Free Space Measurement Method Research Articles

Multi-Functional Reconfigurable Intelligent Surfaces for Enhanced Sensing and Communication.

In this paper, we propose a reconfigurable intelligent surface (RIS) that can dynamically switch the transmission and reflection phase of incident electromagnetic waves in real time to realize the dual-beam or quad-beam and convert the polarization of the transmitted beam. Such surfaces can redirect a wireless signal at will to establish robust connectivity when the designated line-of-sight channel is disturbed, thereby enhancing the performance of wireless communication systems by creating an intelligent radio environment. When integrated with a sensing element, they are integral to performing joint detection and communication functions in future wireless sensor networks. In this work, we first analyze the scattering performance of a reconfigurable unit element and then design a RIS. The dynamic field scattering manipulation capability of the RIS is validated by full-wave electromagnetic simulations to realize six different functions. The scattering characteristics of the proposed unit element, which incorporates two p-i-n diodes have been substantiated through practical implementation. This involved the construction of a simple prototype and the subsequent examination of its scattering properties via the free-space measurement method. The obtained transmission and reflection coefficients from the measurements are in agreement with the anticipated outcomes from simulations.

Orthogonal Adjacent-Order Filtering Element-Based Wideband Transmissive Linear-to-Circular Polarization Converter

This article proposes a unified design method for a highly efficient wideband transmissive linear-to-circular polarization converter composed of two orthogonal adjacent-order bandpass filtering elements for the first time. First, the linear-to-circular polarization converter is constructed to convert two orthogonal linearly polarized (LP) spatial waves split from a 45°-slant incident plane wave into a desired circularly polarized (CP) wave. Taking advantage of the wideband filter synthesis method and phase shifter theory, polarization converter elements with prescribed specifications [e.g., fractional bandwidth (FBW) and Chebyshev passband ripple] are then synthesized by the corresponding low-characteristic-impedance bandpass filtering networks with the loading of shunt short-circuited stubs on quarter-wavelength transmission line (TL) sections. The adjacent-order phase difference function with a natural ±90° value at a center frequency can be simultaneously manipulated by a filtering bandwidth. The theoretical axial ratio (AR) curve is calculated by the synthesized magnitude responses and phase deviation (PD) function. The design guideline is then summarized. Finally, based on the third- and fourth-order bandpass filtering networks, a prototype of the proposed linear-to-circular polarization converter operating at 5 GHz with a designable 70% 3 dB AR bandwidth is implemented, fabricated, and tested by the free-space measurement method. The transmitted CP wave with three in-band AR minima and relatively stable angular performances can be efficiently converted from the incident LP spatial wave as well verified in the experiment. The AR curves and design requirements of different adjacent-order bandpass filtering combinations are further explored according to the proposed design guideline.

Synthesis Design of Bandpass Frequency Selective Surface With Multiple Transmission Zeros Using Slotline Structures

This article presents a synthesis method to design a new type of bandpass frequency selective surfaces (FSSs) with multiple transmission zeros (TZs). To briefly show the concept of our synthesis method, a generalized topology of the bandpass FSS based on cascaded slotline sections paralleled with bandpass resonators is first presented to map with an alternative <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> th-order bandpass filtering network with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula> -inverters and typical LC parallel resonators. Then, two kinds of composite LC parallel resonators, named as LC – LC and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}$ </tex-math></inline-formula> – LC parallel resonators, are introduced and discussed to replace the typical LC parallel resonators, which can produce TZs at skirts of the passband and increase the out-of-band selectivity. The derivations of equivalent relationships between the composite and typical LC resonators are given. Furthermore, for the exploration of bandpass FSS with multiple out-of-band TZs, the realization of these two discussed composite LC parallel resonators is exhibited by inductive and capacitive strip lines loading on slotline structures. The design curves and guidelines are given and discussed correspondingly. To validate our design concept, two FSS examples, a second-order bandpass FSS with four out-of-band TZs and a third-order one with five out-of-band TZs, are synthesized and designed. In final, these two designed bandpass FSS prototypes are fabricated and then measured by the free-space measurement method. The measured results show that the desired stable bandpass filtering performance and independently multiple out-of-band TZs are successfully realized.

Design, development and characterization of resistive arm based planar and conformal metasurfaces for RCS reduction

This paper reports the design, development and characterization of a broadband, polarization insensitive metasurface absorber in planar, cylindrically bent and 90° dihedral surface geometry. Four metallic patches loaded with eight lumped resistors are used, which has been optimized numerically using CST MICROWAVE STUDIO, as a unit cell of developed metasurface absorber, to achieve 20 dB reflection reduction for 51.21% fractional bandwidth (13.42–22.66 GHz) under normal incidence with 0.12 {lambda }_{L} thickness (where {lambda }_{L} corresponds to lower operating frequency). The numerical findings are also verified analytically using equivalent circuit analysis, which exhibits very good agreement. Polarization-insensitive characteristics are achieved using fourfold rotation symmetry of the designed structure. The fabricated prototype of the designed absorber is experimentally characterized, using free space measurement method and ABmm vector network analyzer (VNA) system, and fairly good agreement with numerical-analytical findings are reported. The major novelty of this study is the design and development of a broadband (13.42–22.66 GHz), polarization insensitive metasurface absorber that provides 20 dB reflection reduction numerically as well as experimentally in the whole band, which to the author’s knowledge has not been observed till now. Also, keeping in mind the radar stealth applications, first time we have demonstrated both numerically and experimentally, different geometrical shapes of conformal metasurfaces that can be practically used in actual scenario.

Free-Space Measurement of Dielectric and Magnetic Properties by Double Planar Sample Method in Y-Band

In this work, to measure the complex permittivity and the complex permeability of materials, the double planar sample method is proposed, which is a variant of the classical free-space measurement method. The device under test is a double planar sample instead of a single piece of sample. The greatest advantage of this method is that it eliminates complicated calibration in the measurement of the reflection coefficient. In this novel method, the reflection coefficient is replaced by a second transmission coefficient. Two groups of transmission coefficients are obtained by changing the distance between these double planar samples; then, the complex permittivity and the complex permeability by the transmission coefficient are retrieved. The relative uncertainties of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\varepsilon ^{\prime}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu ^{\prime}$</tex-math></inline-formula> are both 1%. Four materials (polytetrafluoroethylene, silicon, quartz, and leaf of banana) are measured by this method, the results of which are consistent with those reported in the literature. The relative uncertainty of this method and the conventional free-space method is consistent. However, the magnetic loss tangent is very small, and the retrieval method is not sensitive to it.

Development of Resistive-Ink Based Planar and Conformal Metasurfaces for RCS Reduction

This work reports design, development and characterization of both planar and conformal Metasurfaces using resistive Ink based lossy pattern printed on top of FR-4 dielectric substrate for RADAR stealth application. Different geometrical dimensions of the proposed unit cell have been optimized numerically using CST-Microwave Studio, to achieve 20 dB reflection reduction with 60% fractional bandwidth in frequency range 10.5-19.5 GHz under normal incidence with 0.12&#x03BB;_L thickness (where &#x03BB;_L corresponds to lower operating frequency) and 10 dB reflection reduction up to 40&#x00B0; oblique angle of incidence in the given frequency spectrum for both TE and TM polarized wave. An equivalent circuit analytical modelling has also been performed to understand and verify the numerical findings which are nearly in agreement. Polarization insensitive characteristics of developed surfaces is achieved using fourfold rotation symmetry of the designed unit cell. The designed absorbers are fabricated in form of planar, cylindrically bent and 90&#x00B0; dihedral Metasurfaces and are characterized using ABmm Vector Network Analyzer (VNA) system and free space measurement method. A thorough different experimental measurements are performed and it is found that measured characteristics are in agreement with numerical-analytical findings.

Design of assembly‐type frequency selective surface structure using Lego‐type blocks

AbstractA frequency selective surface (FSS) structure was designed using Lego blocks, which can be stacked or assembled to produce various structures. The unit structure of the designed frequency selection surface structure is a patch‐type cross‐dipole and a dipole structure that can be easily fabricated using Lego‐type blocks among the basic structures. Since the unit structure is designed using Lego‐type dielectric blocks and metal blocks, the structure can be easily changed and the resonance frequency can be easily reconfigured. The designed unit structure was fabricated using actual Lego‐type blocks of 4 mm × 4 mm × 5 mm, and the transmission coefficient of the fabricated FSS structure using the free space measurement method was measured. When the measurement result according to the change of polarization and incident angle was compared with the simulation result, the error of the resonance frequency was 1.2% and 0.64% at the maximum for each structure, confirming that the measurement result is relatively similar to the simulation result.

Design, development and characterization of wide incidence angle and polarization insensitive metasurface absorber based on resistive-ink for X and Ku band RCS reduction

This article deals with the design, development and characterization of a broadband, wide incidence angle and polarization-insensitive metasurface, which consists of periodic resistive-ink pattern printed on top of FR-4 dielectric substrate as a RADAR absorber. Geometrical dimensions of the proposed unit cell have been optimized numerically using CST-Microwave Studio to achieve 99% absorptivity for X and Ku frequency spectrum (8–18 GHz) under the normal angle of incidence and to achieve almost 90% absorptivity under the oblique angle of incidence up to 50° within X-Band to Ku-Band spectrum. Furthermore, it is reported that by having fourfold rotation symmetry, the designed structure is insensitive to the polarization of the incident wave. An equivalent circuit analytical modeling has also been performed to understand and verify numerical findings that are nearly in agreement. The designed absorber is fabricated and characterized using ABmm VNA and free space measurement method, for both normal and oblique angle of incidence, and it is found that measured characteristics are in agreement with numerical-analytical findings. Furthermore, the monostatic and bistatic Radar Cross Section reduction capability of the designed absorbing structure for both planar and curved surface (cylinder) is studied extensively to assess its potential application in X-Band to Ku-Band Spectrum.

Designing and analysis of non-symmetric dual layer CRLH metamaterial

The underlying study investigates the artificial metamaterial designing of non-symmetric dual layer combination on the same substrate for the negative characteristics of refractive index, permittivity and permeability. The design is instigated with the investigation of single and dual layers metamaterial design for single or double negative behavior. Apart from the parametric study, the dispersion diagrams of the layers are also evaluated for gap and gapless transactions from left hand to right hand behavior. The combination of symmetric and non-symmetric layers is also analyzed for behavior change in material properties. The permittivity and permeability of the designed non-symmetric dual layer metamaterial are negative up to 10 GHz and are positive above 10 GHz. Moreover, the left and right hand impedance also remains unchanged and matched with that of free space impedance (377 Ω) with a fractional bandwidth of 50%. Hence, the designed material acts as a double positive material for high frequencies above 10 GHz and double negative material for low frequencies below 10 GHz. The design is verified using circuit modeling and full wave electromagnetic simulations. The results obtained are authenticated using the free space measurement method.

An Optimized DTW Algorithm Using the RMSE Approach to Classify the Liquids in Ka-Band

ABSTRACT In this paper, a new optimized algorithm is presented to classify the liquids with high-level accuracy using raw data which show the transmission parameter of liquids. The free space measurement method, one of the microwave spectroscopy methods, has been chosen to collect the parameter for use in detecting illegal and explosive liquids. The aim is to classify the liquids that the passenger can choose with his/her plane journey. To classify and identify these liquids, instead of a multivariate data analysis method, a simpler dynamic time warping algorithm method is proposed. It is advantageous to intervene in this algorithm to perform the classification process better. It will be able to determine the most contradictory similarities among the signals collected from liquids and perform the best classification process. Thanks to this feature, it is expected to give better results than the algorithms proposed earlier. Because, it is thought that the liquids can be classified by the parameter which has unique properties of liquids. Furthermore, this algorithm will allow an intermediate group of suspicious liquids to be detected in another group. The performance of the proposed model is discussed in terms of its potential for reducing the classification problem and composing a database which can be used to identify the un/known materials easily.

Investigation on microwave absorption characteristics of conductive-coated honeycomb absorber

Because microwave absorbers are limited in their absorption performance and bandwidth by their thickness, research on lightweight microwave absorbers having a wide bandwidth has been increasingly conducted. Lightweight structures with periodicity, such as honeycomb structures, have been increasingly attracting attention as they have the potential for application as lightweight broadband absorbers. Aramid or glass fiber-based honeycomb cores are dielectric materials that are often used in industrial applications. If a honeycomb absorber is manufactured by coating a conductive material on a honeycomb core, microwave absorption performance can be expected in a wide frequency range. However, structures with a certain periodicity, such as honeycombs, may bring about unexpected effects due to the shape with respect to the propagation direction of the microwave. In this study, the characteristics of absorbing electromagnetic energy in honeycomb absorbers were investigated, and the possibility of analyzing honeycomb absorbers by the transmission line analysis method was examined. Based on this investigation, a honeycomb absorber was fabricated, and the simulation results were verified by evaluating the performance by the free-space measurement method. Because of unpredictable scattering of the microwave polarized in the in-plane direction, for honeycomb absorbers, it is necessary to keep the coating resistance low or carefully consider the influences through simulation.

Preparation and high-temperature microwave absorbing properties of 6H-SiC/MWCNT/silicon resin composites

Microwave absorbing materials (MAMs) have been widely used in the military and industrial fields. Recently, considerable efforts have been made to develop hybrid composition, multilayer structure absorbers to meet requirements such as strong absorption characteristics, wide absorption frequencies, light weights and oxidation resistance. In this study, a series of flexible composites consisting of 6H-SiC powder (40–50 wt.%), silicon resin (60–50 wt.%) and multiwalled carbon nanotubes (MWCNTs, additional 0.05–0.2 wt.%) were prepared and applied to produce high-temperature microwave absorbers via the spray coating method. The prepared composites were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the microwave absorbing properties of the composites were measured in the 4–18 GHz frequency range by the free space measurement method. The TGA thermograms indicated that the thermal stabilities of the prepared composites increased with increases in the 6H-SiC content but were less affected by increases in the MWCNT content. The actual measurement results showed that the optimum composition was 50 wt.% 6H-SiC, 50 wt.% silicon resin and an additional 0.2 wt.% MWCNT. The minimum reflection loss (RL) value of –58.9 dB was observed at 9.7 GHz with a thickness of 1.6 mm, and the effective absorption bandwidth (RL &lt; –10 dB, for 90% microwave absorption) reached 2.3 GHz. At a high temperature of 450 °C, the absorption peak was located at 10.0 GHz with a reflection loss of – 13.7 dB, and the effective frequency bandwidth still reached 1.6 GHz.

Beam‐switching millimetre‐wave antenna using cantilever‐based FSSs

A beam-switching antenna for millimetre-wave applications at 30 GHz is presented in this study. The radiation reconfigurability for a cylindrical dielectric resonator antenna (DRA) is obtained by using a cantilever-enabled frequency selective surface (FSS). The proposed cantilever-enabled FSS provides reconfigurable bandpass and band-stop performance with good isolation between them. The investigation of designed FSS is performed using electromagnetic simulation and an array of the proposed FSS unit cell is fabricated and measured using the free-space measurement method, which shows a good agreement with the simulation results. Finally, six panels of the 1 × 5 array of the proposed reconfigurable FSS are arranged in a hexagonal configuration around a cylindrical DRA to obtain a reconfigurable radiation pattern in the azimuth plane. A prototype of the proposed radiation reconfigurable DRA is fabricated and the switching radiation patterns are measured at 30 GHz.

건물 내 주파수 선택적 공간 구현을 위한 인쇄전자 기술 기반 필름형 주파수 선택 표면구조 설계

In this paper, a frequency selective surface(FSS) with bandstop operation for radio-frequency spectrum management is presented. The proposed FSS is composed of patterns of fractal-based miniaturized unit cells for stable performance for angles of incidence and polarizations. For practical applications requiring high productivity and environmental compatibility, we fabricated a film-type FSS by screen-printing using Ag ink, rather than a conventional manufacturing method using a printed circuit board. To validate this study, we measured the transmission characteristics of the proposed FSS using the free-space measurement method, and observed the received strength of signal penetrating the FSS film applied to a wall.

Development of Measurement and Extraction Technique of Complex Permittivity Using Transmission Parameter S 21 for Millimeter Wave Frequencies

This study provides an overview of measured S-parameters and its processing to extract the dielectric properties of materials such as Teflon, PMMA, and PVC which are preferred for materials characterization process. In addition, a correction model is presented for transmission parameter (S 21) to obtain the dielectric constant with high accuracy. A non-destructive and non-contact free space measurement method has been used to measure S-parameters of thin samples in the low THz frequency range. S-parameters are measured in free space by vector network analyzer supported with two frequency extenders. Additionally, the parabolic mirrors are used to collimate the generated beam in wide frequency range. Furthermore, a standard filter process is performed to remove the undesired ripples in signal using singular spectrum analyzer before the implementation of extraction process. Newton-Raphson extraction technique is used to extract the material complex permittivity as a function of the frequency in Y-band (325–500 GHz).

Measurement of complex terahertz dielectric properties of polymers using an improved free-space technique

The complex dielectric properties of non-polar solid polymer materials were measured in the terahertz (THz) band by a free-space technique employing a frequency-extended vector network analyzer (VNA), and by THz time-domain spectroscopy (TDS). Mindful of THz wave’s unique characteristics, the free-space method for measurement of material dielectric properties in the microwave band was expanded and improved for application in the THz frequency region. To ascertain the soundness and utility of the proposed method, measurements of the complex dielectric properties of a variety of polymers were carried out, including polytetrafluoroethylene (PTFE, known also by the brand name Teflon), polypropylene (PP), polyethylene (PE), and glass fiber resin (Composite Stone). The free-space method relies on the determination of electromagnetic scattering parameters (S-parameters) of the sample, with the gated-reflect-line (GRL) calibration technique commonly employed using a VNA. Subsequently, based on the S-parameters, the dielectric constant and loss characteristic of the sample were calculated by using a Newtonian iterative algorithm. To verify the calculated results, THz TDS technique, which produced Fresnel parameters such as reflection and transmission coefficients, was also used to independently determine the dielectric properties of these polymer samples, with results satisfactorily corroborating those obtained by the free-space extended microwave technique.

A Free-Space Measurement Method for the Low-Loss Dielectric Characterization Without Prior Need for Sample Thickness Data

A free-space measurement method is presented for the characterization of low-loss dielectric materials at millimeter-wave frequencies that does not require any assumption of a priori knowledge of the sample thickness. The method first employs only maximal and minimal envelopes of measured transmission scattering parameters to determine the real part $\varepsilon _{r}^{\prime }$ of the permittivity of test materials. Subsequently, the thickness of the sample is estimated from $\varepsilon _{r}^{\prime }$ and frequencies for maximal and minimal peaks of the transmission scattering parameter. The calculation of the imaginary part $\varepsilon _{r}^{\prime \prime }$ of the permittivity then easily follows. Our method is examined by measuring two cross-linked polystyrene samples, one polytetrafluoroethylene sample and one polymethylpentene sample in the frequency range of 220–325 GHz at the incident angles of 0°, 10°, 20°, and 30°. Moreover, an explicit uncertainty analysis for the permittivity is derived, and uncertainties of the extracted complex permittivity are reported.

Materials classification by partial least squares using S-parameters

This paper presents a simple and usable algorithm, which is called Partial Least Squares, to classify the samples in different thicknesses and frequency range. This model employed to reconstruct the samples using S-parameters which are collected by Vector Network Analyzer in Free Space Measurement method. The relationship of between S-parameters is showed to classify the materials using proposed model. The Partial Least Squares algorithm has a good potential to classify the different samples with non-contactless and non-destructive measurement method. The classification process will be easy by using the proposed model, due to its practical and simple usage. In addition, the extraction techniques, which are Nicolson Ross Weir, Newton–Raphson, and Genetic Algorithm, are used in order to extract the dielectric constant of samples, in this study. The Newton–Raphson algorithm is selected to obtain the complex permittivity of samples since it gives best results than other extracting techniques and the results are used for classification.

Reduction of S-parameter errors using singular spectrum analysis.

A free space measurement method, which consists of two horn antennas, a network analyzer, two frequency extenders, and a sample holder, is used to measure transmission (S21) coefficients in 75-110 GHz (W-Band) frequency range. Singular spectrum analysis method is presented to eliminate the error and noise of raw S21 data after calibration and measurement processes. The proposed model can be applied easily to remove the repeated calibration process for each sample measurement. Hence, smooth, reliable, and accurate data are obtained to determine the dielectric properties of materials. In addition, the dielectric constant of materials (paper, polyvinylchloride-PVC, Ultralam® 3850HT, and glass) is calculated by thin sheet approximation and Newton-Raphson extracting techniques using a filtered S21 transmission parameter.

Polyurethane foam-based radar absorbing sandwich structures to evade detection

Polyurethane foam-based sandwich structures were developed for radar absorbing properties together with load bearing capability. The sandwich construction of radar absorbing structure comprised glass fiber epoxy matrix composite containing carbon black as front face skin, carbon fiber epoxy matrix composite as back face skin, and polyurethane foam reinforced with carbonyl iron or graphite powder as the core material. The quantity of carbon black in the front face skin composite was varied from 6 wt.% to 8 wt.%, while the loading of carbonyl iron and graphite powder in foam core was varied from 30 wt.% to 55 wt.% and 5 wt.% to 30 wt.%, respectively. The compression molding technique was used to prepare face skins and sandwich structures. Different combinations of the sandwich structures were characterized for a frequency range of 2–18 GHz using free space measurement method. The maximum attenuation of −31.85 dB was observed in a combination containing 6 wt.% carbon black in front face skin and 20 wt.% graphite powder in the foam core. The total reduction in radar cross-section demonstrated almost zero radar signature at nine different frequencies, whereas broad range attenuation of −10 dB was achieved.