhanced selectivity. Additionally, I-shaped and L-shaped slots are etched to better control the position of nulls and enhance the filtering performance. The measured results show that the proposed antenna operates in the 2.45 GHz ISM band with a relative bandwidth of 21.6%. The out-of-band rejection levels on both sides are 30 dB and 29.8 dB, respectively, with selectivity of 183.3 dB/GHz and 200 dB/GHz, respectively. The antenna's wideband and strong anti-interference capabilities make it a promising candidate for the RF front-end system.

A wideband tunable filtering phase shifter with reflectionless characteristics is proposed in this paper. The proposed circuit combines two parallel coupled-line sections, four absorptive branches, a central bandpass section, and four varactor-loaded tuning branches to achieve continuous phase tuning while maintaining wideband reflectionless responses and passband alignment. An odd- and even-mode analysis is carried out to derive the design equations, and a prototype operating at 2.45 GHz is designed, fabricated, and measured. The measured results show a 73.5% 3-dB filtering bandwidth, a 90° in-band phase-shift range with a maximum phase error of ±6.7°, and a 10-dB reflectionless bandwidth from 0.1 to 4.47 GHz.

This paper presents a planar balanced-to-balanced dual-channel combiner with differential- and common-mode reflectionless characteristics. The proposed combiner consists of two filtering sections with different center frequencies and two corresponding absorptive networks. The filtering section is realized by a dumbbell-shaped resonator and a spindle-shaped resonator, and cross-coupling is introduced to increase transmission zeros. The absorptive network is realized by two lambda/4 open-circuited coupled lines with resistors loaded at both ends. The proposed absorptive network is connected in parallel with the filtering section and requires only a single structure to achieve reflectionless characteristics for both differential-mode (DM) and common-mode (CM). To verify the feasibility of this design, a microstrip prototype with dual-channel center frequencies of 2.01 and 2.46 GHz was manufactured based on the simulation results. Actual measurements show that for the lower channel, the DM reflectionless band is 1.35-2.56 GHz and the CM reflectionless band is 1.42-2.42 GHz. For the higher channel, the DM reflectionless band is 1.66-3.01 GHz and the CM reflectionless band is 1.73-2.84 GHz.

A tunable balanced filtering phase shifter with reflectionless and wide stopband characteristics is presented in this paper. The proposed circuit consists of a six-port balanced-to-unbalanced (BTU) quadrature coupler, two identical varactor-tuned reflective loads, and two absorptive filtering networks. The BTU coupler provides quadrature excitation to the reflective loads while suppressing common-mode (CM) noises. The absorptive filtering network is composed of the resistor-terminated coupled lines and stub-loaded resonators, which enable broadband absorption and introduce two transmission zeros near the passband for sharp selectivity. In addition, the BTU coupler yields odd-harmonic transmission nulls through a round-trip recombination process, which works together with the even-harmonic rejection of the absorptive networks to extend the stopband range. A 2.45-GHz microstrip prototype on an F4B substrate is fabricated and measured. The prototype exhibits a 360 degrees phase shift range, simultaneous differential-mode and CM reflectionless operation with 55.92% and 46.94% reflectionless bandwidths, 42 dB in-band CM suppression, and stopband rejection better than 25 dB up to 20 GHz.

This paper proposes a novel unbalanced-to-balanced diplexer with differential-mode reflectionless characteristics. The two channels are realized using two sets of button-ring resonators with different center frequencies, and the absorption network is realized through two sections of parallel-coupled lines with absorption resistors. The microstrip-to-slotline structure is employed, which provides excellent common-mode suppression without affecting differential-mode signal transmission. To verify the feasibility of the theoretical design, a diplexer with center frequencies of 2 and 2.45 GHz, respectively, is designed and fabricated.

A novel dual-port all-band absorptive common-mode filter (A-CMF) is presented in this article, which can not only effectively absorb and suppress common-mode (CM) noise, but also achieve differential-mode (DM) adjustable attenuation behaviors. The differential transmission line is employed to realize CM noise suppression, and the parallel coupled line has the ability to regulate DM in-band matching characteristics. The open-circuited T-stub is loaded to introduce extra transmission zeros, thus achieving the sharp frequency selectivity of the DM signals. The DM attenuation level can be precisely adjusted by changing the value of the variable resistor. Most importantly, dual-port all-band CM absorption characteristics of the A-CMF can be realized by simply placing two pairs of CM absorptive resistors at the horizontally symmetrical plane without additional complex CM absorptive networks. The detailed mixed-mode S-parameters are derived by the classical odd- and even-mode decomposition method, and parameter sweep analysis is used to analyze the working mechanism. To verify the viability, a microstrip prototype of the A-CMF operating at 2.45 GHz is manufactured and measured, which can effectively mitigate electromagnetic interference and improve electromagnetic compatibility performance.

This letter presents a wideband circularly polarized quadrifilar helix antenna (QHA) featuring simultaneously wide 3 dB axial ratio beamwidth (ARBW) and half-power beamwidth (HPBW), addressing the conventional tradeoff between bandwidth and beamwidth. The antenna comprises four pairs of three-stage stepped-impedance folded helical arms, collectively loaded with an arc-shaped metal ring on top. The three-stage stepped-impedance folded helical arm effectively enhance the bandwidth while maintaining a wide HPBW, and the arc-shaped metal ring further broadens the 3 dB ARBW across the wideband. To validate, a prototype was fabricated and measured. Measurements show that the bandwidths corresponding to 10 dB return loss and 3 dB AR reach 44.2% and 35.2%, respectively. Moreover, within a 17.4% bandwidth, the measured HPBW and 3 dB ARBW vary from 132 degrees to 152 degrees and 179 degrees to 265 degrees, respectively, demonstrating its suitability for wideband circularly polarized applications with broad beam coverage and robust AR performance.

A self-decoupled, dual-band, dual-polarized, shared-aperture antenna array is proposed for base station applications. The slotted patch structure and folded stub loading on the 1.69 GHz to 2.2 GHz low-band (LB) antenna effectively suppress LB scattering effects while simultaneously generating a partially reflected wave for the 3.3 GHz to 3.8 GHz high-band (HB) antenna to mitigate coupling between HB elements. Additionally, four circular parasitic elements are arranged around the LB antenna to further reduce coupling among the HB antennas and broaden the LB bandwidth, without requiring additional decoupling layers. A detailed analysis of the decoupling mechanism provided by the slotted patches and folded stubs is conducted. Measurement results confirm that the proposed design achieves excellent decoupling performance, restores the radiation patterns of the HB antennas, achieves an isolation level better than 25 dB, and delivers outstanding performance in both LB and HB frequency bands.

This article proposes a compact bandpass filter (BPF) based on multi-stage spoof surface plasmon polaritons (MST-SSPPs) with miniaturized dispersion-engineered unit cells, featuring a passband covering 0.3 to 1.2 GHz. The unit cell has controllable dispersion characteristics, and the introduction of stepped stubs effectively resolves the long-standing incompatibility between achieving a flat passband and maintaining a steep roll-off skirt in the unit cell-based transmission lines (TLs). While achieving a considerable degree of miniaturization, the unit cell's operating properties can be adapted to various application scenarios through simple adjustment of key geometric parameters. For further miniaturization, this article presents an MST-SSPP TL. Unlike conventional methods for realizing compact SSPPs, the proposed MST-SSPP BPF realizes enhanced wave vectors at the same operating frequency compared with single-stage SSPPs by cascading transition sections of different SSPP structures. In addition, the strong coupling induces an anomalous dispersion region with negative group delay (NGD), which realizes the steep roll-off skirt of the filter. To verify the correctness of the proposed filter, a physical prototype was fabricated and tested. The measurements show good agreement with the simulations. While maintaining good passband performance, the proposed MST-SSPP BPF achieves a compact size of 0.32 lambda g & times; 0.13 lambda g, demonstrating good miniaturization capability compared with other SSPP-based BPFs.

In this paper, a balanced full-passband linear-phase nonreciprocal bandpass filter is reported for the first time. The nonreciprocal characteristic is realized using time-modulated resonators. The full-passband linear-phase characteristic is realized by integrating group delay equalization circuits at both the balanced RF input and output ports of a balanced nonreciprocal bandpass filter. The modulation introduced by the time-modulated resonators achieves reverse isolation of the RF signals while simultaneously altering the group delay in the forward path, resulting in a group delay characteristic that is different from that of conventional bandpass filters. In this design, the group delay is first equalized near the center frequency and then at both the edges of the passband, ultimately achieving a full-passband linear-phase characteristic. The balanced circuit achieves effective common-mode suppression by employing resonators that exhibit different resonant frequencies under common-mode and differential-mode excitations. The fabricated microstrip prototype on an F4B substrate exhibits good agreement between measurement and simulation results, confirming the effectiveness of the proposed design.

This paper proposes a balanced wideband switchable filter with common-mode (CM) reflectionless and selective enhancement characteristics. The proposed balanced switchable bandpass filter consists of a balanced switching filter unit and two T-shaped absorbing networks. The T-shaped absorbing networks not only realize CM reflectionless characteristics but also introduce two transmission zeros, thereby enhancing stopband rejection. By loading six PIN diode circuits into the filter unit, the switching states of the filter can be controlled. Meanwhile, the stepped-impedance lines in the filter unit provide two transmission zeros located near the passband, further improving frequency selectivity. To validate the proposed structure, a microstrip prototype was designed, fabricated, and measured. Experimental results show that the proposed balanced switchable filter exhibits good differential-mode bandpass response and CM reflectionless behavior in the ON-state, as well as excellent OFF-state suppression and CM reflectionless performance in the OFF-state.

In the paper, a four-way wideband filtering power divider (FPD) with an all-port-absorptive feature is presented to fill the gap of FPDs with simultaneous multiway, wideband filtering, good isolation, and all-port absorption features. It consists of two unequal-width three-coupled lines (TCLs), one T-shaped absorptive stub, two composite isolated networks, and two resistors. To obtain wideband filtering responses, the unequal-width TCLs are first utilized. Then, T-shaped absorptive stubs are inserted for obtaining both the input reflectionless and improved isolation between nonadjacent output ports. Composite isolation networks are shunt-connected between adjacent output ports to ensure port-to-port isolation and suppress reflected signals at the output terminals. The resistors are also served for good isolations between adjacent output ports. Theoretical analysis with rigorous closed-form equations is provided. For validation, the four-way prototype was fabricated. Measurements show that the designed FPD exhibits a 10-dB absorption bandwidth of 200% at both the input and output ports with a 3-dB passband bandwidth of 67.3% and out-of-band rejections of larger than 20 dB. Besides, the in-band adjacent and nonadjacent port isolations are 16.7 and 20 dB, respectively.

This paper presents a balanced switchable bandpass filter (SBPF) with differential-mode (DM) reflectionless characteristics in the on-state. The proposed SBPF consists primarily of dual-mode resonators loaded with p-type-intrinsic-n-type diodes, absorptive stubs, and common-mode suppression (CMS) networks. The resonance conditions of the diode-loaded dual-mode resonator in both on/off states, as well as the design principles of the balanced SBPF's, are analyzed and derived. To verify the design concept, a balanced SBPF prototype with a center frequency of 2.6 GHz was designed and manufactured. The measured results show that the proposed SBPF exhibits low DM insertion loss and a wide DM reflectionless bandwidth in the on-state, high DM isolation in the off-state, and effective CMS.