Free-space Measurement System Research Articles

Measuring and Extracting the Complex Permittivity of Porous Ceramic Materials in the Y-Band

ABSTRACT Porous ceramics find extensive applications in aerospace and other fields, and the measurement of their electromagnetic parameters is helpful in optimization of material properties and device designs. In this paper, a free-space 8f quasi-optical measurement system consisting of a vector network analyzer, a 325–500 GHz frequency expansion module, and four off-axis parabolic mirrors have been established to measure the transmission parameter S21 of porous ceramics. The complex permittivity was extracted according to the Newton-Raphson iterative method utilizing its measured S21 parameters based on flat mode. The porous ceramics with different porosity and density were measured and the relationships between their permittivity and porosity and density was established, respectively. These results are beneficial to the quality evaluation and application design of porous ceramic materials in the aerospace field.

Evaluation of electromagnetic characteristics of 3D-printed radar absorbing structures based on three-dimensional period pattern surface

This paper aims to assess the electromagnetic (EM) characteristics of 3D-printed radar absorbing structures (RASs) based on three-dimensional period pattern surface (3D PPS) within the X-band frequency range (8.2–12.4 GHz). The assessment is validated through HFSS software, a 3D printer (Markforged, X7), and a scanning free-space measurement (SFM) system. Firstly, the analysis indicates that the EM characteristics of RASs based on 3D PPS are impacted by impedance matching in the thickness direction and the scattering of EM waves at the edges. The utilization of Onyx (Chopped carbon fiber/Nylon) reveals variations in resonance frequencies and EM wave absorption performance contingent upon the surface shape. Furthermore, significant differences in the variations of EM characteristics are observed even for RASs with the same surface shape but different materials. Secondly, the measurement results of 3D-printed RAS specimens based on PPS using Onyx exhibit similar trends to the analysis results obtained with the SFM system. Ensuring precise measurement results requires specific measurement conditions tailored to each surface shape, along with an examination of the EM characteristic distribution across the entire specimen area. This is crucial due to variations in the size, number, and range of reflected EM waves in the time domain. The analysis and measurement techniques presented in this study hold applicability in designing 3D-printed RASs that demand high EM wave absorption performance, whether under normal or oblique incident EM waves.

Folded-core radar-absorbing sandwich composite with sendust particle-added Ni-plated glass/polyether ether ketone thermoplastic resin in the ultrahigh-frequency band

A novel lightweight folded-core radar-absorbing sandwich composite composed of Ni-plated glass fiber/polyether ether ketone (PEEK) and sendust-particle-filled polyurethane (PU) foam thermoplastic resin is presented. It is designed as a V-pattern folded-core unit cell, which can lead to gradual loss of the incident radar signal intensity and enhanced radio wave absorption performance in the ultra-high frequency (UHF) band (0.3–3.0 GHz). Ni-plated glass fiber was prepared by an electroless plating technique, and a folded-core radar-absorbing sandwich composite by a stamp-forming process. The folded-core was decorated with sendust-particle-filled PU foam to enhance radio wave absorption performance. The radio wave absorption performance of the folded-core sandwich composite was measured using a proposed UHF band single-port free space measurement system. The proposed sandwich composite with a thickness of 81.6 mm achieves a reflection loss below −10 dB (90% absorption) and bandwidth covers in the frequency range of 0.9–3 GHz while maintaining a low density of 0.142 g/cm3. This study presents the new radar-absorbing sandwich composite with a UHF absorber as a low-frequency band, a low density, and excellent radio wave absorption performance.

Electromagnetic characteristics of 3D-printed composites by free-space measurement

This paper aims to assess the electromagnetic (EM) characteristics of 3D-printing composites based on continuous fiber and infill material in the X-band (8.2–12.4 GHz) using the free-space measurement (FSM) system. Firstly, the EM properties of the 3D-printed composite specimens can change depending on type and thickness of the infill material. Especially, Onyx (Chopped carbon fiber/Nylon) and Onyx ESD (Static-dissipative chopped carbon fiber/Nylon) exhibit a change in EM properties according to thickness due to randomly dispersed micro-sized chopped carbon fibers. Secondly, when fabricating a 3D-printed composite specimen containing continuous fiber layers, the top and bottom layers always consist of infill material. As the EM characteristics of both continuous fibers and infill materials depend on their type and thickness, it is crucial to select appropriate materials and thicknesses for 3D-printed composite specimens with continuous fiber layers. The EM characteristics of 3D-printed composites can also aid in designing multi-functional 3D-printed composite structures that require not only mechanical but also EM performance.

Three-dimensional orthogonal woven hybrid fabrics in X band frequency region for electromagnetic shielding effectiveness

The three-dimensional fabric structures with metallic yarns and different fibers offer additional opportunities to enhance the electromagnetic shielding potential. This study involves the manufacturing of a Three-Dimensional Orthogonal Woven (3DOW) hybrid conductive fabric to be potentially used in an electromagnetic shielding field. 3DOW fabric consisting of copper filament-based core-sheath hybrid yarn in the weft direction has been manufactured to obtain maximum shielding effect. Hybrid yarns were produced using the DREF-III spinning technique, having copper filament in the core and sheathing by Polyphenylene Sulfide (PPS) staple fibers. The experiment was designed using Box-Behnken method. The orthogonal fabric samples are tested in an electromagnetic frequency region of 8-12 GHz using Free Space Measurement System (FSMS) to estimate absorbance, reflectance, and transmittance. Hybrid yarn parameters such as PPS fiber diameter, copper filament diameter, and yarn linear density considerably affect electromagnetic shielding. 3DOW fabric’s electromagnetic shielding effect increase with the diameter of the copper core filament, as does PPS yarn’s diameter, showing a promising trend towards shielding. The findings of orthogonal structure can be further extended in other Three-Dimensional structures like multi-layer, angle interlock, draperies, and tops for civilians and soldierly applications.

Method of high-precision free-space distance measurement for noncooperative targets.

Aimed at high-precision distance measurement for noncooperative targets in free space, a spatial distance measurement method is proposed. Based on the concept of optical carrier-based microwave interferometry, this method extracts distance information from the radiofrequency domain. The interference model of broadband light beams is established, and the optical interference can be eliminated by using a broadband light source. A spatial optical system with a Cassegrain telescope as the main body is designed to effectively receive the backscattered signal without cooperative targets. A free-space distance measurement system is built to verify the feasibility of the proposed method, and the results agree well with the set distances. Long-distance measurements with a resolution of 0.033µm can be achieved, and the errors of the ranging experiments are within 0.1µm. The proposed method has the advantages of fast processing speed, high measurement accuracy, and high immunity to disturbances as well as the potential for measurement of other physical quantities.

Free-Space Two-Tier One-Port Calibration Using a Planar Offset Short for Material Measurement

The scattering parameters of a material under test (MUT) are prerequisites for characterizing the material parameters of the MUT. This paper describes a free-space two-tier one-port calibration method using a planar offset short as a free-space calculable reflect standard for measuring the scattering parameters of an MUT from the two successive one-port calibrations of a free-space material measurement system without a precise positioning system in free space. The two-tier one-port calibration method is validated by comparing the measurement results with those of the thru-reflect-line (TRL) calibration method for two reciprocal MUTs (glass plates of 2.780 mm and 4.775 mm thickness) in the W-band (75–110 GHz). Good agreement between the measurement results from the two calibration methods demonstrates that the free-space two-tier one-port calibration method using a planar offset short can be a feasible and effective alternative to the conventional free-space two-port calibration methods.

Free-Space Unknown Thru Measurement Using Planar Offset Short for Material Characterization

Material characterization requires the proper calibration of a material measurement system. This paper describes a free-space unknown thru measurement method using three independent planar metal offset shorts for calibrating a free-space material measurement system. This method is validated by comparing the measurement results with those of the TRL (thru-reflect-line) measurement method for two glass plates of 2.780 mm and 4.775 mm thickness in W-band (75–110 GHz). This can be an affordable and effective alternative to conventional free-space material measurement methods because the precision fabrication of a planar offset short is more feasible and inexpensive than building a precise positioning system in a free-space material measurement system. One can use this measurement method up to a high-frequency range that the fabrication accuracy of a planar offset short is acceptable.

Robotic scanning free-space measurement system for electromagnetic performance evaluation of curved radar absorbing structures

Radar absorbing structures are manufactured for stealth missions and total quality inspection applies to evaluate their performances before assembly to stealth weapon systems. This study adopted a six-axis robot arm to move a target specimen in a scanning free-space measurement system for electromagnetic performance evaluation. The six-axis robot arm completely enables the system to maintain the specimen at the focal point of the antenna and solves the issue of curvature effect on the return loss results of the curved specimens, faced in the two- and three-axis scanning free-space measurement systems. The six-axis robotic scanning free-space measurement system uses a RobotStudio to extract the position and orientation of each target point on the specimen to be evaluated and uses a robotic scanning algorithm to transform all the points into a scan path. The system was verified by variable and constant curvature radar absorbing structure specimens with frequency selective surfaces. Return losses ( S 11s) of the nonsymmetrical cambered airfoil specimen and S-shaped double curvature specimen that could not be accurately evaluated with a three-axis stage-based scanning free-space measurement system were measured by the six-axis robotic scanning free-space measurement system. All the specimen results adhere to the theoretical parameters of the specimen. The transition region results of the S-shaped specimen were studied using effective radius of curvature. Finally, the inspected results of the S-shaped specimen were curvature compensated to check the effect of negative and positive curvature on the specimen resonance frequency performance.

A study on the microwave absorbing honeycomb core embedded with conductive periodic patterned surfaces for the effective dielectric constant

Periodic patterned surfaces can provide effective dielectric constants to the microwave absorbing honeycomb partition wall depending on the size of the pattern. A characteristic study was conducted for a honeycomb-shaped absorber based on the geometric configurations and material properties. The range of the relative permittivity for increasing the absorption bandwidth of the absorber was determined through a parametric study. The imaginary term of the relative permittivity and depth of the wave propagation direction had a significant impact on the bandwidth of the absorber. Moreover, the materials patterned with carbon black- and graphite-based conductive inks using the screen-printing method were bonded to each other at intervals and stacked to form a honeycomb shape. Several absorber specimens were fabricated, and their absorption capabilities were measured using a free-space measurement system. Based on the results, the utility of a film-based honeycomb absorber that can continuously maintain broadband absorptivity was verified.

Rotation included 3-axis scanning free-space measurement and curvature compensation for electromagnetic evaluation of leading-edge and curved stealth structures

Since the last decade, stealth technology has been developing at a much higher speed with much research going on to evaluate the properties of radar absorbing structures (RASs) and scattering parameters of the developed stealth structures at different phases of practical use. A vector network analyzer plays an important role in RAS performance evaluation of stealth structures. Different scientists have used closed cell measurement techniques utilizing coaxial cables and waveguides to evaluate RAS materials at high accuracy, while others have used free-space measurement techniques utilizing radiating antennas to evaluate the RAS performance of larger stealth structures without damaging the specimen. All these conventional evaluation systems can scan a single point at a time. A scanning free-space measurement (SFM) system was an initial step toward automated scanning of large stealth structures. This system was capable of scanning 1000 mm × 1000 mm flat structures using raster scanning with two linear stages. However, the two-dimensional SFM was not applicable to leading-edge and cylindrical structures, and the signal-to-noise ratio (SNR) is highly affected by the specimen curvature. In this paper, a 3-axis scanning free-space measurement (3-axis SFM) system was developed. The system has two linear stages and a rotational stage for raster scanning of the specimen. The system performance was checked by scanning cylindrical RAS specimens with a radius of 100 mm. The results were then compared to the SFM system, which showed that the SNR of the 3-axis SFM is quite high compared to the SFM system. A coordinate-based scanning algorithm was also developed for 3-axis SFM to ensure a fixed interval scan of complex geometrically shaped specimens, such as wing leading edges, at lower incident angles. The algorithm achieved a zero-degree incident angle scan with ±1 degree of accuracy for the NACA-M3 symmetrical leading-edge specimen. Different scan parameters and results are visualized in the developed graphical user interface software. Finally, the inspected results were curvature-compensated to obtain the actual RAS performance of the specimen. The accuracy of the geometry-based perfect electric conductor (PEC) reflection-loss prediction algorithm was further improved by adding a stand-off distance effect and more specimen curvature data to the algorithm. The accuracy of the algorithm was verified by comparing the predicted PEC results to the measured PEC results of the corresponding specimen.

Optimization of electromagnetic shielding of three-dimensional orthogonal woven hybrid fabrics in ku band frequency region by response surface methodology

Electromagnetic shielding (EMS) has become the necessity of the present era due to enormous expansion in electronic devices accountable to emit electromagnetic radiation. The principal target of this paper is to originate three-dimensional (3D) orthogonal fabrics with conductive hybrid weft yarn and to determine their electromagnetic shielding. DREF-III core-spun yarn using copper filament in the core and polyphenylene sulfide (PPS) fiber on the sheath and fabric constructed of such yarn has a promising electromagnetic shielding characteristic. Box–Behnken experimental design has been employed to prepare various samples to investigate the electromagnetic shielding efficiency of 3D orthogonal woven structures. The orthogonal fabric samples were tested in an electromagnetic Ku frequency band using free space measurement system (FSMS) to estimate absorbance, reflectance, transmittance, and electromagnetic shielding. The increase in copper core filament diameter and hybrid yarn linear density enhances the EMS of orthogonal fabric. Statistical analysis has been done to bring out the effect and interaction of various yarn and fabric variables on EMS. Metal filament diameter, orientation, sheath fibers percentage, and fabric constructional parameters significantly affected electromagnetic shielding efficiency. The inferences of this study can be applied in other 3D structures like angle interlock, spacer fabrics for curtains, and coverings for civilians and military applications.

Complex Permittivity Measurements of Steel Fiber-Reinforced Cementitious Composites Using a Free-Space Reflection Method with a Focused Beam Lens Horn Antenna.

To measure the electromagnetic properties of steel fiber-reinforced concrete (SFRC) in the X-band, 1-port measurements were performed using a lens horn antenna in a free-space measurement system. Free-space 1-port calibration with translations of the position of the reflector regarding the characteristics of the focused beam lens horn antenna was applied. The intrinsic impedance and complex permittivity of the SFRC were obtained from the measured reflection characteristics. The steel fiber content increased and the electromagnetic properties of the SFRC gradually changed from a dielectric to a conductor, even in very low frequencies compared to the plasma frequencies of general metal, which are optical frequencies. This is considered to be the plasmon effect of the metallic structure formed by the steel fiber. This result is applicable for analyses of the electromagnetic phenomenon of large structures with fiber content.

High temperature W-band complex permittivity measurements of thermally cycled ceramic-metal composites: AlN:Mo with 0.25 to 4.0 vol% Mo from 25 °C to 1000 °C in air

An apparatus for measuring the W-band (75–110 GHz) complex permittivity of dielectrics at 1000 °C was developed. This apparatus allows for measurements at approximately twice the temperature of previously published high temperature free-space measurement systems while maintaining similar precision. Challenges were addressed related to high temperature measurements, including temperature uniformity, the accuracy of temperature measurements, and preventing temperature related changes to mm-wave measurement systems. The details of complex permittivity extraction from the measured S-parameters are discussed. Sources of error related to permittivity measurement and mathematical models were identified and are discussed in detail herein. Thermally-cycled, mm-wave absorbing, aluminum nitride ceramic composites containing varying levels of molybdenum additives were measured over the range of 25 °C–1000 °C. These measurements were compared to the same composites before thermal cycling. It was found that ceramic composites are largely stable after thermal cycling in terms of dielectric properties despite the presence of visible surface modifications.

Damage detection of CFRP composites by electromagnetic wave nondestructive testing (EMW-NDT)

Damages such as fiber breakage and delamination are likely to occur inside CFRP composites when subjected to external forces, such as impact and fatigue load. These damages are mostly invisible and cause safety hazards during the service of the products. This study proposes a new type of nondestructive testing (NDT) method using electromagnetic wave (EMW) technique, EMW-NDT. It was proven that the proposed EMW-NDT method is effective in detecting damages such as delamination, crack or other defects in CFRP composites. The EMW-NDT method's detection capacity to the delamination size, delamination thickness, and slits in CFRP composites was investigated. A reasonable sensitivity to the damage volume change in delamination was confirmed with a damage area ratio of 12.6%/dB and a thickness change of 5.5 dB/mm. It was found that the incident angle of the EM wave plays a vital role in detecting sensitivity because of the skin effect in CFRP composites. The results confirmed that the proposed method demonstrates good detection sensitivity to delamination size and thickness. In terms of crack damage, the slit and its length were detected and the slit direction was successfully identified in this study based on the characteristics of the electromagnetic interference (EMI) shielding anisotropy in CFRPs. Moreover, the proposed EMW-NDT method with specified designed free-space measurement system is contactless, and no coupling medium is required; thus, it exhibits huge potential to be widely used as a new damage detection technique for CFRP composites.

Numerical simulation study of free space method for measuring sample permittivity

The composition and performance of the material can be expressed by the permittivity. The paper introduces the measurement principle of free space method and TRL calibration principle, the calculation formula of the sample permittivity is theoretically analysed, a horn lens antenna is designed to simulate the emission of plane waves, and numerical simulation software is used for simulation and optimization. In the working frequency range, the voltage standing wave ratio of the antenna is less than 1.49, and the maximum gain is 21.8dB. A free space measurement system is established in the software to measure the sample permittivity with different thicknesses, and compare the calculated value with the actual value. The results show that the root mean square error of the real part is less than 5.3%, and the root mean square error of the imaginary part is less than 1.3%. The correctness and applicability of the formula for calculating the permittivity are also proved.

Embedded optical fiber link interferometer sensors for snapshot surface inspection using the synthetic wavelength technique.

Free-space optical measurement systems can have a direct impact on evaluation systems operational in propagation paths. During propagation via optical fibers, light suffers scattering or interference, causing some output signal loss with an uncertainty outcome. Therefore, this study aims to explore the instant decisions related to the use of single- and multi-mode fiber optics and how they affect the gathering of data from high-speed optical measurement instrument links. The study also seeks to address a number of design methodology aspects and the empirical outcomes related to a surface topography measurement sensor based on fiber optics capable of surface roughness or step-height measurement. The study suggests that the Fourier transform profilometry method (FTP) can overcome the disadvantages of optical metrology sensors (e.g., bulkiness, challenging set-up, high costs, and low speed). However, despite eliminating vertical height problems, the Fourier transform profilometry (FTP) did have some shortcomings for every outcome related to core variables, including the dispersive optical fiber link sensor. The synthetic wavelength method enabled the dispersive optical fiber link sensor to calculate the vertical step height of the selected sample (1 µm). There was improved step-height repeatability, with satisfactory from 20 to 18 nm outcome improvement range. Additional investigations are necessary to establish the compatibility of single- or multi-mode optical fiber sensors with particular instruments, especially those currently preferred for embedded metrology applications.

Planar Offset Short Applicable to the Calibration of a Free-Space Material Measurement System in W-Band

The electrical properties of materials and their dependence on frequency and temperature are indispensable in designing electromagnetic devices and systems in various areas of engineering and science for both basic and applied researches. A free-space transmission/reflection method measuring the free-space scattering parameters of a material under test (MUT) located at the middle of transmit/receive antennas in a free space is suitable for non-destructively testing the MUT without prior machining or physical contact in high-frequency range. This paper describes a planar offset short applicable to the calibration of a quasi-optic based free-space material measurement system in the millimeter-wave frequency range. The measurement results of the dimensional and electrical properties for the three fabricated planar offset shorts with the phase difference of 120° between the reflection coefficients of the planar shorts in the W-band (75–110 GHz) are presented.

Additive Manufacturing of PLA-Based Microwave Circuit-Analog Absorbers

Fused filament fabrication (FFF) was used to additively manufacture (3-D print) two-prototype circuit-analog (CA) absorbers. The CA absorbers consist of a lossy frequency selective surface (FSS), substrate, and ground plane. In this article, the FSS and substrate were manufactured using two different FFF materials to make a cohesive structure manufactured during a single printing procedure. To design the CA absorbers, the complex permittivity of FFF printed polylactic acid (PLA), bronze-, brass-, copper-, and iron-powder infused PLAs, and a graphite-PLA composite was measured using a free-space materials measurement system. Out of the candidate materials measured, graphite PLA and standalone PLA were selected as the FSSs and substrates, respectively. Complex permittivity data from the selected materials were input to Computer Simulation Technology Microwave Studio so that a genetic algorithm could optimize absorber dimensions. Reflectivity of the printed absorbers was measured using a free-space measurement setup. Measured reflectivity data were compared to that from simulations. A simulated-geometric tolerance study corroborated differences noted between ideal models and measured data. The results showed that FFF techniques can be used for CA-absorber designs and that 3-D printing settings can ultimately affect absorber performance.

Broadband Radar Absorbing Structures with a Practical Approach from Design to Fabrication

In this study, a novel broadband radar absorbing volume structure (RAVS) is proposed and demonstrated with a practical point of view from design to fabrication. The proposed RAVS uses a design concept of repeatedly stacked carbon nanotube (CNT) composites and foam cores of the same thickness to improve the applicability to real structures while maintaining absorption performance. The repeatedly stacked CNT composites, which act as electrically lossy materials, result in the multiple scattering of incident electromagnetic waves trapped inside the structure. The trapped incident waves then lose their energy by multiple scattering. Based on this design concept, the RAVS designed through field analysis and parametric study achieved a −10 dB absorption performance from 4 GHz to 16 GHz. With reference to the design values, RAVS was fabricated for verification, and the absorption performance was measured using a free space measurement system. The measurement result showed excellent absorption performance that satisfied −10 dB from 5.8 GHz or less to 14 GHz.