Open-ended Rectangular Waveguide Probes Research Articles

Adaptive segmentation-based evaluation of material properties of dielectric sheets using microwave NDE

ABSTRACT Composite materials/structures are rapidly advancing, offering improved product efficiency, cost-effectiveness, and superior specific properties. These materials find increasing applications in corrosion resistance and load-carrying structures across industries such as aerospace, wind turbines, transportation, and medical equipment. Therefore, ensuring reliable non-destructive evaluation of composites is crucial to address safety concerns and minimise maintenance expenses. The microwave non-destructive evaluation (NDE) method is particularly well-suited for characterising the material properties of composites. In our study, we utilise a large flange open-ended rectangular waveguide (OERW) probe, measuring 152 mm x 152 mm, to assess the complex reflection coefficient of dielectric materials. This measurement enables the extraction of essential material parameters using a forward electromagnetic model combined with an optimised method. To enhance accuracy, we introduce an iterative conjugate gradient descent (CGD) optimiser with adaptive segmentation. This approach effectively avoids local minima and reduces disparities in integrand values, resulting in precise measurements of material properties. Our proposed method provides estimations of the dielectric constant within a range of 1 to 10 and accurately determines sample thickness with a controlled limit of 3% error across all samples. Furthermore, we observe that as the loss-factor value decreases, the error in calculating the loss-factor of samples increases, which is supported by numerical analysis.

Millimeter Wave Thickness Evaluation of Thermal Barrier Coatings (TBCs) Using Open-Ended Waveguide Probes

ABSTRACT Nondestructive testing (NDT) of thermal barrier coatings (TBCs) is a critical and ongoing topic of research and development. In particular, inspection techniques that determine the thickness of ceramic topcoat and thermally grown oxide (TGO) are of interest. This work investigates the utility of open-ended rectangular waveguide probes in the millimeter wave frequency range of 26.5–110 GHz for evaluation of topcoat and TGO thicknesses through a compressive set of electromagnetic (EM) simulations. In addition, these EM simulations are used to illustrate the influence of probe size and TBC substrate curvature on the complex reflection coefficient properties and the subsequent thickness estimation. The impact of volumetric porosity level on the same is also investigated. A standing-wave probe at V-band (50–75 GHz) is constructed and used to measure the topcoat thickness on three button-type TBC samples. This probe eliminates the need for using expensive and bulky vector network analyzers (VNA), which is quite desirous from a practical point-of-view. The experimental results indicate the capability of estimating the topcoat thickness to within ±15 μm (0.6 mils).

IFFT-Based Microwave Non-Destructive Testing for Delamination Detection and Thickness Estimation

Microwave Non-Destructive Testing (NDT) techniques for dielectric coatings are vital processes in many industrial applications due to their superior defect detection capabilities over traditional NDT methods. However, the limited bandwidth and post-processing complexities of these techniques cause them to fall short of accurately estimating the delamination thickness in such structures. In this paper, a novel low-complexity signal processing method is proposed to estimate the delamination thickness in metal-backed dielectric coatings using bandwidth-limited Open-Ended Rectangular Waveguide (OERW) probes. Here, Inverse Fast Fourier Transform (IFFT) process is used to convert the measured complex reflection coefficients at the surface of the coating to the time domain. Next, the amplitude of a specific time step is used to accurately estimate the delamination thickness without extra time-consuming processing. Using a 3 mm-thick macor samples with machined defects, the proposed technique is validated by correctly estimating delamination thickness down to 0.4 mm using an OERW probe with 13.5 GHz bandwidth. Hence, this technique overcomes the challenge of the OERW's limited bandwidth, which highly affects the possibility of tracking shifting in the peak reflection in the time domain. Moreover, the proposed method does not add to the complexity of the NDT process, which makes it suitable for in-situ real time applications. Hence, following this approach would be of great importance to numerous industries, where accurate thickness estimation of minute delamination in coatings is essential to avoid system failure.

Multilayer Structure Technique for Improving Determination of Electromagnetic Properties of Radar Absorbers Based on Two-Layer Method and Flanged Rectangular Waveguide Probe

This paper presents further development of utilization of two-layer method to perform nondestructive electromagnetic properties determination of planar radar absorbers using flanged open-ended rectangular waveguide probe. A multilayer structure of three layers was proposed to improve the measured results of these parameters obtained using two-layer method. These layers were arranged such that the test material is sandwiched between two known low loss materials to provide the needed two independent reflection coefficients necessary to extract them at different conditions of testing. The proposed structure was aimed to decrease the effect of direct backing of test material by metal plate, which influences measurement accuracy if two-layer method is used. The structure permits a suitable electric field interrogation in test material and decreases the influences of both radial and surface waves. FDTD method was adapted for modeling the problem geometry to calculate the reflection coefficients since a probe with finite flange size is used. Measurements were carried out using the proposed technique to determine complex permittivity and complex permeability of several radar absorbers over X-band applications of microwaves. In comparison with both single-layer and two-layer methods results, the measured results of these parameters agreed well with the published data. by companies and literatures.

Multilayer Structure Technique for Improving Determination of Electromagnetic Properties of Radar Absorbers Based on Two-Layer Method and Flanged Rectangular Waveguide Probe

This paper presents further development of utilization of two-layer method to perform nondestructive electromagnetic properties determination of planar radar absorbers using flanged open-ended rectangular waveguide probe. A multilayer structure of three layers was proposed to improve the measured results of these parameters obtained using two-layer method. These layers were arranged such that the test material is sandwiched between two known low loss materials to provide the needed two independent reflection coefficients necessary to extract them at different conditions of testing. The proposed structure was aimed to decrease the effect of direct backing of test material by metal plate, which influences measurement accuracy if two-layer method is used. The structure permits a suitable electric field interrogation in test material and decreases the influences of both radial and surface waves. FDTD method was adapted for modeling the problem geometry to calculate the reflection coefficients since a probe with finite flange size is used. Measurements were carried out using the proposed technique to determine complex permittivity and complex permeability of several radar absorbers over X-band applications of microwaves. In comparison with both single-layer and two-layer methods results, the measured results of these parameters agreed well with the published data. by companies and literatures.

Dual-Polarized Microwave Imaging Probe

Near-field microwave imaging is increasingly finding utility for non-invasive inspection of various critical components in a wide range of industries. Conventionally, microwave imaging is performed using probes of linear electric field polarization such as a rectangular aperture probe excited with its dominant mode. Consequently, detecting polarized targets, e.g., cracks and highly conductive linear inserts, becomes challenging and highly dependent upon the orientation of the target with respect to the irradiating electric field vector. In practice, the orientation of the target could be unknown a priori. Hence, selecting a single suitable polarization is not always feasible. To address this challenge, a dual-polarized Teflon-loaded circular aperture-based microwave probe is proposed herein for imaging applications at 24 GHz. The proposed probe irradiates the target with two orthogonal polarizations and allows reconstructing the image using the average of the reflected signals due to these polarizations. The scattering parameters and electric field distribution of the proposed probe are analyzed using numerical electromagnetic simulations. A prototype of the proposed probe is fabricated and its characteristics are measured experimentally. The efficacy of the proposed imaging probe is demonstrated by acquiring phase and magnitude images of highly polarized targets. The images obtained using the proposed probe are compared to those obtained using a conventional open-ended rectangular waveguide probe. It is shown that unlike the conventional rectangular waveguide probe, the proposed probe could effectively detect polarized targets irrespective of their orientation.

Steel Corrosion Stages Characterization Using Open-Ended Rectangular Waveguide Probe

Safety evaluation of steel structures requires knowledge of corrosion progression stages. The deteriorative stage of corrosion involves multiple parameters, and thus it is difficult to be characterized by the model-based approaches. In this paper, we propose a steel corrosion stages characterization method using microwave open-ended rectangular waveguide (ORWG) probes and a statistical-based principal component analysis (PCA) method. Two ORWG probes operating in successive bands, ranging between 9.5 and 26.5 GHz, are utilized to obtain reflection coefficient spectra from specific sets of corrosion samples; i.e., uncoated corrosion progression, coated corrosion progression, and surface preparation. PCA is applied to extract corrosion progression feature from spectral responses of training samples. The robustness of the PC-based features is analyzed with influences of operating frequency, coating layer, and surface condition. It is found that the corrosion features extracted by the first principal component (PC1) from coated and uncoated corrosion samples are highly correlated to the corrosion progress regardless of probe parameter and coating layer.

Wideband Microwave Reflectometry for Rapid Detection of Dissimilar and Aged ICs

A wideband microwave method is described as a means for rapid detection of slight dissimilarities and aging effects in integrated circuits (ICs). The method is based on measuring the complex reflection coefficient of an IC when illuminated with an open-ended rectangular waveguide probe, at K-band (18–26.5 GHz) and Ka-band (26.5–40 GHz) microwave frequencies. The spatially integrated reflected electromagnetic signature of a given IC is a function of its internal material properties, geometry and metallic deposition of circuit element, and wire bonds. Consequently, dissimilar (including counterfeit) and aged ICs exhibit markedly different reflection properties than their reference and new (nonaged) counterparts. In addition to measuring spatially integrated complex reflection coefficient (over the waveguide aperture), it is also integrated over the operating frequency band (spectrally integrated) , resulting in significant increase in the robustness of the approach. Root-mean-squared-error, defined as the average Euclidean distance between two reflection coefficient vectors, is used to associate a quantitative metric to the complex reflection coefficient difference between two dissimilar ICs. Measurement results on several sets of ICs having the same package (DIP-14) and different functionalities, similar functionalities with slight differences in specifications, and aged ICs are presented. The results clearly indicate the capability of this method to differentiate among ICs having slight differences in packaging material properties and/or electronic circuitry.

Analysis of Flanged Rectangular Waveguide Probe for Nondestructive Absorbing Materials Characterization Using FDTD Simulation

In this paper, the flanged open-ended rectangular waveguide probe technique is studied using Finite Difference Time-Domain simulation (FDTD). Both generally lossy and high loss electromagnetic materials are considered to investigate the influence of probe flange size, operating frequency and sample thickness on complex permittivity (er) and permeability (μr) and thickness measurement. Variations in the probe flange size for different frequencies, material under test type and thickness are simulated. It is found that using of waveguide probe with finite flange affects probe input reflection coefficient substantially in some cases. To verify the obtained simulations results, a series of experiments are conducted for this purpose. Both er and μr of material under test under different measurement conditions are extracted using FDTD modeling and compared with reference data. In order to evaluate the degree of accuracy of this technique, error analysis to various sources of errors and most importantly the effect of finite flange size are also demonstrated by using the measured data compared with the analytical model results. Simulations and measurements results have shown that the consideration of probe flange large enough for the practical purpose to be infinite is restricted by the constitutive parameters (er and μr) and operating frequency as well as the thickness of the material under test. The FDTD simulations and experiments results are presented.

Modified Waveguide Flange for Evaluation of Stratified Composites

Nondestructive evaluation of stratified (layered) composite structures at microwave and millimeter-wave frequencies is of great interest in many applications where simultaneous determination of the complex dielectric properties and thicknesses of multiple layers is desired. Open-ended rectangular waveguide probes, radiating into such structures, are effective tools for this purpose. The technique utilizes a full-wave electromagnetic model that accurately models the complex reflection coefficient as a function of frequency and material properties. While the electromagnetic model assumes an infinite waveguide flange (or ground plane), the measurements are conducted using a finite-sized flange. Consequently, the results of the electromagnetic model and those from measurements may not be sufficiently alike for accurate dielectric property and thickness evaluation. This paper investigates the effect of using an open-ended waveguide with a standard finite-sized flange on the error in evaluating the complex dielectric properties of a composite structure. Additionally, we present the design of a novel flange that markedly reduces this undesired effect by producing very similar electric field properties, at the flange aperture, to those created by an infinite flange. Finally, the efficacy of the design for evaluating the dielectric properties of a layered composite structure is demonstrated as well.

An open-ended substrate integrated waveguide probe for detection and sizing of surface cracks in metals

This paper describes the development of a novel microwave probe system for detection and sizing of surface cracks in metals. The probe, which is based on the substrate integrated waveguide (SIW) technology, consists of five parts, namely, a microwave oscillator, an envelope detector, a microstrip line, a transition section, and an open-ended SIW. To evaluate the performance of the probe, theoretical and experimental results are presented for the scans of two metallic specimens containing long surface cracks. It is shown that the SIW probe is capable of detecting and sizing surface cracks of a few millimeters deep while being relatively compact and light as compared to the conventional open-ended rectangular waveguide probes.

Open-Ended Waveguide Measurement and Numerical Simulation of the Reflectivity of Petri Dish Supported Skin Cell Monolayers in the mm-wave Range

Open-ended waveguide reflectometry is a promising tool for permittivity and other material properties calculation at mm-waves (30–300 GHz). Measurement of the reflection coefficient does not require sample manipulation, allowing in vivo and in vitro non destructive studies on cells. Here we used this technique for measuring the power reflection coefficient (reflectivity) of water and Petri dish supported human skin melanoma and keratinocyte cell cultures, in the 53–72 GHz frequency range. The dependence of the reflectivity on polystyrene or glass thickness of the Petri base plate and on the cell layer thickness was analyzed. Permittivity data were then easily retrieved by using a plane wave-dominant mode approach for formulating the reflectivity at the aperture of the flange-mounted open-ended rectangular waveguide probe. Limits and validity of such an approximate approach were analyzed and compared with full-wave near field formulations for which magnitude and phase of the reflection coefficient must be measured and solved using complicated systems of integral equations and extensive numerical calculation. Finally, Petri dish reflectivity measured by the open-ended waveguide method was compared with that numerically simulated under far-field exposure conditions used in a large number of in vitro studies. Such an analysis showed that, under certain conditions, open-ended reflectivity values approach the far field ones.

Simulation for Microwave Nondestructive Detection of Thickness and Delamination in Layered-Dielectric in Metallic Substrate

Microwaves penetrate inside of low loss dielectric materials and they are sensitive to the presence of internal interfaces and non-uniformities. This allows microwave nondestructive inspection techniques to be utilized for inspecting dielectric in metallic substrate. This article simulated Microwave inspecting thickness and delamination in layered-dielectric in metallic substrate, using open-ended rectangular waveguide probe. Effective reflection coefficient of microwave is used in the detection and evaluation to thickness or delimination in the media. This paper optimized the detection frequency and standoff, which provide a reference for the experimental study.

Output signal prediction of an open-ended rectangular waveguide probe when scanning cracks at a non-zero lift-off

The paper proposes a modeling technique for output signal prediction of a rectangular waveguide probe with finite flange when scanning a surface long crack in a metal at a finite lift-off. The modeling technique approximates the crack–probe interaction with a two-dimensional problem. In this problem, a parallel-plate waveguide with finite flange scans a long crack in a perfect conductor. The method of moments is employed to solve the governing electric field integral equation. The solution provides the reflection coefficient in the parallel-plate waveguide from which the probe output signal is obtained. The main feature of the model is solving the three-dimensional problem in a two-dimensional frame work, thus reducing the degree of complexity and computation time. To validate the accuracy of the model, several simulation results are presented at an operating frequency in the X-band and are compared with their experimental counterparts. To demonstrate the efficiency of the model, we compare our results with those obtained using the well-known HP-HFSS finite element code. It is shown that the proposed model requires less than half of the time taken to solve the same problem running on the same computer.

Near-Field Millimeter-Wave Imaging of Exposed and Covered Fatigue Cracks

In this paper, the efficacy of near-field millimeter-wave nondestructive techniques, using open-ended flange-mounted rectangular waveguide probes, for extracting information of 3-D crack area deformation (i.e., in-plane and out-of-plane deformation) is demonstrated. It is shown that this information can be obtained from indications of unique interference patterns that are generated between the probe and the metal surface during the raster scan of a surface-breaking exposed and covered fatigue crack using a phase-sensitive reflectometer.

Modified $\theta $-Scanning Technique for First/Third-Order Probes for Spherical Near-Field Antenna Measurements

This paper presents a modified thetas-scanning technique for first/third-order probes for spherical near-field antenna measurements. Unlike the traditional thetas-scanning technique, this new scanning technique gives a possibility, due to the cutoff property of the azimuthal spherical modes, to filter out the influence of the undesired third-order azimuthal modes of the probe near the poles of the measurement sphere. Without increasing the measurement time, the technique allows reducing the errors caused by the application of the computationally efficient first-order probe correction technique (incorrectly) to first/third-order probes. The technique is optimal for spherical near-field systems, that employ, for example, an open-ended rectangular (or square) waveguide probe, that closely approximates a first/third-order probe, and apply the well-known first-order probe correction technique. In such systems, compared to the traditional thetas-scanning technique, the modified thetas-scanning technique can be used to significantly reduce the errors caused by the incomplete probe correction.

Innovative Near-Field Microwave Nondestructive Testing of Corroded Metallic Structures Utilizing Open-Ended Rectangular Waveguide Probes

An innovative method utilizing microwave nondestructive testing and evaluation (NDT&E) techniques for enhanced detection and evaluation of rust under paint has been developed. The results are promising, and they show that the use of a layer of dielectric material with known properties to replace the standoff layer (air) yields significant improvements in the measurement sensitivity and resolution for some microwave NDT applications. The goals of this investigation were pursued and achieved using theoretical electromagnetic computer programs describing the interaction of microwave signals radiated out of an open-ended rectangular waveguide with any layered medium. One of the computer programs calculates the aperture reflection coefficient (simulating measurements), and the other computer program calculates the field distribution in any layered media. The experimental results were obtained using an inspection system that was built according to the results of the theoretical investigation, and they confirmed the theoretical results.

Interaction of an Open-Ended Rectangular Waveguide Probe With an Arbitrary-Shape Surface Crack in a Lossy Conductor

This paper presents a modeling technique to predict the output signal of an open-ended rectangular waveguide probe when scanning an arbitrary-shape crack in a lossy conductor with finite conductivity. The loss of the specimen is taken into account by using surface impedance at the surface. The technique discretizes the crack shape and applies the generalized scattering matrix technique to obtain the output signal. To validate the model proposed in this paper, the results of the proposed method are compared with the measurement results and those obtained using a finite-element code. The model is used to obtain appropriate inversion curves for determining crack depth measurement from the probe output signal.

Semiempirical Electromagnetic Modeling of Crack Detection and Sizing in Cement-Based Materials Using Near-Field Microwave Methods

Detection and characterization of cracks in cement-based materials is an integral part of damage evaluation for health monitoring of civil structures. Microwave signals are able to penetrate inside of dielectric materials (e.g., cement-based materials) and are sensitive to local, physical, geometrical, and dielectric variations in a structure. This makes microwave nondestructive testing and evaluation (NDT&E) techniques suitable for inspection and health monitoring of civil structures. Near-field microwave NDT&E techniques offer the added advantage of providing high spatial resolution, requiring simple hardware that may be portable, low power, fast, real time, and robust. Additionally, these techniques are noncontact and one-sided. Besides the need for robust detection, electromagnetic modeling of a microwave probe response to a crack is also an important issue. Such a model can be used to obtain optimal measurement parameters and serve as the foundation for extracting important crack information such as its width and depth. In this paper, the utility of open-ended rectangular waveguide probes for detecting surface-breaking cracks in cement-based materials is discussed. Subsequently, the development of a semiempirical model capable of simulating the crack response is presented. The model described here translates the magnitude and phase of the reflection coefficient as a function of scanning distance into the complex reflection plane and takes advantage of the common shape of these signals for predicting a similar signal from an unknown crack. Finally, this empirical model is used to estimate crack dimensions from a set of measurements.

Near-field microwave image formation of defective composites utilizing open-ended waveguides with arbitrary cross sections

Many nondestructive testing techniques fail to detect the presence of defects inside composite materials due to their complex nature. Microwave near-field nondestructive testing utilizing open-ended rectangular waveguides showed promising results in detecting defects inside these composite structures. Carbon-loaded composites are lossy materials, which, in general, reduce the range of frequency that can be used to inspect these kinds of composites due to the high attenuation. Thus far, open-ended rectangular waveguide based probes have been used to inspect the structural integrity and homogeneity of these structures. These probes were found to be suitable for detecting defects in these composites. However, due to the linear polarization of the fields radiated out of open-ended rectangular waveguide probes and the linear nature of the carbon fibers, high attenuation of the radiated signal is inevitable. Consequently, only close to the surface defects were detected. Circular waveguides have not been used in near-field NDT. Circular waveguides offer many attractive features such as the circular polarization of the radiated waves. This fact might be used in penetrating deeper in high-loss composites. In this paper, the potential of utilizing rectangular and circular waveguides in inspecting defects in carbon-loaded composites will be theoretically investigated. An air void will be introduced inside both types of composites and theoretical images will be formed. Interpretation of the captured images will be done through power patterns in different planes.