Reza Zoughi

Open-ended rectangular waveguide for nondestructive thickness measurement and variation detection of lossy dielectric slabs backed by a conducting plate

Solutions for fields inside a slab of a generally lossy dielectric medium backed by a conducting plate, placed outside a waveguide-fed rectangular aperture, are used for the microwave nondestructive thickness measurement of such dielectric slabs. Upon construction of the waveguide terminating admittance expression from its variational form, an inverse problem is solved to extract the slab thickness form the conductance and susceptance in a recursive manner. A comparison between the experimental and theoretical results showed that the significance of higher order modes is minimal; hence, the dominant mode assumption is, in general, valid for describing the aperture field distribution. The validity of this assumption has led to the construction of a simple integral solution which is fast converging for generally lossy dielectric slabs, and may easily be implemented for real-time applications. Good agreement was obtained between the theoretical and experimental results. Multiple thicknesses of two different dielectric samples were estimated in this way. >

Multimode solution for the reflection properties of an open-ended rectangular waveguide radiating into a dielectric half-space: the forward and inverse problems

Open-ended rectangular waveguides are extensively used in nondestructive dielectric material evaluation. The dielectric properties of an infinite-half space of a material are calculated from the measured reflection properties referenced to the waveguide aperture. This calculation relies on a theoretical and numerical derivation of the reflection coefficient likewise referenced to the waveguide aperture. Most of these derivations assume the dominant mode field distribution across the waveguide aperture. However, when dealing with low permittivity and low loss dielectric materials, there may exist significant errors when calculating the dielectric properties from the measured reflection coefficient. These errors have also shown to be more significant in the upper frequency portion of a waveguide band. More accurate results are obtained when higher order modes are considered in addition to the dominant waveguide mode. However, most studies incorporating higher-order modes have used various approximations when calculating the reflection properties and have not provided a full discussion on the influences of dielectric properties of the infinite-half space and the frequency of operation. This paper gives a rigorous and exact formulation in which the dominant mode and the evanescent higher-order modes are used as basis functions to obtain the solution for the reflection coefficient at the waveguide aperture. The analytic formulation uses Fourier analysis in addition to the forcing of the necessary boundary conditions at the waveguide aperture. The solution also readily accounts for the complex contributions of both TE and TM higher-order modes. Finally, the influences of the dielectric properties of the infinite-half space and the frequency of operation are investigated.

Portable Real-Time Microwave Camera at 24 GHz

This paper presents a microwave camera built upon a two-dimensional array of switchable slot antennas. The camera borrows from modulated scattering techniques to improve isolation among the array elements. The camera was designed to measure vector electric field distribution, be compact, portable, battery operated, possess high dynamic range, and be capable of producing real-time images at video frame-rate. This imaging system utilizes PIN diode-loaded resonant elliptical slot antennas as its array elements integrated in a simple and relatively low-loss waveguide network thus reducing the complexity, cost and size of the array. The sensitivity and dynamic range of this system is improved by utilizing a custom-designed heterodyne receiver and matched filter for demodulation. The performance of the multiplexing scheme, noise-floor and dynamic range of the receivers are presented as well. Sources of errors such as mutual-coupling and array response dispersion are also investigated. Finally, utilizing this imaging system for various applications such as 2-D electric field mapping, and nondestructive testing is demonstrated.

MICROWAVE NONDESTRUCTIVE ESTIMATION OF CEMENT PASTE COMPRESSIVE STRENGTH

Dielectric and reflection properties of our cement paste samples with water-cement ratios (w/c) of 0.31, 0.40, 0.50, 0.60 were measured daily for 27 days using microwave frequencies of 4, 9, 13, and 17 GHz. The dielectric properties of these samples, and hence their reflection coefficients, decreased as a function of increasing w/c. This is opposite to what is expected for physical mixing (no chemical interaction or hydration). The presence of curing as evidenced by this result indicates that microwaves could be used to monitor the amount of curing of a concrete member. The variation in the reflection coefficient of these samples as a fucntion of w/c followed a trend similar to the variation of compressive strength as a function of w/c. Subsequently, a correlation between compressive strength and reflection coefficient of these blocks was obtained. This result can be used to directly and nondestructively estimate the compressive strength of a cement paste block, thus opening up the possibility of nondestructively testing the compressive strength of concrete.

Modeling of surface hairline-crack detection in metals under coatings using an open-ended rectangular waveguide

A surface-breaking hairline crack or a narrow slot in a metallic specimen when scanned by an open-ended rectangular waveguide probe influences the reflection-coefficient properties of the incident dominant mode. Subsequent recording of a change in the standing-wave pattern while scanning such a surface results in what is known as the crack characteristic signal. Since microwave signals penetrate inside dielectric materials, this methodology is capable of detecting cracks under dielectric coatings of various electrical thicknesses as well. To electromagnetically model the interaction of an open-ended rectangular waveguide with a surface-breaking hairline crack under a dielectric coating, the dielectric-coating layer is modeled as a waveguide with a large cross section. Thus, the problem is reduced to a system of three waveguides interacting with each other while the location of the crack is continuously changing relative to the probing waveguide aperture (a dynamic scanning problem). An analysis of modeling the dielectric-coating layer as a dielectric-filled waveguide with a large cross section is given, and its comparison with radiation into an unbounded medium is presented. For obtaining the reflection coefficients of the dominant and higher order modes, the electromagnetic properties of the probing waveguide-dielectric-coating layer junction and the dielectric-coating layer-crack junction are separately analyzed. For each junction, a magnetic-current density M is introduced over the common aperture. Subsequently, the junction formed by the two respective waveguide sections is separated into two systems. A numerical solution employing the method of moments is obtained, and the properties of the junctions are expressed by their respective generalized scattering matrices. Consequently, the generalized scattering matrix for the total system can be evaluated. The convergence behavior of the system is studied to determine an optimal set of basis functions and the optimal number of higher order modes for a fast and accurate solution. Finally, the theoretical and measured crack characteristic signals are compared.

Multimodal Solution for a Waveguide Radiating Into Multilayered Structures—Dielectric Property and Thickness Evaluation

Open-ended rectangular waveguides are widely used for microwave and millimeter-wave nondestructive testing (NDT) applications, such as detecting disbond and delamination in multilayered composite structures, thickness evaluation of dielectric sheets and coatings on metal substrates, etc. When inspecting a complex multilayered composite structure that is made of generally lossy dielectric layers with arbitrary thicknesses and backing, the dielectric properties of a particular layer may be of particular interest (e.g., radome inspection). The same is also true when one is interested in the thickness, or, more importantly, thickness variation, of a particular layer within such complex structures. An essential tool for closely estimating the complex permittivity and/or thickness is an accurate forward electromagnetic model for simulating the reflection coefficient at the aperture of the probing open-ended waveguide. To this end, this paper provides a full-wave accurate forward model for calculating the reflection coefficient from a generally lossy multilayered composite structure possessing an arbitrary number of layers and respective thicknesses while accounting for the influence of higher order modes. This model is subsequently validated through comparisons with a commercial numerical tool and actual measurements. Furthermore, a measurement model is provided, which results in an iterative inverse technique for estimating the complex permittivity and thickness of a dielectric layer. Subsequently, this technique is applied to the measured reflection coefficients of several structures. To evaluate the accuracy of this technique, an analysis on its sensitivity to various sources of errors, and, most importantly, the effect of a finite flange size, is also demonstrated by using the simulated data. Finally, the potential of this model to accurately estimate the thickness of an individual layer, which represents a thin disbond, in a multilayered composite structure is presented.

Millimeter-wave detection of localized anomalies in the space shuttle external fuel tank insulating foam

The Space Shuttle Columbias catastrophic accident emphasizes the growing need for developing and applying effective, robust, and life-cycle-oriented nondestructive testing (NDT) methods for inspecting the shuttle external fuel tank spray on foam insulation (SOFI). Millimeter-wave NDT techniques were one of the methods chosen for evaluating their potential for inspecting these structures. Several panels with embedded anomalies (mainly voids) were produced and tested for this purpose. Near-field and far-field millimeter-wave NDT methods were used for producing images of the anomalies in these panels. This paper presents the results of an investigation for the purpose of detecting localized anomalies in several SOFI panels. To this end, continuous-wave reflectometers at single frequencies of 33.5, 70, or 100 GHz representing a relatively wide range of millimeter-wave spectrum [Ka-band (26.5-40 GHz) to W-band (75-110 GHz)] and utilizing different types of radiators were employed. The resulting raw images revealed a significant amount of information about the interior of these panels. However, using simple image processing techniques, the results were improved in particular as it relates to detecting the smaller anomalies. This paper presents the results of this investigation and a discussion of these results.

Optimum Two-Dimensional Uniform Spatial Sampling for Microwave SAR-Based NDE Imaging Systems

Microwave imaging systems for nondestructive evaluation, based on 3-D synthetic aperture radar (SAR) techniques, utilize either a real aperture, composed of many antennas mounted next to one another, or a synthetic aperture, generated by raster scanning a single antenna. To obtain a quality SAR image, the spatial sampling must be dense enough to accurately sample the electric field reflected from a target. Conversely, the quantity of spatial samples may be optimally reduced, resulting in reduced system complexity and required resources for systems employing real apertures and reduced imaging time for synthetic aperture systems. In the literature, it has been reported that the optimum sampling step size is equal to the theoretical resolution, as per the Nyquist rate. It has also been reported that an image generated using a sampling step size equal to the theoretical resolution may not possess the same spatial resolution as predicted. Also, as expected and reported, resolution is dependent upon the distance between the target and the aperture, aperture dimensions, and antenna beamwidth. However, existing formulations of SAR resolution do not account for all of the physical characteristics of a measurement (e.g., 2-D limited-size aperture, electric field decreasing with distance from the measuring antenna, etc.). This paper presents a theoretical formulation of resolution and a study into optimum uniform spatial sampling by analyzing simulated 3-D SAR images according to metrics representing image quality, namely, half-power resolution and RMS error between practically sampled images and an ideally sampled image. The results of this simulation demonstrate optimum sampling given design requirements that fully explain resolution dependence on sampling step size. Also, it is found that there is additional widening of the 2-D spectral estimation of the data due to the aperture-limited nature of the measurements, which further influences the choice of sampling step size. Subsequently, the simulated results are compared to experimental results corroborating the efficacy of the formulation. Finally, design curves and procedures are proposed for selecting sampling step size as per resolution requirements.

Microwave Detection and Depth Determination of Disbonds in Low-Permittivity and Low-Loss Thick Sandwich Composites

Nondestructive evaluation (NDE) of disbonded low-permittivity and low-loss dielectric multilayered composite media is of considerable interest in many applications. The ability of microwaves to penetrate inside dielectric materials makes microwave NDE techniques very suitable for interrogating structures made of multilayered dielectric composites. Additionally, the sensitivity of microwaves to the presence of dissimilar layers in such materials allows for accurate detection of a disbonded layer. In a multilayered composite, a disbond may occur between any two (or more) layers. The potential of utilizing microwave NDE techniques for the detection and depth estimation of disbonds in a thick sandwich composite is investigated. This study utilizes a theoretical model developed for investigating the interaction of microwave radiation from an open-ended rectangular waveguide sensor with ann-layer dielectric composite medium. The influence of the standoff distance between the sensor and the medium and the operating frequency on the sensitivity of disbond detection and depth estimation is studied to obtain an optimum set of parameters for enhanced detection sensitivity. Results of the theoretical study are presented with a discussion on the optimization process for a thick sandwich composite composed of 13 dielectric layers.

Terrain classification in SAR images using principal components analysis and neural networks

The development of a neural-network-based classifier for classifying three distinct scenes (urban, park, and water) from several polarized SAR images of the San Francisco Bay area is discussed. The principal components (PC) scheme or Karhunen-Loeve transform is used to extract the salient features of the input data, and to reduce the dimensionality of the feature space prior to the application to the neural networks. Using the PC scheme along with the polarized images used in the present study led to substantial improvements in the classification rates when compared with previous studies. When a combined polarization architecture was used, the classification rate for water, urban, and park areas improved to 100%, 98.7%, and 96.1%, respectively. >