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With the rapid development of communication technology, the researches of multi-band filtering circuits have become more and more important. Multimode resonator (MMR) is one of the vital methods to provide multi-resonant modes within a single design. In this paper, a dual-band ultra-wideband bandpass filter (UWB-BPF) using stepped impedance stub-loaded resonators (SI-SLR) is presented. The main advantage of using SI-SLR is to have better performance with multimode behavior and more parameters to control resonant modes. SI-SLR combines the advantages of SIR and SLR structures, which gives a compact, high-performance multiband filter. The proposed filter design has compact size, sharp and flat response with low insertion loss (IL), low return loss (RL), and high band-to-band rejection. The filter is designed for UWB communication in wireless body area networks and fabricated on Arlon substrate with relative permittivity${\\varepsilon_{\\textrm{r}}} = 3.25$, thickness$0.8\\;{\\textrm{mm}}$. The resulted dual-bands are centered at$4{\\textrm{ GHz}}$and$8.3{\\textrm{ GHz}}$with fractional bandwidths$37{\\textrm{% }}$and$48{\\textrm{%}}$. The simulation was carried out using CST Microwave Studio. The filter provides good passband performances, with IL 0.49 dB and 0.31 dB at the center frequency of lower and higher bands, respectively. The band-to-band 40 dB rejection is realized by adding circular spiral at the input/output of the filter.

The proliferation of the Internet of Things (IoT) propels the continuous demand for compact, low-cost, and high-performance multiband filters. This paper introduces a novel low-profile dual-band bandpass filter (BPF) constructed with a back-to-back coupled pair of shielded folded quarter-mode substrate integrated waveguide (SF-QMSIW) multimode cavities. A hybrid structure is obtained by etching a coplanar waveguide (CPW) coupling line in the folded cavity’s septum layer. It serves multiple functions: generating an additional resonance, providing a separate coupling mechanism for the upper passband, and offering the flexibility to control the passbands’ center frequency ratio. Additionally, the unused second higher-order mode is suppressed by integrating embedded split-ring resonators (ESRRs) with an inter-digital capacitor (IDC) structure into the feed lines. A filter prototype has been fabricated and experimentally tested. The measurements confirmed reliable operation in two passbands having center frequencies 3.6 GHz and 5.8 GHz, and exhibiting 3 dB fractional bandwidths (FBWs) of 6.4% and 5.3%, respectively. Furthermore, the group delay variation within both passbands equals only 0.62 ns and 1.00 ns, respectively. Owing to the second higher-order mode suppression, the filter demonstrated an inter-band rejection exceeding 38 dB, within a compact footprint of 0.71λg2 (with λg being the guided wavelength at the lower passband’s center frequency).

A novel multimode resonator is designed in this paper. Incorporating two open-circuited stubs of distinct impedances loaded onto a uniform half-wavelength transmission line, the resonator enables the realization of a dual-band filter—with center frequencies at 2.6 GHz and 4.8 GHz—through both simulation and experimental measurement. Regarding the first passband centered at 2.6 GHz, the device exhibits a return loss |S11| of 13.7 dB, an insertion loss |S21| of 0.37 dB, and a 3 dB bandwidth of 17.3%. As for the second passband with a center frequency of 4.8 GHz, the measured return loss |S11| amounts to 23.6 dB, the insertion loss |S21| measures 0.77 dB, and the 3 dB bandwidth is recorded at 8.75%. Specifically designed for 5G communication systems, the filter achieves three transmission zeros by adopting electrical coupling and 0° feeding, resulting in high selectivity and high isolation. Practical measurements verify that the experimental results are consistent with the simulation results.

With the development of the communication field, more and more communication devices need multiple single-frequency antennas to work simultaneously. However, using multiple single-frequency antennas results in larger sizes. The miniaturized and dual-band communication system has become a new development trend. This paper designs a dual-band filtering patch antenna based on a multimode resonator, which is cascaded. The filter resonator part adopts step impedance resonance for branch shunt design, which can flexibly control the center frequency of two frequency bands. The filtering antenna utilizes a dual-layer substrate structure and achieves an integrated design through coupling probe feeding. This reduces the horizontal dimensions and improves miniaturization. Multiple radiation nulls are introduced outside the two-passband, and roll-off characteristics are evident. In addition, this design realizes high selectivity in the operating band. The antenna has large gains of 1.71 dBi and 6.25 dBi. It has good filtering and radiation characteristics.

In this paper, a novel design of ultra-wideband (UWB) filtering antenna integrated with the multimode resonator (MMR) bandpass filter is proposed, aiming to enhance band-edge selectivity. At the beginning, a MMR bandpass filter is modified and studied. Based on the classic MMR filter, the proposed filter folds the microstrip transmission line to reduce its size while retaining the original filtering performance. Moreover, an open stub and short stub are added to the proposed filter to obtain a transmission of zero. Then, the folded filter with stubs and a UWB bow-tie antenna are integrated together to form a filtering antenna. The open stub and short stub in the MMR structure enhance the antenna’s upper and lower band-edge selectivity, respectively. Series of parameters are studied to analyze their influences on the frequency selection range and band-edge characteristics. Compared with the original UWB dipole antenna, such an integrated approach brings many benefits. Firstly, the UWB filter not only broadens the bandwidth of the device, but also improves band-edge selectivity, which can eliminate the unwanted passband near the operating frequencies. Secondly, the integrated system reduces the size and cost of the devices, which is very important in the miniaturization of wireless systems. In this research, the reflection coefficient (S11) of integrated filtering antenna is lower than −10 dB between 2.92 and 11.51 GHz, and it has a fractional bandwidth of 119%. The measured shape factor is 1.027 (very close to 1), which proves that this design has a better band-edge selectivity. Simultaneously, good radiation characteristics are also attained, with a maximum realized gain of 6 dBi. Theoretical simulation results are similar to the experimental results. The measurement results of the manufactured device effectively validate that its performances have reached the simulation design requirements.

This paper presents a novel design of a modified ultrawideband (UWB) antenna array integrated with a multimode resonator bandpass filter. First, a single UWB antenna is modified and studied, using a P-shape radiated patch instead of a full elliptical patch, for wide impedance bandwidth and high realized gain. Then, a two-element UWB antenna array is developed based on this modified UWB antenna with an inter-element spacing of 0.35 λL, in which λL is the free space wavelength at the lower UWB band edge of 3.1 GHz, compared to 0.27 λL of a reference UWB antenna array designed using a traditional elliptical patch shape. The partial ground plane is designed with a trapezoidal angle to enhance matching throughout the UWB frequency range. The mutual coupling reduction of a modified UWB antenna array enhances the reflection coefficient, bandwidth, and realized gain, maintaining the same size of 1.08 λ0 × 1.08 λ0 × 0.035 λ0 at 6.5 GHz center frequency as that of the reference UWB antenna array. The UWB antenna array performance is investigated at different inter-element spacing distances between the radiated elements. To add filtering capability to the UWB antenna array and eliminate interference from the out-of-band frequencies, a multimode resonator (MMR) bandpass filter (BPF) is incorporated in the feedline while maintaining a compact size. The measurement results showed a close agreement with simulated results. The proposed UWB filtering antenna array design achieved a wide fractional bandwidth of more than 109.87%, a high realized gain of more than 7.4 dBi, and a compact size of 1.08 λ0 × 1.08 λ0 × 0.035 λ0 at 6.5 GHz center frequency. These advantages make the proposed antenna suitable for UWB applications such as indoor tracking, radar systems and positioning applications.

A miniaturized high-isolation quadplexer with wide upper-stopband based on open and short stub-loaded multimode-resonator is proposed in this paper. Based on the theory of multimode-resonator and stepped impedance resonator (SIR), the compact quadplexer is designed by using multimode-resonator and SIR. In order to further miniaturize the size of the circuit, the multimode resonator is employed as the common resonator to replace the common matching network of the quadplexer and the SIRs are curved. Equivalent topology circuit is given to analyze and design the quadplexer. Detailed analyses are given according to the equivalent circuits. The proposed compact quadplexer working at central frequencies of 1.8, 2.4, 2.8, and 3.5 GHz with over 40 dB isolation is finally simulated, fabricated, and measured. The measured results agree well with the simulated ones. The total size of the fabricated quadplexer is 0.36 λ g × 0.42 λ g.

A simpler structure of super wide band (SWB) tunable microstrip band pass filter (BPF) using stub loaded multimode resonator (MMR) is presented here. The MMR is formed by loading a single openended shunt stub at the center with a simple stepped impedance resonator. By incorporating this MMR with two interdigital parallel coupled feed lines, a novel SWB tunable BPF is formed. The BPF is fabricated using FR-4 substrate of 1.6 mm thickness with dielectric constant of 4.4 and simulated using high frequency structure simulator (HFSS) software. The simulated and measured results are in good agreement with each other, with a wide fractional bandwidth (FBW) of 179%. The measured insertion loss is less than -0.9 dB throughout the pass band of 3.1 GHz-15.4 GHz with the return loss higher than 11.5 dB. The group delay of the filter is relatively constant and less than 0.3 ns over the desired pass band.

This paper proposes a novel approach for designing compact ultra-wideband (UWB) band-pass filter with a good tri-notch-band characteristic, which is obtained by using the ringstub multimode resonator (MMR). The equivalent model of the filter is achieved by using odd/even excitation resonance condition. The characteristics of the designed filter are investigated and analyzed by means of IE3D. This filter is designed, analyzed, fabricated, and measured successfully. Experimental and numerical results show that the proposed filter, with compact size of 25×10 mm2 , has an impedance bandwidth range from 3 GHz to 10.6 GHz with the triple notch bands at 4.14 GHz, 6.1 GHz, and 7.1 GHz. The proposed filter can be incorporated into UWB radio systems in order to efficiently enhance the interference immunity from undesired signals.

A compact wide band reconfigurable bandpass filter (BPF) which utilises a hemi-circular flower shaped multimode resonator (MMR) is presented. The proposed MMR provides three resonant modes which fall within the broad frequency spectra. Among these, two modes are even and one is odd. These modes are optimised by varying the dimensions so as to obtain the desired frequency response. The fractional bandwidth is more than 96 per cent. The filter can be operated as multi-band BPF. In OFF condition of ‘Pin’ diode, the centre frequencies are 2.43 GHz, 3.5 GHz, and 5.9 GHz in ON condition of ‘Pin’ diode centre frequencies are 2.43 GHz, 3.5 GHz, 5.9 GHz, 6.5 GHz, and 8.8 GHz which are used for vehicular, WiMAX, intelligent transportation systems and satellite communication respectively. Microstrip filter structures are integrated with ‘Pin’ diodes. Appropriate biasing has been provided by choosing lumped components with precise values. The insertion loss in OFF condition are 0.5 dB, 0.67 dB, and 0.8 dB and in ON condition 0.5 dB, 0.7 dB, 1.2 dB, and 1.9 dB. The measured results agree well with the full-wave simulated results.