Abstract Spectrum lower than 8 GHz still continues to be important in applications such as 5G and beyond, wireless communication systems with regard to achieving wide coverage area and higher data rates. Nevertheless, some limitations have been observed in terms of limited bandwidth, low gain, and mutual coupling in the case of Microstrip Patch Antennas (MPAs) in Multiple-Input Multiple-Output (MIMO). This study develops miniaturized 2-port and 4-port MIMO MPAs in combination with an innovative frequency-selective surface (FSS). A modified patch antenna with a square-ring defected ground structure, orthogonal configuration of radiators, and cross-coupling networks is developed. The superstrate of FSS includes square unit cells having circular and cross-shaped slots to improve directivity and radiation efficiency without compromising impedance matching. The designed antennas exhibit bandwidth ranging from 4.9 to 8.1 GHz (2-port MIMO) and 4.69 to 7.885 GHz (4-port MIMO) with a maximum gain of 5.8 and 11.9 dB, respectively. Excellent performance measures have been observed in the form of envelope correlation coefficient less than 0.002, diversity gain more than 9.99 dB, channel capacity loss below 0.03 bit/s/Hz, and overall reflection coefficient equal to −13 dB. The suggested antenna structure represents a reliable and easy-to-fabricate approach to sub-8 GHz wireless communications as well as 5G mid-band communications.
Abstract This paper presents a compact, ultra-wideband, and high-efficiency rectifier designed for microwave power transmission and energy harvesting applications. The proposed architecture integrates a series-coupled matching network with cascading band-stop structures, enabling both impedance transformation and harmonic control within a compact footprint. Two highly compact rectifiers are designed, fabricated, and measured to verify the proposed method. A broadband rectifier, operating between 1.8 and 3.0 GHz with a relative bandwidth of 50%, achieves above 70% rectification efficiency and a peak efficiency of 79%. The other wideband operation mechanism is analytically investigated: the band-stop network reshapes the diode’s impedance profile by tuning its low- and high-frequency components into a conjugate relationship, while the coupled matching structure transforms the resulting impedance to match the system impedance, thereby realizing ultra-wideband matching. The fabricated rectifier achieves a measured efficiency exceeding 40% across a wide frequency range of 1.1–3.9 GHz, corresponding to a 112% relative bandwidth at a low input power level of 0 dBm. In addition to its wideband and efficient performance, the rectifier features a highly compact form factor of only 18 × 24.8 mm 2 .
Abstract In this work, we propose interconnection structures for substrateless all-silicon dielectric waveguides, a platform for terahertz (THz) hybrid integration that currently relies on traditional tapered interfaces. We demonstrate two dielectric waveguide-to-waveguide interconnect approaches: a truncated termination and a quarter-wave-matched (QWM) slot termination. Both designs maintain a confined coupling point, enabling consistent coupling. Experimental results show maximum interconnection coupling losses of 5 dB $5\,\mathrm{dB}$ 5 d B and 2.5 dB $2.5\,\mathrm{dB}$ 2.5 d B for the truncated and QWM-slot terminations, respectively, across the 220–330 GHz band. The QWM-slot transmission coefficient remains above minus 3 dB $-3\,\mathrm{dB}$ − 3 d B with a misalignment tolerance up to 100 mu m $100\,\mu\mathrm{m}$ 100 μ m , representing a 2.5 times $2.5\times$ 2.5 × improvement over the truncated design. Offset characterization along three axes reveals smooth decoupling behavior over a 1 mm range, validating the potential for broadband, contactless interconnects between dielectric waveguide interfaces. These results establish a compact THz interconnect concept with substantially relaxed translational alignment tolerances, making it well-suited for deployment in scalable 6G multi-chip arrays and THz sensing modules.
Abstract The present work provides a comprehensive analysis of non-linear distortions using Unequally Spaced Multi-Tone (USMT) signals under realistic modulated operating conditions. The investigation focused on USMT stimuli with 2, 4, and 6 tones, with the objective to assess large-signal performance of a pre-matched GaN HEMT device at 29 GHz manufactured by United Monolithic Semiconductors. Experimental measurements were conducted on-wafer and compared with nonlinear Harmonic Balance simulations, incorporating electromagnetic models for passive elements and the manufacturer’s design kit for active components. Furthermore, continuous-wave load-pull simulations were conducted to assess the optimal load impedance for maximum power-added efficiency (PAE). The findings indicate that increasing the number of tones results in a substantial decrease in drain current density due to trapping effects, along with a concomitant degradation in PAE. A corresponding reduction in adjacent channel power ratio (ACPR) was also observed, and an analytical relationship between ACPR and the number of tones was derived. The strong agreement between simulated and measured ACPR values serves to confirm the accuracy of the modelling approach and to validate the reliability of the measurement setup.
Abstract Terahertz near-field imaging enables high-resolution biomedical and material characterization by combining the non-ionizing nature of terahertz radiation with sub-wavelength spatial resolution. However, the scalability of existing solid-state near-field imagers is fundamentally limited by shared illumination schemes and laterally arranged imaging components, restricting pixel miniaturization and array density. This work overcomes these limitations by introducing a novel pixel architecture that vertically integrates the illumination source, near-field sensing element, and power detector within each pixel, replacing conventional lateral implementations. The proposed architecture reduces the pixel footprint while enabling every pixel to operate as an independent terahertz near-field sensor. Based on this concept, a fully monolithic 32 × 32 (1024-pixel) near-field imager was implemented in a 130-nm SiGe BiCMOS HBT technology. The sensor operates in rolling-shutter mode and achieves real-time imaging at up to 40 fps. The chip consumes 280 mW, corresponding to 273 μW per pixel, and achieves a pixel density of 200 pixels/mm 2 , the highest reported for monolithically integrated terahertz near-field imaging systems above 100 GHz. The paper presents the scalable architecture together with the design, characterization, and performance analysis of the individual imaging components and discusses the trade-offs introduced by vertical integration.
Abstract This work proposes two methods for evaluating the intrinsic nonlinear parameters for the microwave ferromagnetic materials, which play an important role in the development of low passive-intermodulation devices. We find that the product of the nonlinear permeability and the skin depth serves as the intrinsic nonlinear parameter governing the passive intermodulation (PIM) effect. Analytical and numerical models are proposed to extract it from the experiments. For the coated transmission line method, an analytical expression for the relationship between PIM and the nonlinear parameter is derived. For the linearity–nonlinearity-separation (LNS) Finite Difference Time Domain (FDTD) full-wave method, a PIM simulation algorithm suitable for thin coating layers is established. Both methods are validated using a nickel-plated microstrip line. The extraction results from the two approaches are in good agreement, confirming the consistency and feasibility of the proposed measurement methods.
Abstract This work presents a broadband two-stage pseudo-differential low-noise amplifier (LNA) fabricated in an advanced 130-nm SiGe BiCMOS technology with ft $f{_t}$ f t / fmax $f_{max}$ f m a x of 470/650 GHz. The design combines Electromagnetic (EM)-based device co-simulations with novel broadband transformer matching networks. The input transformer employs asymmetric broadside-coupled lines to achieve wideband impedance transformation with insertion losses below 2 dB. The novel interstage and output transformers are designed as tapered asymmetric broadside coupled lines, providing a distributed transformation with a high impedance transformation ratio of 5:1 and 4:1, respectively, over a bandwidth (BW) of 100 GHz. The LNA provides a peak small-signal gain of 11.8 dB at 295 GHz with a 111 GHz 3-dB bandwidth spanning 211–322 GHz. The simulated noise figure is between 9.7 dB at 226 GHz and 14 dB at 330 GHz. The LNA has a saturated output power of 4.65 dBm at 280 GHz and an OP 1 dB $OP_{1dB}$ O P 1 d B of 3.6 dBm, including balun losses and consumes 118 mW DC power. It has a measured group delay (GD) variation of plus or minus 3.8 $\pm 3.8$ ± 3.8 ps, making it suitable for broadband wireless communication.
Abstract In recent years, the through-wall imaging (TWI) systems have garnered tremendous interest due to their potential applications in surveillance, search–rescue operations, and security monitoring, enabling the detection and visualization of objects behind opaque obstacles such as walls. The multiple-input multiple-output (MIMO) TWI system has been developed to detect hidden targets behind walls. MIMO-based TWI considerably reduces the time it takes to scan the walls. This study presents an experimental evaluation of a MIMO-based TWI system using a dataset of targets with varying dielectric properties placed at uniform and nonuniform distances behind a wall. The B-scan data are collected for different types of targets for contrast target detection. To effectively suppress clutter and enhance target detectability, we employed a range of advanced signal processing techniques, including singular value decomposition (SVD), SVD-based noise subspace exploitation, the optimum shrinkage algorithm, and background subtraction using data-driven thresholding. The target-to-clutter ratio of the final image was computed to evaluate the effectiveness of each clutter removal technique and to facilitate a comparative analysis of the results. The imaging results demonstrated that the system could detect single and multiple targets, including those composed of different materials.
This paper presents the design, optimization, fabrication, and measurement of three RF-to-DC rectifiers operating at 3.5 GHz for wireless power transfer and RF energy harvesting applications. A low-power rectifier achieves a peak efficiency of 50.5% at -2 dBm, while a high-power rectifier reaches 61.2% at 15 dBm. To extend the operating range, a wide-range rectifier based on a passive dynamic sub-rectifier selection architecture is proposed. The developed system achieves a maximum efficiency of 54.7% at 16 dBm and maintains over 20% efficiency across a 41 dB input power range. Experimental results validate the proposed designs and show good agreement with simulations.
This work proposes a band-switchable electrically small antenna (ESA) system for a wearable health monitoring device. The system operates at multiple 5G-NR bands (875-934 MHz and 1.72-2.2 GHz) using a switchable configuration. The antenna consists of multi-resonant shorted strips that have been miniaturized to fit in meandered form into an area of 63.2 $ imes$25 mm $<^>2$ (0.19 $ imes$0.08 $\lambda<^>2$) available in the intended health monitoring device. To enable operation at two distinct modes, the antenna feedline has been integrated with a switch, which is then matched using lumped matching components. Results indicate that the ESA system operates at 875-900 MHz in Configuration 1, and at 898-934 MHz and at 1.72-2.2 GHz in Configuration 2. The device has been fabricated and integrated into a compact 3D printed casing prior to experimental evaluations. In addition to studying the switchable reflection and radiation performance, the ESA is also evaluated in the presence of a human body phantom. Simulated specific absorption rate levels are less than 0.47 W/kg averaged over 1 g of tissue, which complies with international standards.
With the increasing level of wearable technologies, being intertwined with 5G networks, the necessity of high-speed and wideband communication solutions has gained top priority. The ultra-wideband technology presents a possible avenue for fulfilling these requirements. Following this drive, the proposed work establishes an ultra-wideband antenna system that is explicitly developed and validated to be used in 5G wearables. The designed antenna incorporates a dual-element multiple-input and multiple-output antenna featuring a stepped triangular-shaped radiator, evolved from an initial rectangular-shaped geometry. This transformation enables dual resonances at 4.7 and 7.9 GHz, covering a wide spectrum of 4.0-10.2 GHz. Furthermore, the suggested antenna demonstrates a minimum inter-element isolation exceeding 21.2 dB with a peak gain of 3.15 dBi. The multiple-input multiple-output diversity parameters have been investigated and observed to be within the acceptable limits, while the effectiveness for body-worn applications is further evaluated by analyzing its conformal performance and safety compliance, demonstrating average SAR values of 1.12 and 0.89 W/kg for 1g of tissue mass at 4.7 and 7.9 GHz, respectively. Finally, the antenna is fabricated, and its performance is validated through experimental measurements.
This paper presents a novel, independently controllable dual-polarized reconfigurable intelligent surface (RIS) unit cell for 6G upper mid-band ( 7.125 $7.125$ 7.125 - 24.25 GHz $24.25\,\mathrm{GHz}$ 24.25 G H z ) applications. The proposed unit cell is based on the diagonal placement of mutually orthogonal tunable radiators (loaded with varactor diodes) alongside static dual-polarized radiators. Full-wave electromagnetic simulations of the unit cell demonstrate a phase shift range of 270 ring $270<^>{\circ}$ 270 degrees , a maximum reflection loss of 4.5 dB $4.5\,\mathrm{dB}$ 4.5 d B , and 61 dB $61\,\mathrm{dB}$ 61 d B cross-polarization isolation (XPI) within a 400 MHz $400\,\mathrm{MHz}$ 400 M H z frequency range centered at 15 GHz $15\,\mathrm{GHz}$ 15 G H z . Furthermore, full-wave simulations of a 16 times 16 $16 imes 16$ 16 & times; 16 RIS panel were conducted for various reflection angles under both TE- and TM-polarized illuminations. The results demonstrate a wide-angle beam-steering capability of plus or minus 60 ring $\pm60<^>\circ$ +/- 60 degrees , with a maximum beam-pointing error of only 1.7 ring $1.7<^>{\circ}$ 1.7 degrees . The proposed RIS also exhibits accurate and stable reflection control under wide-angle excitation. A 3 times 3 $3 imes 3$ 3 & times; 3 prototype was fabricated to validate the unit cell's performance. Measurement results agree with the simulations, yielding a 270 ring $270<^>{\circ}$ 270 degrees phase shift, approximately 7 dB $7\,\mathrm{dB}$ 7 d B reflection loss, and 24 dB $24\,\mathrm{dB}$ 24 d B XPI. The proposed RIS is a promising candidate for future 6G communication systems.
This paper proposes an electronically controlled reconfigurable scanned array that uses movable short phase shifters suitable for 6G applications. Excellent performance is demonstrated by the passive electronically scanned array (PESA) operating in the centimeter-wave frequency region (13-17 GHz). Four digitally controlled variable reflect-type phase shifters, each built with a 90 ring $90<^>\circ$ 90 degrees hybrid coupler with a movable short circuit, are integrated with a 2 times 2 $ 2 imes 2$ 2 & times; 2 horn antenna array. The assembly also has E-H feeding networks, which help the antenna array receive a uniform signal distribution. Outstanding performance is demonstrated and verified by time and frequency domain simulation results of the full PESA, which show continuous main-lobe scanning in the azimuth and elevation planes. Over the operating bandwidth, the proposed PESA maintains a steady gain of 14 plus or minus 2 $14 \pm 2$ 14 +/- 2 dBi while achieving a matching level below minus $-$ - 12 dB.
The rapid advancement of the miniaturization techniques for high-frequency transceivers, radars, and communication systems operating in the D-band (110-170 GHz) has positioned circular dielectric waveguides (CDWGs) as a promising transmission-line alternative. Yet, their potential has not been fully exploited. This work fills this gap by introducing novel, compact CDWG-fed lens antenna solutions that deliver excellent gain and impedance matching performance. A low-loss, low-cost, and flexible CDWG made of low-density polyethylene is used to excite dielectric lens antennas of various sizes, contours, and materials, enabling a systematic evaluation of the impact of lens geometry on antenna performance. Through detailed electromagnetic simulations and experimental validations, lenses of different sizes and contours are designed. The proposed CDWG-lens configuration achieves measured broadband matching better than 17 dB and stable high gain across 115-130 GHz. Measured gains of 21-23 dBi for lenses with diameter of 10.6 mm and gain factor of 27-28 dBi for lenses with diameter of 20 mm confirm the high aperture efficiency and novelty of this approach. A strong agreement between the simulation and the measurement results is also observed. These compact and efficient lens antennas combined with the advantages of CDWG feed, can be integrated as a part of advanced system applications in the D-band frequency range like radar sensing and communications.
This paper presents the design and experimental validation of a compact corrugated double-tapered slot Vivaldi antenna (DTSVA) for ultra-wideband (UWB) applications. The proposed antenna provides efficient impedance matching over a wide frequency range of 1-20 GHz. To suppress electromagnetic interference from coexisting narrowband systems, triple band-notched characteristics are realized using spatially distributed resonant elements, including a complementary split-ring resonator (CSRR) and U-shaped slots etched beneath different sections of the feed network, enabling effective rejection of WiMAX (3.3-3.8 GHz), WLAN (5.15-5.85 GHz), and X-band uplink (7.9-8.4 GHz) signals. To enhance forward radiation without increasing the antenna size, corrugated edges and an anisotropic zero-index metamaterial (AZIM) unit cell are integrated within the tapered slot region. Consequently, the proposed antenna achieves a maximum peak realized gain of 12.5 dBi, corresponding to an improvement of up to 3.8 dBi compared to the reference DTSVA. Simulated and measured results show good agreement in terms of impedance bandwidth, radiation characteristics, realized gain, and group delay. Owing to its compact size, enhanced gain, stable time-domain performance, and effective interference suppression, the proposed antenna is well suited for portable indoor and outdoor UWB applications, including imaging, radar sensing, and material characterization.
In this work, a novel hybrid algorithm is introduced that combines the range migration algorithm (RMA) and the back projection algorithm (BPA). The traditional image reconstruction algorithms used for synthetic aperture radar each have trade-offs between computational efficiency and imaging accuracy. The proposed hybrid RMA-BPA approach leverages the computational efficiency of RMA for the initial image reconstruction and object detection, followed by BPA for the refined, high resolution of the cropped region of data. The method of focusing computational resources on the smaller cropped datasets that contain the objects significantly reduces the processing time compared to that of traditional standalone BPA. The hybrid approach's performance was evaluated over three different scenarios, providing a reduction in computation time for each scenario. Due to the algorithm's approach to crop the dataset for the specific object, the increased efficiency varied. The different scenarios each produced different times to compute; however, the most impressive result delivered a 75.2% reduction in computation time compared to traditional BPA, without sacrificing the accuracy of the image. The hybrid approach is especially suited for applications that require precise object detection in healthcare, oil and gas, security, and industrial inspections.
Medical microwave imaging (MMWI) is emerging as a promising noninvasive diagnostic modality, offering a safe, portable, and cost-effective alternative to conventional imaging methods. Despite its potential, MMWI poses significant challenges due to the ill-posed and nonlinear nature of the underlying inverse scattering problem. Two main strategies are typically adopted: qualitative imaging, which yields rapid but approximate reconstructions, and quantitative imaging, which aims to recover the full permittivity distribution of tissues. Both approaches substantially benefit from accurate patient-specific background models that enhance reliability and reduce computational complexity. Building upon preliminary results presented at EuCAP 2025, this work advances the development of patient-specific head models for quantitative brain imaging. The study employs anatomically conformal basis functions and extends the model to include cerebrospinal fluid, which represents a strong scatterer with markedly distinct dielectric properties. The proposed framework improves reconstruction accuracy and convergence efficiency, thereby contributing to the advancement of quantitative MMWI toward practical clinical applications.
This study presents the design and analysis of a small, high-gain, high-isolation 4-port multiple-input multiple-output (MIMO) array antenna engineered for 5G millimeter-wave (mmWave) applications at 28 GHz. The suggested antenna array, with dimensions of 30 & times; 30 & times; 0.8 mm(3), is optimized on a Rogers RT/Duroid 5880 substrate to provide an efficient and broadband response. Each element in the array is designed to resonate precisely at 28 GHz, achieving a remarkable return loss of -30 dB, indicating exceptional impedance matching and minimum reflection losses. The antenna has a broad impedance bandwidth of 2.2 GHz (about 7.85%), guaranteeing dependable performance throughout the designated 5G mmWave spectrum. The simulated and observed outcomes show a maximum gain of 13.50 dBi, guaranteeing robust link quality for high-data-rate transmission conditions. The antenna exhibits an isolation greater than 27 dB over the working band, underscoring its appropriateness for MIMO applications by efficiently mitigating mutual coupling effects. The key performance parameters, including the envelope correlation coefficient (ECC < 0.005) and diversity gain (nearing 10 dB), validate the antenna's efficacy for multi-antenna systems. This work introduces a novel MIMO antenna solution characterized by small dimensions, high gain, low mutual coupling, and extensive operating bandwidth, successfully fulfilling the rigorous requirements of next-generation 5G mmWave communication systems.
