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The miniaturization and decoupling of wideband stacked patch antennas is first presented in this communication. The miniaturization is achieved by loading spoof surface plasmon polariton (SSPP) structures on nonradiation edges of the stacked patches. Meanwhile, the wideband operation is realized by controlling the difference in propagation constants of the stacked patches. Due to the large propagation constant of the SSPP structures, the size of the antenna is reduced to $0.18\times 0.18~\lambda _{0}^{2}$ , which achieves 72% footprint reduction as compared with the conventional patch antennas on the same substrates. Moreover, both the simulation and measurement show that the proposed miniaturized antenna exhibits a wide bandwidth of more than 16%. To explore its applications in compact multi-input-multi-output (MIMO) systems, a miniaturized wideband decoupling scheme is further proposed by introducing a coupling null at the lower resonant frequency using an SSPP-loaded defected ground structure (DGS). Experimental results show that high isolation larger than 20 dB is achieved over the wide bandwidth with a center-to-center spacing of $0.28~\lambda _{0}$ .

This article presents a framework for frequency modulated continuous wave (FMCW) multiple input multiple output (MIMO) radar vital sign monitoring to achieve accurate heart rate (HR) in home application scenarios and low SNR environments. The framework consists of two main parts. The first part includes a 4-D cardiac beamformer, which is a beamformer composed of the distance dimension, azimuth and zenith angles, and the time-frequency dimension. A novel time-domain FMCW radar interference-resistant (IR) phase extraction method is proposed and combined with a spectrum-based cardiac signal quality evaluation model, ultimately focusing the radar array beams on the position of the human heart. The second part involves a modified adaptive HR trace carving technique. This technique involves the analysis of the time-frequency domain vital sign signal, utilizing forward accumulation and backward propagation to reconstruct faint heartbeat trajectory information concealed beneath respiratory harmonics and noise. To validate the proposed framework, a K-band 2T4R FMCW MIMO radar prototype was designed and fabricated. In a total of 20 overnight sleep experiments, the overall RMSE of respiratory rate and HR estimation obtained by the radar prototype based on the proposed framework were 0.53 and 2.68 bpm, respectively. The proposed framework demonstrates robust IR and the capability to operate in extremely low SNR conditions, which holds significant potential in long-term vital sign monitoring applications.

2-D transition metal dichalcogenides (TMDCs) have great potential in ultra-thin photodetectors. However, the atomically thin thickness leads to weak light absorption, thus hindering their applications. Herein, this work demonstrates that the modification of MoSe2/WSe2 heterojunction photodetectors with gold (Au) nanorods can dramatically enhance the photoelectric performance of heterojunction photodetectors. The Au nanorods are integrated into TMDCs heterojunctions of WSe2/MoSe2 (Au/WSe2/MoSe2) by a dry-transfer method. The responsivity of its photodetector is dramatically improved from visible to infrared spectral range, with a maximum enhancement of 97-fold at 750 nm and a large enhancement of 42-fold at 1100 nm. The enhancement is attributed to the broadband surface plasmon resonance supported by the Au nanorods. Combining TMDCs heterojunction with surface plasmon metal nanostructures provides a new strategy for realizing photodetectors with high response in a broad spectral range.

A simple wideband decoupling method for the laminated resonator antenna (LRA) array is proposed in this article. For compactness, the LRA elements are placed right next to each other with zero edge spacing (ES), sharing the same middle via a fence on one side. To neutralize the strong space coupling that existed between the antenna elements, the extra cavity coupling is introduced by disconnecting the middle via the fence with the upper ground. With an appropriate dimension of the middle via fence, the electric field coupled through the cavity path is approximately equal in magnitude to that coupled through the space path, but in the opposite direction. Therefore, the isolation between LRA elements is enhanced substantially. By utilizing different shapes of middle via fence, the coupling in both the E- and H-plane placed LRA arrays is reduced significantly. The entire antenna system is very compact and simple without using additional structure. For verification, the decoupling $1\times 2$ LRA arrays are fabricated and measured. Wide overlapping bandwidths (S $_{\mathbf {11}} \lt -10$ dB and S $_{\mathbf {21}} \lt -25$ dB) in both E- (16%) and H-planes (12%) are achieved. Moreover, the effectiveness of the decoupling method is also successfully validated in the $1\times 4$ LRA arrays, showing its potential in multiple-input-multiple-output (MIMO) systems.

Robust contactless Doppler cardiogram (DCG) detection in practical scenarios encounters challenges arising from radar hardware performance limitations and signal processing effectiveness. In this work, through an analysis of IQ mismatch signals, we proposed a novel vectors analytic demodulation (VAD) method, which employing the original radar signal to generate four basis vector signals, to linearly obtain the target’s differential displacement signal. Simulation experiments confirm the efficacy of VAD algorithm in achieving efficient linear phase demodulation under IQ mismatch and time-varying dc offset, surpassing the performance of existing linear demodulation algorithms. Subsequently, based on the demodulated differential displacement signal, we develop a DCG-based QRS and T wave (DCG-RT) detector to enhance the characteristic waveforms corresponding to QRS- and T-wave for automatic cardiac timing like QRS-wave to QRS-wave (RR) interval and QRS-wave to T-wave (RT) interval calculation. This algorithm achieves easily identifiable cardiac feature waveforms with minimal computational resources. Using a 24-GHz Doppler radar and medical device, the synchronous acquisition experiments were conducted on five subjects. The comparative analysis of the enhanced signals extracted by the algorithm with PCG, ECG, and other signals reveals a remarkable 98% accuracy in cardiac timing detection.

In this article, we rederive the formulas and discuss constraint rules for the S-parameters’ retrieval and the wideband extraction of per unit length resistance, inductance, conductance, and capacitance, and propagation constants of multiconductor transmission lines. To effectively solve the mode aliasing problem and improve the efficiency and the robustness in extracting above characteristic parameters of the quasitransverse electromagnetic mode multiconductor transmission lines, a wideband improved mode tracking algorithm based on the Hermitian inner product is proposed. Simulation experiments are designed and the extracted results by the proposed algorithm are compared with those from various commercial software (HFSS, Cadence Sigrity PowerSI, and Polar Si9000). The excellent data consistency, the better high-frequency data continuity, as well as the wider frequency intervals powerfully prove that the proposed extraction algorithm has the superb high-frequency stability and smaller computation amount, while extracting the wideband parameters correctly.

In this article, an efficient method for electro-thermal analysis of substrate integrated waveguide (SIW) filters is proposed. First, an analytical model of electromagnetic (EM) losses for the SIW filters is developed to obtain the heat source. Subsequently, the electro-thermal analysis is performed with the in-house developed solver based on the finite difference method (FDM), where the equivalent thermal model of via arrays is introduced to improve the computational efficiency. To ensure accuracy, the temperature-dependent (TD) material properties of both the conductor and dielectric are considered. Moreover, Aitken’s dynamic relaxation method is used to accelerate the convergence of the electro-thermal simulation. Finally, the accuracy and efficiency of the proposed method are validated through several typical examples. The results obtained by our method exhibit good agreement with the simulation conducted by the commercial multiphysics solver.

A novel design methodology for asymmetric coupled line tandem couplers with arbitrary terminal real impedances and arbitrary power dividing ratio is proposed in this article. By reasonably equating the quarter-wavelength asymmetric coupled line section with multiple single quarter-wavelength transmission lines in the vicinity of the central frequency point, an equivalent branch line model of the asymmetric coupled line tandem coupler is established and analyzed. Relationships between four terminal real impedances, four image impedances, and the power dividing ratio ${r}_{p}$ , for satisfying conditions of the perfect isolation, perfect match of ports, and power dividing requirement simultaneously, are deduced. The simplified analysis method and design formulas for symmetric coupled line tandem couplers are also presented and verified from the simulation level. Based on the extraction of unit length resistance, inductance, conductance, and capacitance parameters from full-wave simulation S-parameters of coupled lines, procedures for quickly and accurately obtaining the initial structural parameters values are provided in detail, significantly reducing the burden of subsequent optimizations. Two tandem couplers, respectively, with power coupling degrees (dB) of 2 dB ( $r_{p}\approx 0.5849$ ) and 3 dB ( $r_{p}$ = 1) were designed and fabricated. Both couplers are very compact ( $0.0073\lambda _{g}^{2}$ for 3 dB), maintain good orthogonality between the through and coupled ports, achieve the broadband property without introducing additional circuits or components, and strictly meet the expected coupling degree. The great consistency between the simulated and measured results powerfully proves the rationality of the proposed analysis theory, as well as the validity of our design methodology.

A quarter-wave patch antenna (QWPA) suffers from the fundamental disadvantage of a high cross-polarization level (XPL) in the H-plane. In this letter, we present a new method to overcome this issue by integrating a gradient spoof surface plasmon polariton (SSPP) structure on the side edges of QWPAs. In the proposed scheme, the SSPP structure is gradient loaded from the open end to the short end to suppress the cross-polarization radiation from the side edges. Both the single-ended and differentially driven QWPAs are investigated and compared based on the proposed method. Simulated results indicate that SSPP-loaded QWPAs exhibit remarkable miniaturization and XPL reduction as compared with the conventional ones. To validate the feasibility of the proposed method, a differential prototype is fabricated and measured. Experimental results show that the proposed antenna achieves a distinct cross-polarization reduction of over 20 dB and a compact size of 0.088 × 0.095 λ02 at the center operating frequency.

Annually, a significant number of premature infants suffer from apnea, which can easily cause a drop in oxygen saturation levels, leading to hypoxia. However, infant cardiopulmonary monitoring using conventional methods often necessitates skin contact, and they are not suitable for long-term monitoring. This article introduces a non-contact technique for infant cardiopulmonary monitoring and an adjustable apnea detection algorithm. These are developed using a custom-designed K-band continuous-wave biomedical radar sensor system, which features a DC-coupled adaptive digital tuning function. By using radar technology to detect chest motions without physical contact, it is feasible to extract significant biological information regarding an infant's respiration and heartbeat. The proposed algorithm utilizes an adaptively adjusted threshold and personalized apnea warning time to automatically measure the total number of apneic events and their respective durations. Experiments have been conducted in clinical environment, demonstrating that both the accurate cardiopulmonary signals and the apneas of varying durations can be effectively monitored using this method, which suggest that the proposed technique has potential applications both inside and outside of clinical settings.

In this letter, a miniaturized active electric probe, which simultaneously achieves high performance in terms of wide bandwidth, high spatial resolution, and high sensitivity, is proposed for radiation source localization. This probe is manufactured utilizing a printed circuit board (PCB) process, occupying a compact area of 588 mm2. High spatial resolution is achieved through an ultra-narrow induction tip with an effective width of 0.1 mm, the PCB process limit. Sensitivity is improved by integrating an amplification circuit with a 17 dB gain. Wide bandwidth is achieved by implementing a wideband amplifier of 10 MHz to 27.8 GHz and a horizontal tapered substrate integrated coaxial line transition structure. Through the measurement results, the proposed probe can operate from 10 MHz to 27.8 GHz. At 27.8 GHz, the spatial resolution of the proposed probe is 0.41 mm. The sensitivity has a 17 dB improvement compared with the reference passive probe.