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In this article, a general and effective method for designing compact bandpass filters (BPFs) with extensible orders and channels on a modified patch resonator is presented, for the first time. A comprehensive demonstration and validation of the proposed method are conducted, employing a circular patch resonator. First, the electric-field distributions under different mode resonances of a circular patch resonator are investigated. The TM00 modes can be efficiently excited by loading a metal via at the center of the circular patch resonator. Interestingly, when dividing the modified circular patch resonator by magnetic walls along the radii, the resonant frequencies of the TM00 mode in the resulting subresonators, are almost identical. These subresonators effectively suppress the excitation of higher order modes. Building upon this principle, second-, third-, and fourth-order BPFs with compact size and good stopband rejection are presented. Furthermore, by introducing multiple perturbed metal vias, BPFs with extensible channel numbers are developed. Dual-, tri-, and quad-channel BPFs with constant and compact size, wideband, and wide frequency range isolation are designed for verification. All measured results are in good agreements with the simulated ones, providing compelling evidence for the feasibility of the proposed design method.

This article proposes a novel microwave displacement sensor based on a filtering phase shifter (FPS), which can provide displacement sensing and intrinsic filtering functions simultaneously. The FPS comprises a third-order Chebyshev bandpass filter (BPF) network and a pair of transmission lines. With the derived closed-form formula of magnitude and phase response of the proposed FPS structure, variable insertion phase and constant filtering response characteristics are applied to design the linear displacement sensors for the first time. The displacement sensor mainly consists of double layers, where a pair of fixed transmission lines connect two ports in the first layer as a stator and an FPS structure in the second layer as a movable contact slider. The insertion phase drifts linearly along the displacement, while the bandpass filtering response remains unchanged. A proof-of-concept microstrip sensor prototype operating at 2.98 GHz is implemented and tested to validate the proposed method experimentally. The measured results are in good agreement with the simulated ones, which shows that the proposed sensors can achieve an average phase sensitivity of about 12.8°/mm with a constant bandpass filtering response of 29.5% 3-dB fractional bandwidth (FBW).

This article presents a design method for multilayer packaged single- and dual-band dual-channel bandpass filters (DCBPFs) based on square substrate integrated waveguide (SIW) cavities. By exploiting one or multiple pairs of orthogonal degenerate modes in square SIW resonators, two single-band and one dual-band DCBPFs with high channel isolations are implemented. By arranging feeding ports and coupling apertures, a highly selective single-band DCBPF with multiple types of operating states is demonstrated. In order to construct a dual-band DCBPF design, balanced feeding ports are introduced to excite the operating modes and suppress unwanted spurious modes between the dual passbands, thus realizing a satisfactory dual-band frequency response. To validate the design method, two single-band and one dual-band prototypes are designed, fabricated, and measured. The results exhibit excellent filtering performance and high channel-to-channel isolations.

This work proposes a novel UHF RFID-assisted axle counting system for wireless train detection. In the system, the RFID tag antennas are proposed to be attached to the train compartments. When the train travels across the signal indicator next to the railway, the reader detects the RFID tag, and the axle counter counts the number of passing train axles. To overcome the traditional reader antenna with the blind reading zone that may introduce safety problems, a new null-filled monopole UHF RFID reader antenna is proposed. The monopole is connected to the metal ring, which changes the antenna's surface current and electric field distribution, and eliminates the radiation blind reading zone at the antenna zenith. The measured S11 of the proposed antenna is better than −10 dB in 0.84–1.02 GHz, achieving a relative bandwidth of 19.6%. The antenna has a zenith gain of 1.5 dBi, with horizontal radiation gain better than −3.55 dBi and an HPBW of 141° at 0.92 GHz. The null-filled monopole antenna can provide a potential solution for the reader in the proposed UHF RFID-assisted axle counting system under wireless train detection.