Devendra K. Misra

An X-Band Microwave Life-Detection System

An X-band microwave life-detection system has been developed for detecting the heartbeat and breathing of human subjects lying on the ground at a distance of about 30 m or located behind a cinder block wall. The basic principle of the system is to illuminate the subject with a low-intensity microwave beam, and then from the back-scattered microwave signal, extract the heart and breathing signals that modulate it. The circuit description of the system and some experimental results are presented. Potential applications of the system are noted.

Noninvasive electrical characterization of materials at microwave frequencies using an open-ended coaxial line: test of an improved calibration technique

Dielectric measurements using a probe consisting of a coaxial transmission line with an open-circuit end placed against the sample are discussed. For the 2.99- or 3.6-mm (OD) probes considered, a simple lumped parameter model shows errors above 1 GHz that increase greatly with frequency. An approximate model based on measured probe impedances from 1 to 18 GHz with samples consisting of water, methanol, and dioxane-water mixtures is evaluated. This model is more accurate than the lumped-parameter model and is better suited for calibration of the automatic network analyzer (ANA). The errors introduced in dielectric measurements by the use of approximate models for the probe are discussed. The technique succeeds because of partial cancellation of errors in modeling the probe in ANA-based measurements. >

An experimental technique for in vivo permittivity measurement of materials at microwave frequencies

A procedure for making in vivo dielectric measurements with an open-ended coaxial line probe is discussed. The theory behind the technique is discussed, along with a correction method to account for the system imperfections. Experimental results are compared with data for the lumped parameter model approach and a mathematically sound integral equation method available in the literature. The results show good agreement, with the exception of the lumped parameter technique, which deviates at higher frequencies. The limitations of this technique are considered. >

The co-axial aperture electromagnetic sensor and its application in material characterization

An open-ended co-axial line has been used as an electromagnetic sensor or probe for various industrial and scientific applications. These applications are based on the principle that the characteristics of the echo signal produced by the co-axial opening depend upon the sample material terminating the probe. Thus, if the aperture admittance (or reflection coefficient) characteristics can be precisely formulated, then the electrical parameters of the sample can be found. Several admittance models have been reported during the last two decades. A comparative study of these models is reported in this review. Furthermore, the inverse process of calculating the complex permittivity of materials is summarized.

Static analysis of an open-ended coaxial line terminated by layered media

A spectral domain formulation for the aperture fields of an open-ended coaxial line terminated by layered dielectrics is considered. This analysis is simplified for the case of electrostatic fields. The static capacitance and fields over the annular aperture are determined. The results of this study may be used in nondestructive electrical characterization of layered dielectrics, such as biological media. >

On the measurement of the complex permittivity of materials by an open-ended coaxial probe

A number of papers have appeared on the characterization of materials using a coaxial probe. In most cases, approximate admittance models have been used for this purpose. The present work compares two rigorous mathematical models for the aperture admittance. One of these relations is based on the variational principle while the other utilizes the method of moments. Mullers method is then used to determine the complex permittivity of materials. The two procedures have been tested in the frequency range of 1-18 GHz. >

A study on the coaxial aperture electromagnetic sensor and its application in material characterization

An open-ended coaxial line has been studied by many researchers in order to use it as an electromagnetic sensor for the characterization of materials. These studies include static as well as quasi-static formulations under various simplifying assumptions. In this paper, we use a full-wave formulation of the problem and solve an integral equation to evaluate the aperture field distribution. The electromagnetic field distributions in the material medium as well as the aperture admittance are evaluated subsequently. The effects of a conducting flange over the coaxial opening and the finite sample size are also studied. The results are compared with the experimental data as well as the data obtained by other approaches. An application of this coaxial sensor for the characterization of materials up to 40 GHz is demonstrated. >

A study on coaxial line excited monopole probes for in situ permittivity measurements

A coaxial line excited monopole probe has attracted many researchers for in situ permittivity measurement of materials. In most of these cases, a delta gap excitation is assumed for the probe in relating the measured input impedance to the permittivity of the material. Since an open-ended coaxial line is more accurately represented by a magnetic frill generator, we employed this model of excitation for the present study. This paper reports the results obtained by solving the Pocklingtons integral equation numerically using a magnetic frill excitation and piecewise sinusoidal Galerkin method. A comparison of these results with the available experimental and theoretical data is presented and some limitations of this approach are discussed.

In situ electric probe method for determining the dielectric properties of materials

An attempt is made to use the induced-EMF (electromagnetic field) method for input impedance determination of an antenna in order to compute the dielectric behavior of the medium over a large range of probe lengths. Numerical results obtained by this approach are compared with those of other existing theories valid for different probe lengths as well as with available experimental data. A fairly close agreement is found among these results. >

A study on the open-ended coaxial line method for measuring the permittivity of materials at microwave frequencies

The authors report the results of a study on the open-ended coaxial line method for measuring the complex permittivity of materials at microwave frequencies. The method is based on the quasi-static analysis of an open-ended coaxial line terminated by a semi-infinite medium. The complex permittivity of a 40% dioxane-water mixture at 25.1 degrees is evaluated for measured admittance over a frequency range of 1 to 18 GHz. The results obtained compare favorably with the available experimental and theoretical data. The sensitivity of the measurement to the standard used is shown by measurements of 20% dioxane-water mixture using two different sets of standards. The limitations of this technique are also discussed.<<ETX>>