Microwave imaging using ultra wideband frequency‐domain data

Abstract

This article describes an ultra wideband (UWB) microwave system that uses the frequency-domain reflection coefficient data acquired by a UWB antenna to form a microwave image of a homogenous cylindrical dielectric body containing cylindrical targets. Both hardware and signal processing aspects of the system are described. The system uses a frequency-domain image reconstruction algorithm using the different data obtained for adjacent positions of the scanning antenna so that the effect of air-image body interface is removed. The algorithm is tested in examples of a cylindrical dielectric container including a vegetable oil and targets in the form of smalldiameter plastic straws filled with water. Both simulated and measured results are presented. The successful detection and location of the targets is achieved by visual inspection of the formed microwave images. V C 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54: [13][14][15][16][17][18] 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26465Key words: microwave radar; microwave antenna; ultra wideband INTRODUCTIONRecent years have shown an increased interest in using active microwave techniques to obtain internal images of various dielectric objects. A prominent example is the ground penetration radar to detect and locate voids, pipes, and other objects buried underground, and its recent extension, the ''breast radar'' for detection of a tumor in the woman's breast [1][2][3][4][5].There have been two major approaches to see the inside of a dielectric body using active microwave techniques. One of the approaches uses the so-called inverse scattering technique using a single or multiple frequency signal data acquired by antennas surrounding an imaged body [1,[6][7][8][9]]. An alternative is an ultra wideband (UWB) radar approach using short duration pulses, which are transmitted and received by UWB antennas [1][2][3][4][5]. The two approaches encounter different challenges concerning hardware implementation and image reconstruction. Both face the same trade-off between penetration and in-depth resolution, with high microwave frequencies offering high resolution images but small penetration, and low microwave frequencies providing large penetration but poor resolution. The inverse scattering approach does not require any a priori information to produce the complex dielectric constant distribution of an imaged object as it solves the problem by comparing the data obtained from a forward full electromagnetic (EM) solution with the measured data. However, it needs a considerable effort to overcome challenges of an ill-posed nonlinear problem. The UWB radar approach does not offer an exact EM image reconstruction as it looks for locations of an increased scatter inside an image body. It requires an estimate of average dielectric properties of the body to produce a spatial scatter map. Its main dare comes from a large magnitude signal caused by reflections at the body interface that masks small return signals of internal targ...

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