This paper presents a planar microwave sensor for the sensitive and accurate characterization of complex permittivity of solid materials. To realize the proposed sensor, the combination of a parallel interdigital capacitor and a dual wide gap resonator was etched on the ground plane and coupled with a transmission line in the top plane of a printed circuit board. The main advantage of the proposed sensor configuration lies in the generation of a high-intensity coupled resonating electric field suitable for the sensitive measurement of complex permittivity of solid materials. The high-intensity electric field enhances the coupling and field interaction, and thereby produces a high-accuracy permittivity characterization of a solid material exposed to the maximum field of the proposed sensor. Our developed microwave permittivity sensor, which characterized the complex permittivity of several materials by exploiting the measured shifts in resonance frequencies, exhibited a high sensitivity (up to 37% shift in resonance frequency for 20% change in permittivity) at least 1.5 times higher than previously reported microwave permittivity sensors. In addition, the proposed sensor exhibited 99.9% and 99.7% sensing accuracies for real and imaginary parts of permittivity, respectively, and the measurement results revealed an excellent (0.06758) and high sensitivity (67.58 MHz per unit change in real permittivity).

This study demonstrates a wideband four-element multiple input multiple output (MIMO) antenna employing a loaded metamaterial superstrate. The proposed MIMO antenna configuration has four slotted rectangular microstrip antennas arranged on a compact square substrate to attain excellent impedance matching over a wide bandwidth of 41 % (3.33 - 5.04 GHz). The suggested four-element MIMO antenna has a compact whole area of 50 × 50 × 1.6 mm ³ (0.65 λ₀ × 0.65 λ₀ × 0.002 λ₀ ), while λ ₀ is the wavelength of free space at 4 GHz. The suggested design with a novel 3×3 metamaterial superstrate structure reduces mutual coupling of no less than -23 dB among all ports. The metamaterial superstrate is positioned above MIMO elements at 15 mm (0.2 λ₀ ) for the purposes of isolation and gain enhancement. Satisfactory parameter values are also determined for the competitive MIMO performance, such as envelope correlation coefficient (ECC), diversity gain (DG), channel capacity loss (CCL), and total active reflection coefficient (TARC). The anticipated design has been fabricated and measured. The comparison of simulation outcomes with the experimental findings is satisfactory.

This paper presents an interdigital capacitor (IDC)-based high sensitivity microwave sensor for the multidimensional (complex permittivity and thickness) characterization of solid materials. The proposed sensor was implemented by etching a slot resonator with an IDC on the ground layer of 0.76 mm-thick RF-35 substrate with relative permittivity of 3.5 and loss tangent of 0.0018. The IDC fingers were configured as a spur-line structure to create an intense electric field zone that interacted significantly with the material under test (MUT), resulting in high sensitivity. A sensor prototype with optimized dimensions generated dual resonant frequencies of 2.35 GHz and 5.79 GHz and was used to measure the MUT’s complex permittivity and thickness, respectively. Experimental results revealed that the resonant frequencies of the developed sensor exhibited high sensitivities (276 MHz/ $\varepsilon _{r}$ for permittivity and 143 MHz/mm for thickness) and resolutions (0.27678 for permittivity and 0.1439 for thickness) of solid materials. Moreover, the sensing accuracies were 99.9% for real permittivity and 97.7% for imaginary permittivity. The demonstrated high performance of the developed sensor validates its applications in the multidimensional characterization of various solid materials.

In this paper, the extraction of the life activity spectrum based on the millimeter (mm) wave radar is designed to realize the detection of target objects and the threshold trigger module. The maximum likelihood estimation method is selected to complete the design of the average early warning probability trigger function. The threshold trigger module is designed for the echo signal of static objects in the echo signal. It will interfere with the extraction of Doppler frequency shift results. The moving target detection method is selected, and the filter is designed. The static clutter interference is filtered without affecting the phase difference between the detection sequences, and the highlight target signal is improved. The frequency and displacement of thoracic movement are used as the detection data. Through the Fourier transform calculation of the sequence, the spectrum value is extracted within the estimated range of the heartbeat and respiration spectrum, and the heartbeat and respiration signals are picked up. The proposed design uses Modelsim and Quartus for CO-simulation to complete the simulation verification of the function, extract the number of logical units occupied by computing resources, and verify the algorithm with the vital signs experiment. The heartbeat and respiration were detected using the sports bracelet; the relative errors of heartbeat detection were 0–6.3%, the respiration detection was 0–9.5%, and the relative errors of heartbeat detection were overwhelmingly less than 5%.

This paper presents a four-port multiple-input multiple-output (MIMO) antenna based on frequency selective surface (FSS) for X-band communication. The suggested MIMO antenna is composed of four-slotted circular microstrip patch elements, which have been planned on an FR-4 substrate with a low profile (0.91λ0 × 0.91λ0 × 0.045λ0). The suggested antenna reduces mutual coupling (MC) and enhances gain based on the FSS technique. The band-stop FSS transmission coefficient is below -10 dB over the 7.2 to 9.1 GHz range. Additionally, for transverse magnetic (TM) and transverse electric (TE) polarization, the planned FSS offers a steady frequency response for incidence angles up to 40°. The suggested MIMO antenna bandwidth is from 7.8 GHz to 9.08 GHz (15.4%), with a peak gain of 5.2 dBi. The MC < -20 dB is attained over the entire operating bandwidth. In addition, the suggested antenna envelope correlation coefficient (ECC) is < 0.005, diversity gain (DG) is < 9.99, and channel capacity loss (CCL) < 0.3 bps/Hz is obtained in the operating frequencies band, approving good MIMO diversity performance.

Simultaneous characterization of permittivity and permeability of magnetodielectric materials using a single resonator-based microwave sensor is challenging due to their similar impacts on the sensor’s response. This work presents an interdigital structure-based planar microwave resonator sensor featuring high-intensity electric (E)-field zones and magnetic (M)-field zones to characterize permittivity and permeability. The proposed sensor’s sensing E-field and M-field zones were implemented using the interdigital fingers configured as a spur line structure and interdigital electrodes, respectively, generating a 5.9-GHz resonance frequency with a high Q-factor (1700). The resonance frequency of the designed sensor exhibited high sensitivities (324 MHz/ $\Delta \varepsilon _{r}$ for permittivity and 825 MHz/ $\Delta \mu _{r}$ for permeability) for characterizing single/multilayers magnetodielectric materials with excellent resolutions (0.324 for permittivity and 0.825 for permeability). The demonstrated high sensitivity and resolution results validate the performance of the interdigital structure-based microwave resonator sensor for simultaneous characterization of multilayer magnetodielectric materials.