Aaron D. Benally

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.

Microwave Nondestructive Determination of Sand-to-cement Ratio in Mortar

Abstract Microwave nondestructive testing methods have shown great promise for the inspection of cement-based materials. Previously it has been shown that the magnitude of reflection coefficient from cement paste specimens can be correlated to their water-to-cement (w/c) ratios and more importantly to their compressive strengths. For mortar specimens, the sand-to-cement (s/c) ratio plays an important role in its physical, mechanical, and microwave reflection properties. To determine the w/c ratio and other properties of mortar specimens, one also needs to know its s/c ratio. To illustrate the ability of microwave reflection measurements for this purpose, two sets of four mortar specimens are produced with w/c ratios of 0.50 and 0.60 and with s/c ratios of 1.0, 1.5, 2.0, and 2.5. The microwave reflection coefficients of these specimens are measured using an open-ended rectangular waveguide sensor, in the G- (3.95–5.85 GHz), J- (5.85–8.2 GHz), and X-band (8.2–12.4 GHz) frequency ranges. It is shown that a s...

Embedded modulating dipole scattering for near-field microwave inspection of concrete: preliminary investigations

Nondestructive evaluation of concrete structures is an important practical issue in the construction industry. Evaluation encompasses many issues including concrete constituent property, compressive strength and chloride contamination determination to name a few. To this end, a combination of modulated scattering technique and near-field microwave nondestructive evaluation technique, is used to determine its potential for evaluating dielectric properties of a hardened mortar specimen. This technique utilizes a small resonant PIN diode-loaded dipole scatterer embedded inside the mortar while using an open-ended rectangular waveguide probe operating at 7 GHz to detect this dipole which is modulated at low frequencies. The results of this preliminary experiment, and its future ramifications for nondestructive concrete inspection structures are provided in this paper.

Microwave nondestructive detection of chloride in cement based materials

Preliminary results pertaining to the near-field microwave nondestructive detection and evaluation of chloride in cement paste and mortar specimens are presented. The technique used for this purpose utilizes an open-ended rectangular waveguide at the aperture of which the reflection properties of the specimens are measured. It is shown that the magnitude of reflection coefficient is a useful parameter for detecting chloride in these specimens. Furthermore, the difference in the amount of chloride present in these various specimens, at the time of mixing, can also be determined. Reflection property measurements were conducted in S-band (2.6 GHz-3.95 GHz) and X-band (8.2–12.4 GHz) for two sets of four mortar specimens with 0.50 and 0.60 water-to-cement ratio and varying salt (NaCl) contents added to the mixing water used in producing these specimens. It is shown that the reflection properties of these materials vary considerably as a function of their chloride content. Also, by monitoring the daily variatio...

Application of near-field microwave sensing techniques for segregation detection in concrete members

In this presentation, a simple, low-cost near-field microwave nondestructive inspection technique for segregation detection in concrete members is presented. This process employs information from the measured magnitude of reflection coefficient at the aperture of an open-ended rectangular waveguide sensor. These measurements, whose results will be presented, were conducted using a Hewlett-Packard HP8510B network analyzer. However, in practice a simple and relatively inexpensive inspection apparatus constructed from discrete microwave components can easily be employed. It is shown that the standard deviation of magnitude of reflection coefficient measurement is linearly correlated with the aggregate density in concrete. Furthermore, for concrete in which the aggregate has segregated, this measurable parameter will change as a function of vertical position of the microwave scan. Results correlating the microwave measurements to the actual aggregate density of a well consolidated concrete specimen and a spec...

Two-Port Network Analyzer Dielectric Constant Measurement of Granular or Liquid Materials for the Study of Cement Based Materials

The construction industry has a keen interest in using a nondestructive, real-time, reliable and inexpensive technique for the in-place evaluation of the compressive strength of concrete structures. Compressive strength of concrete is usually determined by drilling a core and testing it in a laboratory. This method is relatively expensive, and it may take a few days for the results to be known. In addition, this method is destructive. Consequently, several nondestructive techniques have been developed for this purpose. These include: pulse velocity method, surface hardness, penetration, pullout, breakoff and maturity techniques1.

Near-field microwave reflection property analysis of concrete using open-ended rectangular waveguides at S- and X-band

Near-field nondestructive reflection property characterization of concrete based materials using open-ended rectangular waveguide probes is presented. The statistical distribution of multiple reflection property measurements are used as a tool to determine the water-to-cement (w/c) ratio as well as the coarse aggregate volumetric distributions in several concrete mixtures. These results may be used to determine the compressive strength of concrete. In this effort, concrete specimens with 0.50 w/c ratio were used. For these specimens sand-to-cement (s/c) ratio of 1.0 was used along with coarse aggregate-to-cement (ca/c) ratios of 1.0, 1.5, and 2.0, respectively. The microwave reflection coefficient of these specimens were measured at x-band (10 GHz) and S-band (3 GHz) using an HP8510 vector network analyzer. The results of these measurements, from the point of view of their probability distribution functions are presented, in addition to discussions regarding their implementation for nondestructive characterization of concrete based materials.

Dielectric properties of concrete at S and X bands: a near-field investigation

When inspecting concrete structures with microwaves (radars, embedded microwave sensors, modulated scattering techniques, etc.) the dielectric properties of the concrete are considered as a ground truth data and must be known. During the past three years, extensive microwave near-field measurements of the reflection properties of concrete specimens with varying water-to-cement (w/c) ratios, sand-to-cement (s/c) ratios and coarse aggregate-to-cement (ca/c) ratios have been conducted. These experiments were conducted using open-ended rectangular waveguide probes radiating into a half-space of these concrete specimens. These measurements were conducted at S- (2.6 - 3.95 GHz) and X-bands (8.2 - 12.4 GHz). Moreover, an electromagnetic model, which took into account the presence of higher-order modes at the waveguide aperture, was also used to model this process. Finally, a root finding technique was applied to calculate the effective dielectric properties of the concrete specimens. This paper presents the results of these measurements and calculations as they related to determining the dielectric properties of concrete. Since concrete is a heterogeneous material, the results from many locations in a specimen are reported rendering effective dielectric properties showing the mean and standard deviation of the measurements and calculations at these frequency bands. The results of the dielectric constant can also be used to predict the reflection properties of concrete when using a standoff distance (i.e. non-contact measurements) or when using other types of microwave sensors.

A Multi-Mode Solution for Analysis of the Reflection Coefficient of Open-Ended Rectangular Waveguides Radiating into a Dielectric Infinite Half-Space

The open-ended rectangular waveguide probe is a powerful tool for characterizing the dielectric and reflection properties of various materials and structures [1–11]. In these applications the measurable parameter, Г(i.e. the reflection coefficient), is used to determine the sought-for parameters. The said reflection coefficient is defined as the ratio of the reflected electric field at the aperture of the waveguide to that of the incident electric field.