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.

In this paper, a dual-band bandpass filter with a wide upper stopband is proposed and designed by integrating stepped-impedance resonators (SIRs) and a low-pass filter. The operating center frequencies of the designed dual-band filter are targeted at 2.5 GHz and 5.35 GHz, respectively, to meet the frequency requirements of typical wireless communication scenarios. Notably, the filter achieves a wide upper stopband ranging from 6.1 GHz to 25 GHz, which can effectively suppress unwanted high-frequency interference signals within this frequency range and avoid mutual interference with other high-frequency communication systems. And it exhibits insertion losses of 0.12 dB (2.5 GHz) and 0.6 dB (5.35 GHz) in its two passbands to ensure minimal useful signal attenuation. The simulation results agree well with the measured results.

A quad-channel diplexer is designed in this paper. The diplexer is composed of four stub-loaded step impedance resonators and a common feeder T-joint, which realizes four passbands with center frequencies of 2.6 GHz, 3.48 GHz, 4.8 GHz, and 6.3 GHz. The dual-band filter can be formed by coupling the stepped impedance resonator with the stub load, so two dual-band filters with good performance can be constructed. At the input and output end, a 0-degree feed is used to generate transmission zeros, which improves the high selectivity. When two dual-band filters are combined, a good impedance matching is obtained, and the |S₂₃| > 20 dB between the two dual-band filters achieves good isolation. The simulation results are consistent with the measured results.