Integrated and Independent Solid Microwave Sensor With Dual-Band Bandpass Filter Through Unified Mux–Demux Structure

Multifunctional, integrated, and independent microwave sensor (MS) with dual-band bandpass filter (BPF) for solid material characterization and radio frequency (RF) communication is presented. The proposed device is developed using a multi-coupled line (CL) resonator structure to integrate both multiplexer (MUX) and demultiplexer (DEMUX) functionalities. On the input side, the circuit behaves like a MUX, divided into three main sections: Path I, Path II, and the MS structure. Paths I and 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 (RR) configuration, positioned between Paths I and 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 (FBWs). 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 (
$\varepsilon _{r}$
) ranging from 1 to 9.5 were used as samples. The MS achieved a normalized sensitivity (NS) 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 two-port single-layered substrate design.