Showing papers on "Equivalent circuit published in 2020"

Impedance Modeling and Stability Analysis of Grid-Connected DFIG-Based Wind Farm With a VSC-HVDC

Abstract: A new type of subsynchronous oscillation (SSO) has been observed recently in double-fed induction generator (DFIG)-based wind farm integrated via voltage source converter-based HVdc (VSC-HVdc) system. However, the mechanism of this emerging oscillation is not entirely understood. In this paper, the impedance models of DFIG with and without considering the phase-locked loop (PLL) dynamics are both derived. Then, the impedance-based simplified equivalent circuit of the multiple DFIGs interfaced with VSC-HVdc system is established. This model can be further represented as the RLC series resonance circuit to quantify the start-oscillating condition intuitively. The theoretical analysis results show that DFIGs behave as an inductance in series with a negative resistance at the resonance point, whose interaction with wind farm side VSC (WFVSC) (regard as a resistance–capacitance) constitutes an equivalent RLC resonance circuit with negative resistance. Therefore, the oscillation tends to occur due to the negative damping. In addition, the impact of various factors including number of grid-connected DFIG-wind turbines (WTs), wind speed, and parameters of PI controllers and PLL on the SSO characteristics is analyzed based on the proposed simplified model. Finally, the correctness of the theoretical analysis is validated by both the time-domain simulation and hardware-in-loop experiments.

Online Fault Diagnosis of External Short Circuit for Lithium-Ion Battery Pack

Abstract: Battery safety is one of the most crucial issues in the utilization of lithium-ion batteries (LiBs) for all-climate electric vehicles. Short circuit, overcharge, and overheat are three common field failures of LiBs. In this paper, online fault diagnosis for external short circuit (ESC) of LiB packs is investigated. The experiments are carried out to obtain and compare ESC characteristics of 18650-type NMC battery pack and single cell. Based on the analysis of experimental results, a two-step equivalent circuit model is established to describe the ESC process and an online model-based scheme is proposed to diagnose ESC faults of battery packs. The proposed scheme is evaluated by experimental data. The results show that it can effectively diagnose ESC faults in 3.5 s after their occurrences with the terminal voltage error less than 25 mV. The proposed scheme has shown great generalization ability. ESC faults of battery packs under different number of cells connected in series and unavailable current information can also be diagnosed at the terminal voltage error less than 48 and 60 mV, respectively.

A Novel Slot-Array Defected Ground Structure for Decoupling Microstrip Antenna Array

Abstract: In this article, a novel slot-array defected ground structure (DGS) for decoupling microstrip antenna array is proposed. The slot-array DGS is etched surrounding each antenna element on the ground plane and parallel to the radiating edges of each antenna element. The decoupling mechanism is elucidated via an equivalent circuit model and the coupled current field analysis, which reveals slot-array DGS has the spatial band-stop characteristic and changes the direction of the partially coupled current, respectively. Both characteristics of the slot-array DGS contribute to mutual coupling reduction. Three practical design examples of applying slot-array DGS to single-linearly polarized (LP), dual-LP, and compact circularly polarized (CP) antenna array are given to illustrate the design process and considerations. The simulated and measured results show that about 50 dB isolation enhancement is obtained by using the slot-array DGS when the edge-to-edge spacing between CP antenna elements is 0.057 wavelength. Additionally, a wheel-shaped absorber based on the electromagnetic loss material is designed and fabricated to reduce the backward radiation caused by slot-array DGS. The absorber has an absorptivity of more than 95% in the frequency range of 1.2–1.35 GHz and suppresses the backward radiation over 12.5 dB in the plane phi = 0° and 16.1 dB in the plane phi = 90° without deteriorating other antenna performances.

Pseudo-two-dimensional model and impedance diagnosis of micro internal short circuit in lithium-ion cells

Abstract: The early detection of micro internal short circuit (ISCr) cells can provide sufficient response time for preventing accidents such as spontaneous thermal runaway in battery packs of electric vehicles, and greatly improve safety. Because the existing models describing ISCr are mainly equivalent circuit models and three-dimensional physics-based models, we build a pseudo-two-dimensional model of micro ISCr cells to make up for the gaps. Using the calculation results of this model, we reveal the phenomenon of electric quantity depletion and the variation of internal electrochemical parameters when a micro ISCr occurs in the cell. We find the effective electrical conductivity of the separator is a crucial parameter describing the ISCr severity and determine reasonable values for this effective conductivity for fault diagnosis and battery design. Moreover, we propose an impedance-identification method that can be used for ISCr diagnostics. Through the simulation and experimental results, we find that the impedance of micro ISCr cells is different from that of normal cells and shows a certain regularity with the increase of ISCr severity.

Theoretical modeling of triboelectric nanogenerators (TENGs)

Abstract: Triboelectric nanogenerators (TENGs), using Maxwell's displacement current as the driving force, can effectively convert mechanical energy into electricity. In this work, an extensive review of theoretical models of TENGs is presented. Based on Maxwell's equations, a formal physical model is established referred to as the quasi-electrostatic model of a TENG. Since a TENG is electrically neutral at any time owing to the low operation frequency, it is conveniently regarded as a lumped circuit element. Then, using the lumped parameter equivalent circuit theory, the conventional capacitive model and Norton's equivalent circuit model are derived. Optimal conditions for power, voltage, and total energy conversion efficiency can be calculated. The presented TENG models provide an effective theoretical foundation for understanding and predicting the performance of TENGs for practical applications.

A novel fractional variable-order equivalent circuit model and parameter identification of electric vehicle Li-ion batteries

Abstract: Accurate Li-ion battery modeling is integral to the design of effective battery management systems in electric vehicles. However, the voltage-current (U-I) characteristic of Li-ion batteries presents strong nonlinearity. The application of fractional-order models to create lower-order models to represent physical systems (e.g., the battery characteristics for the state of charge estimation) is interesting and timely. In this paper, a novel fractional variable-order equivalent circuit model (FVO-ECM) is proposed to represent the nonlinear U-I characteristic of Li-ion batteries; its parameter identification is achieved and verified by charge and discharge tests. Compared with the integral-order equivalent circuit model and the fractional constant-order model, the proposed FVO-ECM can identify battery nonlinear characteristics most accurately.

An online SOC and capacity estimation method for aged lithium-ion battery pack considering cell inconsistency

Abstract: For lithium-ion battery packs, especially aged lithium-ion batteries, the inconsistencies in State-of-Charge (SOC), model parameter and capacity between cells cannot be ignored. In order to accurately estimate the SOC and capacity of each cell in the lithium-ion battery pack online, a "Special and Difference (S&D)" model, i.e. a serial-connected battery pack model, is established based on a second-order equivalent circuit model as cell model. The multi-time scale extended Kalman filter algorithm is proposed based on “S&D” model to estimate the SOC, model parameter and capacity of each cell in the battery pack. The proposed algorithm involves three time dimensions: a short time scale which contains special cell's SOC and model parameter estimation, a middle time scale which contains the remaining cells’ SOC and model parameter estimation, and a long time scale which contains all cells’ capacity estimation. The multi-time scale extended Kalman filter algorithm for aged battery pack is verified under two dynamic conditions. The results show that the SOC estimation error of each cell in the battery pack is within 5% in the whole testing period and it is within 3% when the later capacity estimation process keeps stable. In addition, the number of the cells with maximum and minimum capacity can be accurately identified after the middle stage of the capacity estimation process, which is significant for the consistency management of the battery pack.

Blessing and Curse: How a Supercapacitor’s Large Capacitance Causes its Slow Charging

Abstract: The development of novel electrolytes and electrodes for supercapacitors is hindered by a gap of several orders of magnitude between experimentally measured and theoretically predicted charging time scales. Here, we propose an electrode model, containing many parallel stacked electrodes, that explains the slow charging dynamics of supercapacitors. At low applied potentials, the charging behavior of this model is described well by an equivalent circuit model. Conversely, at high potentials, charging dynamics slow down and evolve on two relaxation time scales: a generalized RC time and a diffusion time, which, interestingly, become similar for porous electrodes. The charging behavior of the stack-electrode model presented here helps to understand the charging dynamics of porous electrodes and qualitatively agrees with experimental time scales measured with porous electrodes.

Structure of the Electrical Double Layer Revisited: Electrode Capacitance in Aqueous Solutions

Abstract: The structure of the electrical double layer at the interface of planar electrodes and aqueous solutions is investigated. Electrical impedance spectroscopy is used to measure the impedance of aqueous solutions of sodium chloride and two different surfactants over a wide range of concentrations. The electrode capacitance is directly inferred from the admittance spectra as well as by regression of the impedance spectra to an equivalent circuit. It is found that the electrode capacitance remains on the same order of magnitude over the entire range of investigated concentrations. This is contradictive to the predictions of the Gouy-Chapman-Stern theory which predicts that, at low concentrations, the electrode capacitance should be determined by the diffuse layer. It is concluded that the Stern layer capacitance always dominates the electrode capacitance, even at very low concentrations, and the establishment of a diffuse layer capacitance requires an ionic strength of around 1 mM.

On the Design of Ultrawideband Circuit Analog Absorber Based on Quasi-Single-Layer FSS

Abstract: In this letter, an ultrawideband and single-layer circuit analog absorber with a low profile and small unit size is proposed. The absorber is composed of a Rogers 4003 dielectric with two square loop arrays, respectively, printed on the top and bottom, and a metal ground below. Lumped resistors are embedded on the four edges of the square-loop arrays to introduce resistance loss. Equivalent circuit, input impedance, and current distribution are then presented to provide great insight into the existence of three resonances and wideband. The absorber offers a fractional absorption bandwidth of 132.3% (4.91:1) with at least 10 dB reflection reduction. The thickness and the unit size are, respectively, reduced to $0.075\lambda _{L}$ (wavelength at the lowest frequency) and $0.13\lambda _{L}$ , leading to a good performance under oblique incidence. A good agreement between the calculated, simulated, and measured results validates the proposed design.

Miniaturized-Element Frequency-Selective Rasorber Design Using Characteristic Modes Analysis

Abstract: A dual-polarization frequency-selective rasorber (FSR) with two absorptive bands at both sides of a passband is presented. Based on the characteristic mode analysis, a circuit analog absorber is designed using a lossy frequency-selective surface (FSS) that consists of miniaturized meander lines and lumped resistors. The positions and values of resistors are determined according to the analysis of modal significances and modal current. After that, the presented rasorber is designed by cascading of the lossy FSS and a lossless bandpass FSS. Equivalent circuits of the FSR are modeled, and surface current distributions of both FSSs are illustrated to explain the operation mechanism. Measurement results show that, under the normal incidence, a minimum insertion loss of 0.27 dB is achieved at a passband around 6 GHz, and the absorption bands with an absorption rate higher than 80% are 2.5–4.6 GHz in the lower band and 7.7–12 GHz in the higher band, respectively. Our results exhibit good agreements between measurements and simulations.

Estimating Power Transformer High Frequency Model Parameters Using Frequency Response Analysis

Abstract: Frequency response analysis (FRA) has become a widely accepted technique by worldwide utilities to detect winding and core deformations within power transformers. The main drawback of this technique is its reliance on the personnel level of expertise more than standard or automated codes. To establish reliable FRA interpretation codes, accurate high frequency transformer model that can emulate the frequency characteristics of real transformers in a wide frequency range is essential. The model can be used to investigate the impact of various winding and core deformations on the transformer FRA signature. The transformer equivalent high frequency electric circuit parameters can be calculated based on design data, which are rarely available, especially for old transformers. As such, this paper presents an artificial intelligence technique to estimate these parameters from the transformer FRA signature. The robustness of the proposed technique is assessed through its application on three, 3-phase power transformers of different ratings, sizes, and winding structures to estimate their high frequency electric circuit parameters during normal and fault conditions. Results show that the proposed technique can estimate equivalent circuit parameters with high accuracy and helps interpret the FRA signature based on the numerical changes of these parameters. The main advantage of this approach is the physical meaning of the model parameters facilitates reliable identification of various faults and hence aids in establishing reliable interpretation codes for transformer FRA signatures.

Estimating the State-of-Charge of Lithium-Ion Battery Using an H-Infinity Observer Based on Electrochemical Impedance Model

Abstract: The lithium-ion batteries in the electric vehicles are nonlinear systems with complex electrochemical dynamics, and estimation of battery state-of-charge (SOC) is affected by factors such as environmental temperature and battery current. Considering the above problems, the accurate estimation of battery SOC has always been a difficult and the critical issue of battery management system (BMS). In this paper, the constant phase element (CPE) is introduced to the traditional time domain circuit model by analyzing the electrochemical impedance spectra of lithium-ion batteries. Accordingly, an equivalent circuit model based on electrochemical impedance is constructed by using fractional order theory, which has specific physical significant, leading to the improved estimation accuracy to represent battery voltage. Moreover, the polarization resistance in the model is replaced by Butler-Volmer (BV) equation, which can solve the problem caused by large current and temperature variation during the actual operation of electric vehicles. Next, based on the model, an $\text{H}\infty $ observer is designed for battery SOC estimation, and the proposed SOC observer is tested by real-time experimental data of battery. The efficiency of the proposed model and observer are validated by some simulations and experiment tests.

A Self-Decoupled Antenna Array Using Inductive and Capacitive Couplings Cancellation

Abstract: The concept of a self-decoupled antenna array using the cancellation of two opposite couplings is proposed in this article. A pair of such antennas can be closely placed with inherent high isolation without using an extra decoupling structure between the antennas. A pertinent equivalent circuit model is presented to illustrate the physical mechanism of this new concept. It is found that the inductive and capacitive couplings between the antennas can be well canceled out with each other by properly adjusting the antenna dimensions. A demonstrating antenna array with a spacing of $0.024\lambda _{0}$ at the working frequency of 3.5 GHz and its counterpart array are first studied. The measured results show that although the proposed antenna array occupies a slightly larger size than its counterpart array, it presents better performance compared with its counterpart antenna array in port isolation (from 10 to 20 dB), total efficiency (from 68% to 80%), and envelope correlation coefficient (ECC) (from 0.14 to 0.04) throughout the desired frequency band of 3.3–3.8 GHz. A 3-D self-decoupled antenna array is designed to show that the proposed antenna can be in a compact form factor. Another self-decoupled array and its counterpart working at 2.14 GHz (long-term evolution (LTE) band 1) are studied through multi-input multi-output (MIMO) over-the-air (OTA) test when the arrays are integrated with an LTE module, showing significant improvement on the data throughput.

Hardware-in-the-loop Implementation of ANFIS based Adaptive SoC Estimation of Lithium-ion Battery for Hybrid Vehicle Applications

Abstract: This paper presents an effective method to estimate the state of charge (SoC) of a Lithium-ion battery. This parameter is very crucial as it indicates the performance and health of the battery. The battery SoC estimation equivalent circuit provided in MATLAB has been modified by adding the 3- RC pairs in series with its internal resistance. The values of the RC pairs have been calculated mathematically by solving the circuit model, based on charging and discharging dynamics of the battery. The values of these parameters have also been optimized using a “lsqnonlin” function. The SoC of the battery is estimated using the combination of coulomb counting and open-circuit voltage methods to minimize the error in estimation. The obtained SoC is further corrected for errors using ANFIS based algorithms. The effect of temperature has also been accounted for modelling the battery and in SoC estimation. These obtained SoCs for 3 cases, i.e. without RC/with RC pairs and then tuned with ANFIS based optimization are compared for the same load. The parameter calculation method adopted here results in an efficient and accurate model that keeps track of correct battery SoC. The complete system is validated in real-time using hardware-in-the-loop laboratory setup.

A Wideband Multifunctional Absorber/Reflector With Polarization-Insensitive Performance

Abstract: A wideband and polarization-insensitive absorber/reflector with multifunctional characteristics is proposed in this communication. The designed structure comprises a switchable layer, an air spacer, and a metal ground. The switchable layer contains active resonant elements, whose switching properties are realized by controlling p-i-n diodes. Bias networks for the p-i-n diodes are integrated into the structures and no dedicated bias lines are required. Thus, the negative effect of extra bias lines can be alleviated. In order to achieve switchable and polarization-insensitive properties simultaneously, a super-element configuration is developed by combining four similar unitary elements. One unique feature of the designed device lies in its real-time and multifunctional capability to switch among multiple working states. In addition, the switchable bandwidth of the proposed structure covers a much wider frequency band compared with previous switchable absorber/reflector designs. Distributions of electric field and surface current as well as an equivalent circuit model are presented to gain the necessary physical insight into the working mechanism of the switchable structure. Finally, a prototype of the proposed design is fabricated and measured for validation.

Design of Waveguide Filters With Cascaded Singlets Through a Synthesis-Based Approach

Abstract: In this article, we introduce a synthesis-based design approach for waveguide filters, including singlets. As is known, a singlet block comprises one resonator coupled to source and load plus an additional coupling between source and load. In this way, a pole–zero pair is produced, thus allowing the introduction of a transmission zero in the frequency response. Structure making that uses cascaded singlets (especially implemented in waveguide technology) has been successfully used to realize compact and quasi-in-line pseudoelliptic filters. The design of this configuration suffers, however, from a severe drawback; while an isolated singlet can be exactly synthesized, when these blocks are put in cascade, nonresonating nodes (NRNs) are required to allow the cascade connection. Consequently, the resulting topology includes both cross couplings and NRNs, for which no exact synthesis solution is till now available (all the design solutions proposed in the literature are based on optimization). The new design procedure here proposed overcomes this drawback, allowing the synthesis of the equivalent circuit of the cascaded-singlet topology. Once this circuit is obtained, the dimensioning of the singlets can be carried out by exploiting a full-wave simulator according to the procedure described in the article. The proposed design approach allows a very accurate initial dimensioning of the filter, which represents an excellent starting point for final adjustments/optimizations. A detailed design example is presented in the article showing how to practically implement the novel procedure. This new approach is also experimentally validated by a waveguide filter prototype, previously designed by optimization and fabricated. It is shown how this filter can be redesigned with the novel procedure, obtaining very similar physical dimensions in a noticeably shorter time.

Novel Low-RCS Circularly Polarized Antenna Arrays via Frequency-Selective Absorber

Abstract: New electromagnetic (EM) structures are demonstrated to realize low-radar-cross section (RCS) antennas by making effective use of frequency-selective absorber (FSA). According to the well-known reciprocity principle, the two-layered FSA can be considered as an actual receiving antenna and then transformed to a circularly polarized (CP) antenna. At the radiation state, a truncated patch resonator on the bottom layer is fed by a coaxial probe so as to produce CP wave, and it further excites the slots on the upper layer, resulting in its radiation toward free space, and when the antenna at the stealth state, the detective incident wave can be effectively absorbed outside the radiation band, thus achieving RCS reduction. The design strategies are then explained and verified with the aid of the corresponding equivalent circuit models. Two examples of $2 \times 2$ and $4 \times 4$ antenna arrays were designed to validate the flexibilities of the proposed design method. Finally, the $4 \times 4$ antenna array was fabricated and measured, and reasonable agreement is achieved.

Microwave-to-Optical Transduction Using a Mechanical Supermode for Coupling Piezoelectric and Optomechanical Resonators

Abstract: The successes of superconducting quantum circuits at local manipulation of quantum information and photonics technology at long-distance transmission of the same have spurred interest in the development of quantum transducers for efficient, low-noise, and bidirectional frequency conversion of photons between the microwave and optical domains. We propose to realize such functionality through the coupling of electrical, piezoelectric, and optomechanical resonators. The coupling of the mechanical subsystems enables formation of a resonant mechanical supermode that provides a mechanically-mediated, efficient single interface to both the microwave and optical domains. The conversion process is analyzed by applying an equivalent circuit model that relates device-level parameters to overall figures of merit for conversion efficiency η and added noise N. These can be further enhanced by proper impedance matching of the transducer to an input microwave transmission line. The performance of potential transducers is assessed through finite-element simulations, with a focus on geometries in GaAs, followed by considerations of the AlN, LiNbO3, and AlN-on-Si platforms. We present strategies for maximizing η and minimizing N, and find that simultaneously achieving η > 50 % and N < 0.5 should be possible with current technology. We find that the use of a mechanical supermode for mediating transduction is a key enabler for high-efficiency operation, particularly when paired with an appropriate microwave impedance matching network. Our comprehensive analysis of the full transduction chain enables us to outline a development path for the realization of high-performance quantum transducers that will constitute a valuable resource for quantum information science.

Novel Mesoscale Electrothermal Modeling for Lithium-Ion Batteries

Abstract: This paper devises an innovative mesoscale electrothermal model for Li-ion batteries. This model manipulates the mesoscale calculation grid in finite element analysis as independent small cell sandwiches and establishes a lumped equivalent circuit model for each cell sandwich. Then, such electrical models are arranged in parallel to form a multilayer equivalent circuit to simulate electrical characteristics of a whole battery, through capturing the current and terminal voltage of each constituent cell sandwich. This modeling idea overcomes the entrenched disadvantage of heat generation models with lumped parameters, i.e., the unavailability of heat generation distribution inside a battery. Besides the current and terminal voltage, the temperature and state of charge dependent open-circuit voltage and entropy coefficient are incorporated into a Newman's heat generation model to estimate the heat generated in the calculation grid. The battery temperature distribution is eventually derived by solving the heat conduction equation with thermal conductivity as a function of the battery temperature. We leverage the developed electrothermal model to track the temperature evolution of an 18 650 Li-ion battery at different ambient temperatures and discharge rates, for the first time. Experimental results demonstrate that the electrothermal model can precisely emulate the battery thermal dynamics with an average error of 0.72 °C. Moreover, a comparative study shows that the proposed model outperforms common resistance-based thermal models that do not consider the heat generation distribution and the interdependence between the battery temperature and thermal conductivity.

An Ultrathin and Ultrawideband Metamaterial Absorber and an Equivalent-Circuit Parameter Retrieval Method

Abstract: In this article, we propose an ultrathin and ultrawideband metamaterial (MTM) absorber based on periodically arranged metallic square spirals. The design, characterization, and measurement of the proposal are presented as its equivalent circuit. The lumped elements of the equivalent circuit are extracted using a proposed algorithm based on the least-square method, which presents a straightforward and promising approach and shows a good matching with the electromagnetic simulation. The unit cell of the proposed structure has the square spiral mounted on an FR-4 substrate in front of a conductive plate. The simulated results show an absorptivity of more than 90% from 11.4 to 20.0 GHz, covering the $Ku$ -band for transverse magnetic (TM) and transverse electric (TE) polarizations, and this broadband feature is confirmed by the experimental measurement. Furthermore, the proposed MTM absorber is $\lambda /16.4$ in thickness at the lowest frequency of absorption. The proposed MTM has proper response under oblique incidence from 0° to 50° and shows great performance with regard to the absorbers previously presented in the literature.

Equivalent Circuit Design Method for Wideband Nonmagnetic Absorbers at Low Microwave Frequencies

Abstract: An accurate equivalent circuit (EC) design approach for wideband nonmagnetic absorbers operating at the low microwave frequency (1–10 GHz) is presented. Following the impedance matching approach, this communication introduces an EC model based on the simulated data and synthetic asymptotes for single- and double-layer frequency-selective surface (FSS)-based nonmagnetic absorbers. Two simple and commonly used resistive FSSs, i.e., square patch and single square loop, are considered in this communication. Compared to the full-wave simulations, the proposed EC model shows more than 95% accuracy. By employing the proposed model and genetic algorithm-based optimization, several designs of broadband absorbers are demonstrated. The presented single- and double-layer FSSs show 126% and 161% fractional bandwidth, respectively, with the total thickness close to the Rozanov limit. The results confirm that the proposed method is a simple and efficient way of designing thin wideband absorbers using single- or double-layer FSS configurations.

Identification of recombination losses and charge collection efficiency in a perovskite solar cell by comparing impedance response to a drift-diffusion model.

Abstract: Interpreting the impedance response of perovskite solar cells (PSCs) is significantly more challenging than for most other photovoltaics. This is for a variety of reasons, of which the most significant are the mixed ionic-electronic conduction properties of metal halide perovskites and the difficulty in fabricating stable, and reproducible, devices. Experimental studies, conducted on a variety of PSCs, produce a variety of impedance spectra shapes. However, they all possess common features, the most noteworthy of which is that they have at least two features, at high and low frequency, with different characteristic responses to temperature, illumination and electrical bias. The impedance response has commonly been analyzed in terms of sophisticated equivalent circuits that can be hard to relate to the underlying physics and which complicates the extraction of efficiency-determining parameters. In this paper we show that, by a combination of experiment and drift-diffusion (DD) modelling of the ion and charge carrier transport and recombination within the cell, the main features of common impedance spectra are well reproduced by the DD simulation. Based on this comparison, we show that the high frequency response contains all the key information relating to the steady-state performance of a PSC, i.e. it is a signature of the recombination mechanisms and provides a measure of charge collection efficiency. Moreover, steady-state performance is significantly affected by the distribution of mobile ionic charge within the perovskite layer. Comparison between the electrical properties of different devices should therefore be made using high frequency impedance measurements performed in the steady-state voltage regime in which the cell is expected to operate.

Ultrathin 3-D Frequency Selective Rasorber With Wide Absorption Bands

Abstract: A novel ultrathin 3-D frequency-selective rasorber (FSR) with wide lower and upper absorption bands is presented. The wide absorption band is achieved by using a commercial ferrite absorber and the ultrathin profile is realized using a slow wave structure. The ferrite absorber is very thin and operates over a wide frequency band due to its strong magnetic loss. The attractive low insertion loss feature is obtained through bypassing the ferrite absorber with a series $L$ – $C$ circuit at the passband frequency. An equivalent circuit model is proposed to analyze the physical mechanism of the proposed 3-D FSR and to formulate the design equations. A prototype of the proposed 3-D FSR is fabricated and measured. Tested results show a bandwidth (BW) of 26.6% for the transmission band with insertion loss less than 3 dB and a fractional bandwidth of 163.6% for reflection coefficient less than −10 dB can be obtained. A key feature of the proposed FSR is that its thickness is only $0.086\lambda _{c}$ , where $\lambda _{c}$ is the free-space wavelength at the center frequency of the passband. Moreover, simulated results show that the proposed structure exhibits stable frequency responses under oblique incidence up to 45°.