Microwave Structures Research Articles

A Novel Method for Intrusive Stochastic Estimation of Geometric Tolerance Effects in Finite Element Electromagnetic Analysis

The robust design of microwave, millimeter-wave, or Terahertz components and structures incorporating manufacturing process tolerances would enhance the fabrication yield and thereby reduce the overall production cost. Based on the spectral stochastic finite element method (SSFEM) for material variations, a novel geometrical SSFEM (GSSFEM) is proposed to analyze dimensional uncertainty in 3-D microwave models. The Jacobian for Piola mapping is represented as a function of stochastic variables to capture mesh level uncertainties. Polynomial chaos expansion is used to approximate the electric field as a random process. The technique is validated by applying it to waveguide problems with uncertainty in geometric dimensions. GSSFEM results are compared with analytical formulations of the transmission coefficient for a physical insight and further with the Monte Carlo simulations for quantitative evaluation. The results using the proposed approach are in excellent agreement with conventional methods and are found to be faster than sparse grid stochastic collocation, while the scaling of the computational requirements with the number of degrees of freedom and stochastic dimension is as good as this efficient scheme. Furthermore, this approach has been employed to analyze uncertainty in multiple geometric parameters to compare their sensitivity. The proposed GSSFEM can be employed for analyzing the impact of fabrication tolerance of passive waveguide components at microwave frequencies and beyond.

ANNs for Fast Parameterized EM Modeling: The State of the Art in Machine Learning for Design Automation of Passive Microwave Structures

Artificial neural networks (ANNs) are information processing systems, with their design inspired by studies of the ability of the human brain to learn from observations and generalize by abstraction. Researchers have investigated a variety of important applications utilizing the ability of ANNs to perform the modeling and optimization of microwave components and circuits, such as high-speed very large-scale integration (VLSI) interconnects [1]-[3], spiral inductors [4], microwave field-effect transistors (FETs) [5], [6], heterojunction bipolar transistors [7], [8], high-electron mobility transistors [9], [10], filters [11]-[14], power amplifiers [15]-[17], oscillators [18], transmitters [19], receivers [20], digital predistortion [21], microelectromechanical systems [22], wireless power transfer [23], and multiphysics design [24], [25].

Magnetic dipolar modes in magnon-polariton condensates

For dipole-carrying excitations observed in a high-quality resonator, strong-coupling modes can appear as composite bosons with the spontaneous formation of quantized vortices in the condensed phase of a polariton fluid. In this paper, we show that magnon-polaritons can be realized due to magnon condensation caused by magnetic dipole-dipole interaction. In a quasi-2D ferrite disk placed in a microwave cavity, one observes quantum confinement effects of magnetic-dipolar-mode (MDM) oscillations. These modes, characterized by energy eigenstates with rotational superflows and quantized vortices, are exhibited as spinor condensates. Along with the condensation of MDM magnons, electric dipole condensation is also observed. At the MDM resonances, transfer between angular momenta in the magnetic insulator and in the vacuum cavity, demonstrates generation of vortex flows with fixed handedness. The unique topological properties of the polariton are manifested by curved wavefronts and supperradiance effects in microwave structures. In an environment of scattering states of microwave waveguide, electromagnetic (EM) waves can carry topological phases of MDM resonances.

Microwave Bragg structures in a half-open coaxial line

Bragg microwave structures in a coaxial line have found application as converting elements in devices for dielectric control of liquid and bulk medias. A feature of the measuring conversion of the complex dielectric constant in coaxial structures is the introduction of the substance under study into the inner space of the line. The latter circumstance imposes certain restrictions on the design of the external conductor for express control. In this regard, the consideration of half-open coaxial lines based on an external conductor in the form of longitudinal conducting rods is an urgent problem. The article proposes a method for calculating the wave impedance of a half-open coaxial line using a numerical calculation of electromagnetic fields. Based on the results of modeling Bragg microwave structures in a half-open coaxial line, recommendations are made for the construction of conversion elements based on them.

A Model-Order Reduction Approach for Electromagnetic Problems With Nonaffine Frequency Dependence

The aim of this paper is to present a novel model-order reduction (MOR) technique for the efficient frequency-domain finite-element method (FEM) simulation of microwave components. It is based on the standard reduced-basis method, but the subsequent expansion frequency points are selected following the so-called sparsified greedy strategy. This feature makes it especially useful to perform a fast-frequency sweep of problems that lead to systems of equations exhibiting a nonaffine frequency dependence. This property appears, for example, when the excitation of the problem is characterized by a frequency-dependent waveguide mode pattern, or when the computational domain includes materials with frequency-dependent permittivity or permeability tensors. Moreover, the new MOR scheme can be also used to accelerate the frequency sweep of problems with many excitations, for which the standard reduction algorithms tend to be time-consuming. Its effectiveness and accuracy is verified through analysis of three microwave structures: planar microstrip branch-line coupler, three-port waveguide junction with ferrite post, and an eighth-order dual-mode waveguide filter.

Sommerfeld Integrals and Their Relation to the Development of Planar Microwave Devices

This paper deals with the mathematical expressions called Sommerfeld integrals. Introduced by A. Sommerfeld in 1909, they are mathematically equivalent to inverse Hankel transforms. The original historical goal of these integrals was to provide an accurate mathematical description of the electromagnetic phenomena involved in long-distance wireless radio and telegraphy. However, their scope was quickly enlarged thanks to the so-called spectral-domain stratified theory, and now they are ubiquitous in the mathematical models associated to many electromagnetic technologies, ranging from EMC lightning modelization and ground penetrating radar to optical and plasmonic integrated devices and going through the familiar microwave and millimeter-wave planar structures using printed circuit technology. In all these areas, Sommerfeld integrals can provide direct evaluations of the involved electromagnetic fields or they can be used as Green's functions in the frame of integral equation formulations. Other disciplines involving stratified media, like seismology and geological prospection, also benefit from these integrals. After discussing the most canonical Sommerfeld integral, appearing in the so-called Sommerfeld identity, this paper reviews three classical structures, namely, the original Sommerfeld problem involving two semi-infinite media, and the microstrip and stripline geometries. It is shown that Sommerfeld integrals provide a unifying treatment of these three problems and that their mathematical features have a direct translation in terms of the physical properties exhibited by the electromagnetic fields that can exist in them.

Improved Modeling of Microwave Structures Using Performance-Driven Fully-Connected Regression Surrogate

Fast replacement models (or surrogates) have been widely applied in the recent years to accelerate simulation-driven design procedures in microwave engineering. The fundamental reason is a considerable—and often prohibitive—CPU cost of massive full-wave electromagnetic (EM) analyses related to solving common tasks such as parametric optimization or uncertainty quantification. The most popular class of surrogates are data-driven models, which are fast to evaluate, versatile, and easy to handle. Notwithstanding, the curse of dimensionality as well as the utility demands (e.g., so that the model covers sufficiently broad ranges of the system operating conditions), limit the applicability of conventional methods. A performance-driven modeling paradigm allows for mitigating these issue by focusing the surrogate setup process in a constrained domain encapsulating designs being of high quality w.r.t. the assumed figures of interest. The nested kriging framework capitalizing on this idea, renders the constrained surrogate using kriging interpolation, and has been shown to surpass traditional approaches. In pursuit of further accuracy improvements, this work incorporates the performance-driven concept into the fully-connected regression model (FRCM). The latter has been recently introduced in the context of frequency selective surfaces, and combined deep neural networks with Bayesian optimization, the latter employed to determine the network architecture and hyper-parameters. Using two examples of miniaturized microstrip couplers, our methodology is demonstrated to outperform both conventional modeling techniques and nested kriging, with reliable models constructed over multi-dimensional parameters spaces using just a few hundreds of samples.

Microwave Spectroscopy Based Classification of Rat Hepatic Tissues: On the Significance of Dataset

With the advancements in machine learning (ML) algorithms, microwave dielectric spectroscopy emerged as a potential new technology for biological tissue and material categorization. Recent studies reported the successful utilization of dielectric properties and Cole-Cole parameters. However, the role of the dataset was not investigated. Particularly, both dielectric properties and Cole-Cole parameters are derived from the S parameter response. This work investigates the possibility of using S parameters as a dataset to categorize the rat hepatic tissues into cirrhosis, malignant, and healthy categories. Using S parameters can potentially remove the need to derive the dielectric properties and enable the utilization of microwave structures such as narrow or wideband antennas or resonators. To this end, in vivo dielectric properties and S parameters collected from hepatic tissues were classified using logistic regression (LR) and adaptive boosting (AdaBoost) algorithms. Cole-Cole parameters and a reproduced dielectric property data set were also investigated. Data preprocessing is performed by using standardization and principal component analysis (PCA). Using the AdaBoost algorithm over 93% and 88% accuracy is obtained for dielectric properties and S parameters, respectively. These results indicate that the classification can be performed with a 5% accuracy decrease indicating that S parameters can be an alternative dataset for tissue classification.

Dielectric Spectroscopy of Fluids and Polymers for Microwave Microfluidic Circuits and Antennas

The permittivity response from 0.5 to 40 GHz of ethyl acetate (C4H8O2), ethylene glycol (CH2OH)2, castor oil (C57H104O9), liquid paraffin, and other fluids commonly used for microwave microfluidic antennas and circuits is reported. The response of selected polymer solids, such as polydimethylsiloxane (PDMS), acrylonitrile butadiene styrene (ABS), acrylic, nylon, and others, used in microfluidics and 3-D printing of microwave structures was also studied from 0.5 to 20 GHz. Permittivity measurements were taken with a coaxial probe. Using a nonlinear least-squares fitting routine, the Cole–Cole dielectric relaxation model parameters are found for each material under test, thereby providing an analytic expression for their permittivity response.

Synthesis of open structures starting from closed‐cross‐section waveguide devices

This study presents a new methodology, based on the field modal expansion theory, to derive and analyse millimetre wave (mm-wave) ‘open’ cross-sections structures starting from standard closed cross-section design. The method is of great interest since W-band open devices offer many advantages over traditional closed structures in terms of risks for manufacturing, tuning and power handling in critical high-power devices. A hybrid scalar product, or ‘overlap’ integral, between the ‘open’ and ‘closed’ orthogonal basis of the open structure and the reference closed one has been defined, so introducing a figure of merit to evaluate if the derived open structure can have the same performances of the closed one in terms of fields confinement and distribution. As an example of the methodology, an ‘open’ version of a Gaussian horn and of a rectangular-to-circular WR10 waveguide transition have been successfully derived starting from classic ‘closed’ design. In order to fully test the method, the mm-wave rectangular-to-circular open-waveguide transition has been manufactured and measured (thanks to tapered close-to-open transitions and standard WR10 interfaces). The measurements well agree with full-wave numerical simulations and provide experimental validation of the proposed approach and fabrication technology. The method can be applied to other open microwave structures as well.

InSb pixel loaded microwave resonator for high-speed mid-wave infrared detection

High-speed mid-wave infrared (MWIR) detectors are of significant interest for a wide range of applications in communication and spectroscopy. High-speed photodetection is conventionally achieved using reverse biased semiconductor p-i-n junction devices. However, implementing such structures in the mid-infrared (mid-IR) is challenging due to both the limited available material platforms and challenges associated with device design. Here we demonstrate a high-speed MWIR detector architecture that uses a microwave resonator circuit loaded with a photoconductive indium antimonide (InSb) pixel, originally grown highly lattice-mismatched on a GaAs substrate. Time domain measurements demonstrate sub-nanosecond detector response at temperatures from 77 K to room temperature. Frequency response measurements demonstrate that optical signals modulated at frequencies as high as 2 GHz can be well resolved up to room temperature. Time domain circuit simulations support the experimentally measured short minority carrier lifetimes in the InSb pixel, while also suggesting a circuit time constant of ~200 ps, which is ultimately the limiting factor for the bandwidth of the presented detector architecture. Our results provide an alternative approach for the development of contact-free, high-speed infrared detectors capable of directly interfacing with microwave components and structures for a range of RF/mid-IR applications.

Thermal Frequency Drift of 3D Printed Microwave Components

Fabrication of microwave slot array antennas and waveguide bandpass and notch filters using 3D printing has significant advantages in terms of speed and cost even for parts with high mechanical complexity. One disadvantage of Stereolithography (SLA) 3D printed, copper plated microwave components is that some SLA resins have a high Coefficient of Thermal Expansion (CTE), quoted in micrometers per meter per degree or 10−6 per degree. Compared to typically used metals such as aluminum (CTE 24 × 10−6·K−1) and copper (CTE 17 × 10−6·K−1), SLA resin can have CTE above 100 × 10−6·K−1. Resonant structures experience significant frequency drift with temperature changes on the order of 10–50 °C. The issue of 3D printed microwave structures changing frequency characteristics significantly with temperature shift has not been addressed or reviewed in current literature. We measured and simulated the effect of temperature change on a slot array, cavity notch filters, and post loaded waveguide bandpass filters. We tested several types of SLA resin, different plating techniques, and also Direct Metal Laser Sintering (DMLS) and Binder Infusion metal 3D printing. Performance as a function of temperature is presented for these alternatives.

High coherence superconducting microwave cavities with indium bump bonding

Low-loss cavities are important in building high-coherence superconducting quantum computers. Generating high-quality joints between parts is crucial for the realization of a scalable quantum computer using the circuit quantum electrodynamics (cQED) framework. In this paper, we adapt the technique of indium bump bonding to cQED to realize high-quality superconducting microwave joints between chips. We use this technique to fabricate compact superconducting cavities in the multilayer microwave integrated quantum circuit (MMIQC) architecture and achieve single photon quality factors over 300 × 106 or single-photon lifetimes approaching 5 ms. To quantify the performance of the resulting seam, we fabricate microwave stripline resonators in multiple sections connected by different numbers of bonds, resulting in a wide range of seam admittances. The measured quality factors combined with the designed seam admittances allow us to bound the conductance of the seam at gseam≥2×1010/(Ω m). Such a conductance should enable the construction of micromachined superconducting cavities with a quality factor of at least a billion. These results demonstrate the capability to construct very high-quality microwave structures within the MMIQC architecture.

Means for monitoring the dielectric parameters of liquid media based on quasiperiodic Bragg microwave structures in a coaxial waveguide

The main characteristic of the converter element in resonant microwave sensors that determines the sensitivity of the conversion is the quality factor of the resonance line. In structures with periodic inhomogeneity of the classical topology, the quality factor of the circuit at high values of the signal-to-noise ratio may turn out to be low, which entails the emergence of restrictions on the maximum achievable conversion parameters. One of the ways to increase both the Q factor and the modification of other properties of the microwave structure that affect the overall efficiency of the converter element is the use of quasiperiodic elements. The coaxial cable, where the holes in the dielectric act as inhomogeneity, is presented, forming structure with phase shift. Using the method of flow graphs, we obtained the analytical dependence of the reflection coefficient for this structure. The analysis of the frequency response of the microwave reflection coefficient was carried out for different values of the phase shift and the location of the phase shift section along the length of the cable. A numerical and physical experiments were carried out to determine the frequency dependence of the microwave reflection coefficient with a π-phase shift for various values of the automobile fuel permittivity introduced into the cells.

Microwave spectra and structures of 2-fluoro-4-picoline

Rotational spectrum of the ground state of 2-fluro-4-picoline has been investigated by pulsed jet Fourier transform microwave spectroscopy. Besides the parent species, all the mono-substituted 13C and 15N isotopologues in natural abundance have been also measured and assigned. The rotational spectrum displays the features of the quadrupole coupling effect of the 14N nucleus with a near-free internal rotation of CH3 top. The V3 barrier (191 (60) J/mol) to the internal rotation of CH3 has been estimated from the unexpected low values of the inertial defects.

3D and 4D printing for optics and metaphotonics

Abstract Three-dimensional (3D) printing is a new paradigm in customized manufacturing and allows the fabrication of complex optical components and metaphotonic structures that are difficult to realize via traditional methods. Conventional lithography techniques are usually limited to planar patterning, but 3D printing can allow the fabrication and integration of complex shapes or multiple parts along the out-of-plane direction. Additionally, 3D printing can allow printing on curved surfaces. Four-dimensional (4D) printing adds active, responsive functions to 3D-printed structures and provides new avenues for active, reconfigurable optical and microwave structures. This review introduces recent developments in 3D and 4D printing, with emphasis on topics that are interesting for the nanophotonics and metaphotonics communities. In this article, we have first discussed functional materials for 3D and 4D printing. Then, we have presented the various designs and applications of 3D and 4D printing in the optical, terahertz, and microwave domains. 3D printing can be ideal for customized, nonconventional optical components and complex metaphotonic structures. Furthermore, with various printable smart materials, 4D printing might provide a unique platform for active and reconfigurable structures. Therefore, 3D and 4D printing can introduce unprecedented opportunities in optics and metaphotonics and may have applications in freeform optics, integrated optical and optoelectronic devices, displays, optical sensors, antennas, active and tunable photonic devices, and biomedicine. Abundant new opportunities exist for exploration.

Bilinear Z‐transform perfectly matched layer for rotational symmetric microwave structures with magnetised ferrite

In order to efficiently simulate the symmetrical structures in the cylindrical coordinate system, the implementation of the complex frequency-shifted perfectly matched layer based on the body of revolution finite-difference time-domain algorithm is proposed for analysing the magnetised ferrite microwave structures. To improve the computational efficiency in the magnetised ferrite domain compared with the existing calculation methods, the bilinear Z-transform for the simulation of the dispersion relationship of the magnetised ferrite is proposed. The proposed implementations can not only improve the computational efficiency but also obtain a considerable absorbing performance. Numerical examples based on the magnetised ferrite microwave structures are carried out to demonstrate the effectiveness and efficiency of the proposed formulations.

Rapid Optimization of Compact Microwave Passives Using Kriging Surrogates and Iterative Correction

Design of contemporary microwave components is—in a large part—based on full-wave electromagnetic (EM) simulation tools. The primary reasons for this include reliability and versatility of EM analysis. In fact, for many microwave structures, notably compact components, EM-driven parameter tuning is virtually imperative because traditional models (analytical or network equivalents) are unable to account for the cross-coupling effects, strongly present in miniaturized layouts. At the same time, the cost of simulation-based design procedures may be significant due to a typically large number of evaluations of the circuit at hand involved. In this paper, a novel approach to expedited design closure of compact microwave passives is presented. The proposed procedure incorporates available designs (e.g., existing from the previous design work on the same structure) in the form of the kriging interpolation models, utilized to yield a reasonable initial design and to accelerate its further refinement. An important component of the framework is an iterative correction procedure that feeds the accumulated discrepancies between the target and the actual design objective values back to the kriging surrogate to produce improved predictions. The efficacy of our methodology is demonstrated using two miniaturized impedance matching transformers with the optimized designs obtained at the cost of a few EM simulations of the respective circuits. The relevance of the iterative correction is corroborated through the comparative studies showing its superiority over rudimentary gradient-based refinement.

Energy Tunneling: A Way to Achieve Highly Sensitive Material Detection With Sub-Wavelength Resolution

The phenomenon of microwave energy tunneling (ET) has been reported in guided microwave structures loaded with either epsilon-near-zero (ENZ) materials or resonant metallic wires [1]-[14]. A bit of confusion sometimes can arise over the term tunneling, which is often used by physicists, so it seems appropriate here to point out the difference between ET and the well-known particle tunneling encountered in quantum mechanics (Figure 1).

Fast multi-objective design optimization of microwave and antenna structures using data-driven surrogates and domain segmentation

Purpose The purpose of this paper is to investigate the strategies and algorithms for expedited design optimization of microwave and antenna structures in multi-objective setup. Design/methodology/approach Formulation of the multi-objective design problem-oriented toward execution of the population-based metaheuristic algorithm within the segmented search space is investigated. Described algorithmic framework exploits variable fidelity modeling, physics- and approximation-based representation of the structure and model correction techniques. The considered approach is suitable for handling various problems pertinent to the design of microwave and antenna structures. Numerical case studies are provided demonstrating the feasibility of the segmentation-based framework for the design of real-world structures in setups with two and three objectives. Findings Formulation of appropriate design problem enables identification of the search space region containing Pareto front, which can be further divided into a set of compartments characterized by small combined volume. Approximation model of each segment can be constructed using a small number of training samples and then optimized, at a negligible computational cost, using population-based metaheuristics. Introduction of segmentation mechanism to multi-objective design framework is important to facilitate low-cost optimization of many-parameter structures represented by numerically expensive computational models. Further reduction of the design cost can be achieved by enforcing equal-volumes of the search space segments. Research limitations/implications The study summarizes recent advances in low-cost multi-objective design of microwave and antenna structures. The investigated techniques exceed capabilities of conventional design approaches involving direct evaluation of physics-based models for determination of trade-offs between the design objectives, particularly in terms of reliability and reduction of the computational cost. Studies on the scalability of segmentation mechanism indicate that computational benefits of the approach decrease with the number of search space segments. Originality/value The proposed design framework proved useful for the rapid multi-objective design of microwave and antenna structures characterized by complex and multi-parameter topologies, which is extremely challenging when using conventional methods driven by population-based metaheuristics algorithms. To the authors knowledge, this is the first work that summarizes segmentation-based approaches to multi-objective optimization of microwave and antenna components.