Karl J. Bois

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...

Dielectric Mixing Models for 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 techniques [1]. The major disadvantage of these techniques is their limited accuracy, and the fact that they are not totally nondestructive.

Simple and accurate determination of complex permittivity and skin effect of FR4 material in gigahertz regime

In this paper, a simple and accurate measurement method of dielectric properties for printed circuit board (PCB) materials will be presented. Here a method is proposed that only requires measurement of scattering parameters for two transmission lines of different lengths etched in FR4 with identical cross-sections. Additionally, the connectors associated with these lines are readily de-embedded fiom the measurement (e.g. SMA connectors or plated-through holes (PTH)). Moreover, the conductor losses can he easily separated from the dielectric losses. Results of dielectric properties for three different types of FR4 material from varying vendors will be provided up to 8.5 GHz.

The digital designer's complete lossy transmission line model

Presents a technique for simulation of the lossy behavior of transmission line elements that addresses the most common problems that digital designers encounter when called upon to simulate links operating above 500 Mb/sec. Models that seek to represent the distributed losses of the transmission lines with a multiple section RLC network run into complexity related problems when the number of sections exceeds about 100. The SPICE integration routine begins to insert its own oscillations and may fail to converge, yielding no answer or at best a loss of time when the user tries various methods to achieve convergence. In addition, the time to evaluate the SPICE model can become an annoyance or a real time sink for the designer. Alternative methods using S-parameters or Laplace transforms are often compute intensive as well. The model presented addresses the above problems by reducing the number of elements to a minimum with innovative circuit design, and by using appropriate approximations. It can be implemented in any SPICE version, and it works unchanged in both AC and transient modes.

A Decision Process Implementation for Microwave Near-Field Characterization of Concrete Constituent Makeup

Determination of concrete compressive strength is one of the most important factors in the construction industry. Concrete is a mixture of cement, water, sand (fine aggregate) and rock (coarse aggregate) of various sizes. Water and cement chemically bind to form the cement paste phase of concrete, which primarily influences the compressive strength. The aggregate content mainly acts as a filler but also influences the compressive strength of concrete. Currently, there is no single method for nondestructive determination of compressive strength of concrete. As a result, the only reliable method of compressive strength measurement is to core a specimen from an existing structure and perform a compressive failure test. In recent years, studies on the reflection properties of concrete with varying material content have been performed using microwave nondestructive testing techniques. These techniques, employing open-ended rectangular waveguide probes, have shown tremendous potential for determining concrete constituent makeup. The results have demonstrated that microwave reflection property measurements of concrete as a function of varying constituent makeup, possess statistical properties that provide for a unique “fingerprint”. In this paper a simple decision process is proposed, based on the maximum likelihood ratio, which uses the a priori statistical distribution of microwave reflection measurements for material content determination of concrete specimens. The results show that by simply performing two sets of microwave reflection property measurements at 3 and 10 GHz, the constituent makeup of an unknown concrete specimen can be very closely determined. Subsequently, the compressive strength of the specimen could be easily determined from a simple lookup table relating constituent makeup and compressive strength, which are readily available in the industry.

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...

Frequency domain analysis of the multi-tap driver in high speed links

This paper presents a method, based upon channel frequency response, for determining tap settings for multitap link driver circuits. Optimization is based upon creating a flat magnitude response over frequency = 0 to 1/2 data rate.

A simplified cross coupling model for multiple balanced transmission lines

Modeling of the coupling between transmission lines in the practical sense has been an elusive quarry. Since the favorite tool of signal integrity designers is SPICE, the ultimate goal should be translation of the mathematics of coupled mode analysis into a simple SPICE formulation. This investigation presents a method to simulate the parasitic coupling between transmission lines in structures where complete isolation is impractical. The conventional mathematical analysis of such coupling does not suggest a simple SPICE solution, so a farther analysis of cause-effect and elimination of unimportant terms resulted in a set of models which uses the transmission line capacitance matrix to create a direct input of parameters to a SPICE subcircuit. This SPICE subcircuit is utilized as an attachment to a previously designed lossy transmission line circuit. The resultant circuit models cross coupling originating from any number of culprits affecting one victim. The circuit requires one victim line and one culprit line, where the assumption is made that all culprit lines carry the same signal. The technique is applicable to single ended and differential pair transmission lines. An HSPICE subcircuit was included to illustrate the ultimate form of the model.