Polarization diversity is an antenna feature that has been widely researched to increase the capacity of a wireless communication system. The conceptual theory of polarization states that all polarization vectors can be decomposed into two orthogonal electric field vector modes. This study proposes a novel framework to bridge the gap between circular polarization (CP) technique and filtering antenna design using two orthogonal resonators with the ability to represent any polarization. The proposed method combines the techniques of antenna–filter and CP design into a single integrated step. To achieve the second-order antenna–filter integration and generate CP, the two orthogonal resonators modeled as a rectangular radiator and a $\uplambda $ /4 resonator combined. The CP filtering antenna operates at a frequency of 4.65 GHz, with an impedance bandwidth of 4.7%, an axial ratio (AR) bandwidth of 3.2%, and a gain of 6.2-dBi. To validate the CP filtering antenna, simulations and measurements were performed, and the results were found to be in good agreement.

This study presents a novel design of two filtering antennas with a +45° and -45° slant polarization, using the co-design of a third-order filter and a rectangular patch antenna. Slant polarization is obtained by directing the surface current that flows to the radiator by applying two parallel inverted resonators in the transmission line. The resonators consist of two parallel strips with alternate open and short circuits at each end and the 45° and -45° polarization is determined from these short circuit positions. The rectangular patch antenna is fed using proximity coupling, and it is parallel to the resonators. The filtering antennas provide not only 45° slant polarization, a flat gain response but also a controllable bandwidth. Operating at 4.65 GHz for 5G application, the antennas have a controllable bandwidth range from 5.9% to 8.4% without affecting the polarization and center frequency. The gain responses are filter like and attain the maximum value of 6.82 and 6.69 dBi at 4.7 GHz. This design needs no extra component or rotation of the radiator and transmission line because the inverted resonators can switch the polarization to be +45° and -45°. The results of the 45° and -45° polarization filtering antenna simulation and experiments are consistent with each other.

Multifunctional, integrated, and independent microwave sensor (MS) with dual-band bandpass filter (BPF) for solid material characterization and radio frequency communication is presented. The proposed device is developed using a multi-coupled line resonator structure to integrate both multiplexing (MUX) and demultiplexing functionalities (DEMUX). On the input side, the circuit behaves like a MUX, divided into three main sections: Path I, Path II, and the MS structure. Path I and Path II sections feature a pair of coupled-line configurations for the lower-band BPF (LB-BPF) with ladder-based resonators and upper-band BPF (UB-BPF) with hairpin-based resonators, respectively. Additionally, the MS structure employs a ring resonator configuration, positioned between Path I and Path II. On the output side, the circuit behaves like a DEMUX, combining back into a single port. As a result, the dual-band BPF has frequency centers of 1.80 GHz (LB-BPF) and 2.56 GHz (UB-BPF) suitable for 4G and 5G applications, respectively. Moreover, the dual-band BPF exhibits independent characteristics, with easily controllable frequency centers and fractional bandwidths. Furthermore, for the MS function, the frequency of maximum transmission (MT) and the transmission zero (TZ) were utilized for MS evaluation. Solid materials with low permittivity (ε _r ) ranging from 1 to 9.5 were used as samples. The MS achieved a normalized sensitivity of 2.60% and 1.98% for MT and TZ, respectively. Interestingly, the integration of LB-BPF, UB-BPF, and MS shows independent performances and does not influence each other. This behavior is linked to distinctive electric field locations. Finally, compared to prior structures, the proposed design offers several advantages, including: 1) multifunctional capability by combining a dual-band MS and dual-band BPF, 2) independent and simultaneous characteristics, and 3) full integration with a simple structure based on 2-port single-layered substrate design.

JCM is an open access journal on the science and engineering of communication.

In this paper, filter and antenna integration is studied to produce a compact device for wireless front-end equipment. Filtering antennas are advanced due to their selectivity performance, represented by a flat gain response. Two filtering antennas are proposed to improve the selectivity using different orders. The first antenna is based on second-order filter and the other on third-order filter. Both antennas are designed to operate at 4.65 GHz for mid-band 5 G application with a bandwidth of 6.45%. The first antenna integrates a rectangular radiator and an interdigital resonator based on second-order filter. It obtained a bandwidth impedance of −10 dB for 300 MHz and a maximum gain of 6.48 dBi. Meanwhile, the second design consists of a rectangular radiator and two interdigital resonators based on third-order filter as the feedline. Having the same bandwidth as the first design, the second design achieved a flat gain of 6.37 dBi in the operational bandwidth. The second antenna design showed better selectivity with sharper gain than the first design. The two antennas were fabricated and measured for validation. The simulation and measurement results showed good agreement.

In this paper, a compact tri-band bandpass filter (TBPF) is proposed. The proposed TBPF employs a multicoupled-line structure to achieve an independence between the inter-passbands. In addition, the triple-band bandpass characteristics are generated separately and independently using two pairs of quarterwavelength resonators with L- and U-shaped configurations. The two pair of resonators were set backto-back creates a second order filter and coupling to the stub transmission lines. The pairs of U resonators create mode splitting results two resonances frequency at 1.87 GHz and 5.82 GHz while two pairs of L resonator create bandpass with a wide $-3$ dB bandwidth of 21.6% at 3.35 GHz center frequency. The two of three passband frequencies can be set and designed independently. Consequently, the proposed TBPF achieves a 46.4% size reduction compared to the previously reported TBPF. To validate its performance, the filter was fabricated and measured. The measurement results are in close agreement with the simulations. Overall, the proposed TBPF exhibits compact dimensions and independently controlled inter-passband characteristics.