Microwave Band Research Articles

Crystal structure and dielectric properties of novel temperature-stable SrLn2O4 (Ln=Ho, Tm) microwave dielectric ceramic

In this paper, a comprehensive investigation was conducted to the factors affecting the crystal structure and microwave dielectric properties of the SrLn2O4 (Ln= Ho, Tm) ceramics. Rietveld refinement results of the XRD patterns conclude that SrLn2O4 (Ln= Ho, Tm) ceramics belong to orthorhombic system with Pnam symmetry. The effects of porosity, packing fraction and lattice energy on the properties of SrLn2O4 (Ln= Ho, Tm) ceramics were precisely analyzed. Excellent properties for SrTm2O4 ceramics (εr = 14.49, Q×f = 41138GHz, τf = -36.69 ppm/℃) and SrHo2O4 ceramics (εr = 14.87, Q×f = 24021GHz, τf = -17.03 ppm/℃) can be achieved due to good densities and high packing fraction and lattice energy. In addition, CaTiO3 is proved to be well compatible with SrLn2O4 (Ln= Ho, Tm) ceramics. Near-zero τf were obtained in SrTm2O4 + 1.8wt% CaTiO3 ceramic and SrHo2O4 + 1.2wt% CaTiO3 ceramic, making them ideal candidates for applications in microwave band.

Magnetic properties and dual band microwave absorption in conjunction with red shifted resonance of γ-Fe2O3 linked with 2-BOP dispersed on graphene oxide

Magnetic properties and dual band microwave absorption in conjunction with red shifted resonance of γ-Fe2O3 linked with 2-BOP dispersed on graphene oxide

Measuring Precipitation via Microwave Bands with a High-Accuracy Setup.

The urgent need for timely and accurate precipitation estimations in the face of ongoing climate change and the increasing frequency and/or intensity of extreme weather events underscores the necessity for innovative approaches. Recently, several studies have focused on estimating the precipitation rate through induced attenuation of radio frequency (RF) signals, which are abundant in modern communication systems. Most research has concentrated on frequencies exceeding 10 GHz, as attenuation at lower frequencies is minimal, posing measurement challenges. This study aims to confront this limitation by introducing a high-precision experimental setup capable of detecting this subtle attenuation at frequencies under 10 GHz. The setup includes a transmitter and receiver optimized for operation at 2.07, 4.63, and 6.22 GHz, where minimal worldwide research exists. A power resolution below 10-5 dB in preliminary measurements demonstrated its effectiveness in quantifying signal attenuation due to precipitation across the specified frequencies. Moreover, a strong power law relationship was observed between signal attenuation and precipitation rate for all three frequencies, while, as expected, the higher the frequency, the more pronounced the signal attenuation was.

All‐Sky Microwave Radiance Observation Operator Based on Deep Learning With Physical Constraints

AbstractSatellite data assimilation relies on the radiative transfer models (RTMs) to establish the relationships between model state variables and satellite radiances. However, atmospheric radiative transfer calculations are computationally expensive, especially when involving multiple‐scattering calculations in cloudy areas. In recent years, deep learning (DL) models have been increasingly applied to emulate and accelerate physical models. This study, for the first time, explores DL techniques to emulate all‐sky radiative transfer in microwave bands. The FengYun‐3E (FY‐3E) Microwave Humidity Sounder‐2 (MWHS‐2) was selected as the target instrument due to its comprehensive spectral coverage, with the radiative transfer for TOVS scattering module (RTTOV‐SCATT) serving as the reference model. Three DL architectures were trained and compared, including multilayer perceptron (MLP), Bidirectional Long Short‐Term Memory with Attention (BiLSTM‐Attention), and Transformer. The BiLSTM‐Attention architecture demonstrated superior performance in both clear‐sky and cloudy radiance simulations. This may be attributed to its bidirectional recurrent structure resembling physical radiative transfer processes and the attention mechanism's ability to link MWHS‐2 channels with corresponding vertical layers. Although DL models achieve high accuracy in forward prediction, they often struggle with instability in Jacobian calculations. To address this issue, the trained BiLSTM‐Attention model was fine‐tuned using the reference model Jacobians as physical constraints. The fine‐tuned BiLSTM‐Attention model accurately characterized radiance sensitivities to temperature, water vapor, and hydrometeors under different cloud conditions, indicating its potential to serve as a radiance observation operator in data assimilation and physical retrieval applications.

Observations of the 3.4 mm line of HCN in C/2020 T2 (Palomar)

Abstract The observations of comet C/2020 T2 (Palomar) were carried out using the 3.4 mm wavelength microwave band before perihelion from January 22 to July 5, 2021 and after perihelion on July 13, 2021. During this period, the comet was located at a heliocentric distance of between rh=2.840 AU and 2.055 AU. The consecutive long-term monitoring of outgassing of C/2020 T2 was conducted with Purple Mountain Observatory (PMO) 13.7-m telescope and the Institut de RadioAstronomie Millimétrique (IRAM) 30-m telescope in the atmospheric radio window. The hyperfine triplet components of hydrogen cyanide (HCN (1-0) F=0-1, F=2-1 and F=1-1) of the J=1-0 vibrational ground-state transitions, as the primary tracer of molecular gas, were unambiguously identified in this comet C/2020 T2. Combining all data, we derived the positive signal of line width corresponds to the coma expansion velocity vexp from ∼0.2 km s−1 to ∼0.4 km s−1. The mean gas molecular production rates of HCN were derived QHCN = (2.92 ± 0.51) × 1025 molecules s−1 at PMO 13.7-m, and QHCN = (6.26 ± 1.55) × 1025 molecules s−1 at IRAM 30-m, respectively. Overall, studying the gas composition of the Long-Period Comet revealed abundant information about the missing link between interstellar molecular clouds and the outer regions of a distant protoplanetary accretion disk, and the relationship between amorphous ice sublimation mechanism and heliocentric distance.

Measuring high-efficiency perfect composite vortex beams with reflective metasurfaces in microwave band.

Optical vortex beams carrying orbit angular momentum have attracted significant attention recently. Perfect vortex beams, characterized by their topological charge-independent intensity profile, have important applications in enhancing communication capacity and optimizing particle manipulation. In this paper, metal-insulator-metal copper-coin type reflective metasurfaces are proposed to generate perfect composite vortex beams in X-band. We introduce the qualified equivalent circuit model based on the theory of transmission line to design the meta-atom of the structure. The experiments are performed to measure the far-field and near-field perfect composite vortex beams and evaluate their orbital angular momentum purity at different frequencies. The experimental results agree well with the theoretical predictions. This work provides new ideas and methods for generating high-quality metasurface-based perfect composite vortex beams in the microwave region, paving an ideal path for microwave communication systems, optical manipulation and radar detection.

Recovering NDVI over lake surfaces: Initial insights from CYGNSS data enhanced by ERA-5 inputs

The escalating water pollution in many lakes has led to more frequent occurrences of algal bloom disasters in recent decades. The severity of these disasters can be assessed through remote sensing techniques, specifically using the Normalized Difference Vegetation Index (NDVI) for measurement. However, NDVI observations using optical sensors are often affected by cloud and fog in areas with numerous water bodies, such as Taihu Lake. Sensors operating in the microwave band can effectively mitigate this issue, particularly the emerging Global Navigation Satellite System Reflectometry (GNSS-R), which offers high temporal resolution and cost-effectiveness. In this paper, we propose a new method to recover lake-surface NDVI on cloudy days, utilizing GNSS-R observables and auxiliary meteorological data in conjunction with a machine learning regression algorithm called Bagging Tree. We also examine the effective range of GNSS-R data within this application scenario. Meanwhile, the Weighted Linear Regression-Laplacian Prior Regulation Method (WLR-LPRM) image gap-filling algorithm is used as a benchmark to evaluate recovery accuracy. The regression coefficient of NDVI retrieved using the proposed method is 0.95, with a root mean square error (RMSE) of 0.021 and a mean absolute error (MAE) of 0.010. Compared to the previous work on GNSS-R algal bloom detection with overall accuracy of 0.82, this work shows significant improvement in both accuracy and utility. The recovery of lake surface NDVI provides detailed insights into algal blooms, including quantifiable metrics such as the amount and spatial distribution, which are crucial for effective monitoring and management. Additionally, the recovered image textures exhibit high clarity and closely resemble the reference NDVI images. Experimental evaluation using simulated and actual cloud blocks indicates the model’s robustness to recover NDVI under varying cloud cover conditions. In summary, this study demonstrates the capability of GNSS-R aided by supplementary data for recovering missing NDVI values on lake surfaces when optical observations are absent for the first time.

Carbon based Composite Materials for Microwave Absorption in Low Frequency S and C Band: A Review

AbstractWith the rapid advancement of 5G technologies and electronic devices, there is a growing demand for microwave absorbing materials, especially those effective in low frequency microwave bands (2–8 GHz), making it an essential requirement. In recent years, many innovative microwave absorbers have been developed for low frequency absorption, with carbon/magnetic composite materials becoming particularly promising due to their various loss mechanisms and optimized impedance matching characteristics over a wide frequency range. However, there is currently a lack of a comprehensive review that summarizes the findings on low frequency absorbers. This review article thoroughly examines the current research on carbon/magnetic composite materials with efficient absorption performance in the low‐frequency S and C bands. It provides a detailed discussion of the microwave absorption performance of these composite materials. Furthermore, the composite design strategies, synthesis techniques, microstructure performance relationship, and microwave attenuation mechanisms are summarized. Lastly, the challenges and future outlook for low frequency microwave absorbing materials are addressed. This review article aspires to provide new insights into designing and synthesizing composite materials to accomplish effective low‐frequency microwave absorption, thereby promoting practical applications.

Analysis of 3D Printed Dielectric Resonator Antenna Arrays for Millimeter-Wave 5G Applications

This paper explores the potential use of fused deposition modeling (FDM) technology for manufacturing microwave and millimeter-wave dielectric resonator antennas (DRAs) for 5G and beyond communication systems. DRAs operating at microwave and millimeter-wave (mmWave) frequency bands were simulated, fabricated, and analyzed in terms of manufacturing quality and radio frequency (RF) performance. Samples were manufactured using a 3D printer and PREPERM® ABS1000 filament, which offers a stable dielectric constant (εr = 10 ± 0.35) and low losses (tan δ = 0.003) over wide frequency and temperature ranges. Surface profile tests and microscope measurements revealed discrepancies in the dimensions in the xy-plane and along the z-axis, consistent with the observed shift in resonant frequency. Despite these variations, reasonably good agreement between RF-simulated and measured results was achieved, and the DRA array successfully covered the intended mmWave band. However, challenges in achieving high precision may restrict applications at higher mmWave bands. Nevertheless, compared with conventional methods, FDM techniques offer a highly accessible and flexible solution with a wide range of materials for home and micro-manufacturing of mmWave DRAs for modern 5G systems.

A Series of Ultra-low Permittivity ALaP4O12 (A = Li, Na, K) Metaphosphate Microwave Dielectric Ceramics for Ultra-wideband Dielectric Resonant Antenna Application.

The employment of ultra-low permittivity materials in the configuration of antennas has been demonstrated to augment the antenna bandwidth and diminish signal delay effectively. This study presents three ultra-low permittivity metaphosphate microwave dielectric ceramics (MWDCs). The ALaP4O12 (A = Li, Na, K) metaphosphate ceramics, which all belong to the monoclinic crystal system, exhibit extremely low permittivity (εr ≈ 5) and excellent quality factor (Q·f > 10,000 GHz) at a low sintering temperature (T < 950 °C). Terahertz time-domain spectroscopy indicates that the εr of ALaP4O12 at terahertz frequencies is comparable to that observed in the microwave band and its value remains stable over an extensive frequency range. Furthermore, the relationship between the crystal structure and the dielectric properties of ALaP4O12 has been analyzed through the lens of chemical bond theory. The highest Q·f value observed for LiLaP4O12 can be attributed to the high chemical bond strength and stability of its crystal structure. The lowest ionic polarizability per unit volume is exhibited by NaLaP4O12, which results in the lowest pore-corrected permittivity. The thermal expansion of the chemical bonds within KLaP4O12 is considerable, resulting in the highest coefficient of thermal expansion. Finally, the performance of a LiLaP4O12-based dielectric resonator antenna (DRA) excited by slot-coupled microstrip lines was designed and optimized by using different ceramic radius-to-height (RH) ratios. It was found that when the RH ratio of DRA reached 1.85, both the fundamental mode (HEM11δ) and the higher-order mode (HEM12δ) of DRA were simultaneously excited. The two modes overlap significantly, resulting in an ultra-wideband (UWB) of 46.8% (bandwidth = 7.93 GHz). Concurrently, the maximum radiation efficiency and gain of the DRA, obtained from the simulation, are 97.9% and 7.92 dBi, respectively. The findings of this study may inform the investigation of ultra-low permittivity phosphorus-based MWDCs and UWB DRAs.

Miniaturized tri-band integrated microwave and millimeter-wave MIMO antenna loaded with metamaterial for 5G IoT applications

This study presents a revolutionary tri-band combined microwave (MW) and millimeter wave (MMW) multiple-input-multiple-output (MIMO) antenna. The antenna is built on a Rogers RT-5880 substrate with dimensions of 32 × 32 × 1.6 mm³. This integration increased the isolation from 18 dB to 27 dB in the MW band and 28 dB–40 dB in the MMW band. The effective bandwidths for the MW and MMW bands were initially 3.3–3.7 GHz, 5.45–9.2 GHz, and 24–28 GHz, respectively. The MTM increased these bandwidths to 2.6–4.1 GHz, 5.3–11.3 GHz for the MW band, and 22.5–29.3 GHz for the MMW band. In addition, the realized gain increased significantly, from 1.5 dBi, 3.5 dBi, and 6.5 dBi to 4.5 dBi, 6.1 dBi, and 11 dBi at 3.5 GHz, 6.9 GHz, and 28 GHz. The antenna's overall efficiency increased by 10–12 % across all operational bandwidths. The suggested antenna has excellent diversity performance, with an envelope correlation coefficient of less than 0.0002, a diversity gain greater than 9.998, and a channel capacity loss of less than 0.12 bits/s/Hz. The proposed antenna addresses the increasing demand for IoT devices by efficiently supporting both MW and MMW frequency bands, offering high performance in wireless communication and modern infrastructures. The proposed design not only improves connectivity and efficiency in IoT devices but also supports the Sustainable Development Goals (SDGs) by contributing to the development of smart city infrastructure (SDG 11) and fostering innovation and sustainable industrial practices (SDG 9), thereby promoting sustainable development and industrial innovation.

A scalar method for measuring the reactive element in microwave and millimeter-wave bands

This paper introduces a novel scalar method to determine the value of reactive elements by only measuring the amplitude value of the reflection coefficient. The methodology is illustrated using a Smith Chart, integrating reactive elements in series or parallel with a standard resistance element, and deriving a corresponding equation. Simulation results closely align with theoretical predictions. This approach extends beyond reactive elements, applying universally to π/T-network circuits. It obviates the need for Vector Network Analyzers (VNA) in measurement, reduces equipment requirements, and holds substantial importance for measuring diverse types of reactive microwave elements.

Enhanced breast tumor localization with DRA antenna backscattering and GPR algorithm in microwave imaging

With the rapid rate of increasing breast cancer cases across the globe and restrictions of existing early stage detection techniques, microwave imaging based cancer and malignant cell diagnosing methodology is finding its way. Microwave imaging technology has higher accuracy of detection in the variation of tissue properties. In this article, a dielectric resonator based wide band antenna is designed and investigated for microwave imaging (MWI) of early breast cancer detection due to its improved accuracy. The designed antenna has compact size, broad frequency spectrum, high gain and broadside radiation characteristics. To attain the wider bandwidth from (6.5 GHz–12.5 GHz), two rectangular shaped dielectric slabs are used. Wide band microwave frequency band supports the high resolution images. A C-shaped defected ground structure is used to tune the impedance matching. Dielectric resonator based antenna has higher gain and reduces the squints in antenna impedance. Two-asymmetrical dielectric slabs contribute in the excitation of different resonating modes which in turn enhance the bandwidth. It also contributes to excite multiple resonance mode that in turn enhance the bandwidth. Antenna performance is first numerically and experimentally verified in free space. Then, simulation based performance is examined for the microwave imaging. Numerical phantom with equivalent tissue properties is designed with and without the tumor. Unhealthy cells have higher water percentage and larger dielectric constant as compared to healthy cells. It causes the different in the back scattered signal of antenna, which is analyzed to diagnose the tumor or cancer. Here, a maximum deviation of 16 dB is observed in the backscattered signal. Furthermore, surface of the breast tissue is scanned along the x-axis and y-axis at different angels. The collected signals are used to reconstruct the microwave image of the interior. Mean value data of backscattered signals is used to locate the existence of unhealthy cells and GPR algorithm is used to calculate the depth of a 4 mm tumor in breast tissue. Proposed structure has shown efficient performance for microwave imaging.

Comparison of ultra-wide band microwave system and ultrasound in live cattle to predict beef carcase subcutaneous fatness

Ultrasound and ultrawide band microwave system (MiS) were directly compared in their ability to scan live cattle to predict carcase traits. Commercial beef cattle (n = 315) were scanned on farm 0–14 days prior to slaughter. Traits measured were subcutaneous fatness at the P8 site (over the gluteus muscle on the rump, at the intersection of a line through the pin bone parallel to the chine and perpendicular through the 3rd sacral crest) and subcutaneous fatness at the rib fat site (between 12th & 13th rib, ¾ of the length ventrally over the longissimus muscle). The precision of prediction of carcase traits was slightly better using MiS. MiS prediction of P8 fat depth had an average RMSEP of 2.48 mm and R2 of 0.65. The MiS could predict carcase rib fat with an average RMSEP of 2.28 mm and R2 of 0.56. The accuracy of prediction was very similar between the two technologies. When predicting P8, the average bias was smallest using MiS at 0.157 mm, but the average slope was smallest using ultrasound at 0.03 mm. When predicting rib fat, MiS had the smallest average bias at 0.204 mm, and smallest average slope deviation at 0.06 mm. The MiS predicted P8 and rib fat carcase traits with similar precision and accuracy as ultrasound.

The 3Cat-4 Spacecraft Thermal Analysis and Thermal Vacuum Test Campaign Results

3Cat-4 is the fourth member of the CubeSat series of UPC’s NanoSat Lab, and it was selected by the ESA Academy’s Fly Your Satellite! program in 2017. This mission aims at demonstrating the capabilities of nano-satellites, and in particular those based in the 1-Unit CubeSat standard, for challenging Earth Observation (EO) using Global Navigation Satellite System-Reflectometry (GNSS-R) and L-band microwave radiometry, as well as for Automatic Identification Systems (AIS). The following study presents the results of the thermal analysis carried out for this mission, evaluating different scenarios, including the most critical cases at both high and low temperatures. The results consider different albedos and orbital parameters in order to establish the optimal temperatures to achieve the best mission performance within the nominal temperatures, and in all operational modes of the satellite. Simulation results are included considering the thermal performance of other materials, such as Kapton, as well as the redesign of the optical properties of the satellite’s solar panels. The correlation with the thermal model and the TVAC test campaign was conducted at the ESA ESEC-GALAXIA facilities in Belgium.

Fast inverse design of microwave and infrared Bi-stealth metamaterials based on equivalent circuit model

This work proposed a fast inverse design method for microwave and infrared (IR) bi-stealth metamaterials based on the equivalent circuit model (ECM). Using this method, we designed a microwave and IR bi-stealth metamaterial by deploying a multilayered structure of the indium tin oxide (ITO) film based metasurface. First, the IR emissivity of the ITO film was calculated in the framework of the ECM. Then, an ITO metasurface was proposed to implement low IR emission and high microwave transmission simultaneously. Based on the ECM of the square patch, the ECM of the whole metamaterial was established at the microwave band. An inverse design program was built by incorporating the ECM with genetic algorithm (GA). Structure parameters of the metamaterial were optimized by GA to achieve the broadest microwave stealth bandwidth for the given thickness. Finally, the sample of the optimized bi-stealth metamaterial was prepared and tested. The calculated, simulated, and measured results are in good agreement, showing that such a metamaterial has an IR emissivity of 0.18 in the band from 3 to 14 μm and an efficient microwave stealth band from 4.8 to 17 GHz with a thickness of 4.9 mm. The proposed method will benefit the design and application of microwave and IR bi-stealth metamaterials.

Multiband electromagnetically induced transparency-like on metamaterials

In recent years, the electromagnetically induced transparency-like (EIT-like) of metamaterials has been widely concerned due to obtain high Q, which has potential applications in the field of sensing and slow light. In this paper, an EIT-like metamaterial structure in the microwave band was investigated, which can realize to multi-band EIT-like phenomena. The equivalent circuit method and the electric field distribution are used to study the EIT -like effect’ mechanism, and the multi-band EIT-like phenomenon is discussed by analyzing its structural parameters. The results show that the multi-band EIT in the proposed structure is obtained by coupling multiple bright modes to multiple dark modes. The change of transmission spectrum is discussed by changing the environmental permittivity. The results have potential applications in areas such as refractive index sensing, filters.

Detection of malignant breast tissue using SAR observation with microwave imaging and convolutional neural network

The research paper introduces a deep learning approach for distinguishing between benign and malignant breast tissues using Ultra Wide Band (UWB) microwave imaging. A multi-resonant monopole microstrip patch antenna was crafted and placed on a female breast phantom model containing a tumor. The phantom’s dielectric properties were adjusted to mimic those of benign and malignant tissues. The Specific Absorption Rate (SAR) was analyzed by rotating the antenna at various angles throughout the phantom’s trajectory. Images capturing the SAR distribution over the breast phantom at different antenna resonances were obtained. The popular Convolutional Neural Network (CNN) based AlexNet model was employed for autonomous feature learning from SAR images. To address the challenge of overfitting with a limited dataset, the study utilized image augmentation and transfer learning methods. The proposed model achieved an impressive 98.59% accuracy in classifying benign and malignant breast phantom images, surpassing the performance of three other CNN models—VggNet16, GoogleNet, and ResNet. Notably, this microwave imaging system based on SAR images detected tumors as small as 1 mm with an accuracy of 97.08% outperforming existing malignancy screening techniques.

Microwave Band Polarization-Decoupled Multi-Structured Waves Generation Metasurface

Microwave Band Polarization-Decoupled Multi-Structured Waves Generation Metasurface

Stabilized conformal planar cavity with continuous adjustable resonant frequency

A conformal planar cavity with filling dielectrics is conformally transformed from a concave cavity by transformation optics. The proposed conformal planar cavity offers remarkable advantages, including high stability, high Q factor, tunable resonant frequency, and small mode volume. In contrast to concave cavities, it offers a significant advantage by being integrable onto a chip. Additionally, when compared to planar cavities based on metasurfaces, which lack the capability for continuous resonant frequency adjustment, the proposed conformal cavity offers a tunable resonant frequency that can be continuously adjusted by simply changing the cavity length without the need for redesigning the filling dielectrics. Only dielectrics with permittivity ranging from 1 to 1.7 are required inside the conformal cavity, which can be easily obtained at various frequency bands. The tunability and stability of the conformal planar cavity is numerically verified, and experimental demonstrations at microwave band are performed.