Enhancement Of Short-Range Microwave Images Produced By Backward Propagation

AbstractImages of a simple object were digitally computed by a backward propagation algorithm from phase and intensity data measured overa large numerical aperture that was spaced from the object by about half the aperture width. Images were produced for distinct object orien-tations, and multiplying the image transparencies improved resolution.IntroductionMicrowave imaging has several diagnostic applications that include the determination of antenna aperture distributions, the analysis ofreflections by radomes, and the identification of buried dielectric anomalies.'" To determine the detailed structure of the reconstructed fields,the lateral and longitudinal resolutions must approximate the wavelength. Consequently, data are gathered over large numerical apertures atshort distances from the object. We call this distance range. Typical range values are between 0.1 and 3 meters, depending on the configura-tion of the object and the wavelength. These ranges are small compared to those mapped by synthetic aperture radar (SAR), which probablyis the most familiar form of microwave imaging.2,3The methods for short range imaging differ from those of SAR. The latter utilizes pulsed radiation for detection of distant targets so itproduces a map with range as one co- ordinate. The second co- ordinate is orthogonal to range. For short range imaging, continuous wavesare often used because the shortest practical pulses, or order 10-9 sec, give longitudinal resolution of approximately 15 cm. For concave ob-jects, pulsed systems can produce false images from multiple reflections. Moreover, continuous -wave systems are usually simpler and cheaperthan pulsed systems.This paper describes some experimental and numerical work on the enhancement of microwave images formed at short ranges. Longi-tudinal and lateral resolution were studied for a simple object, a pair of parallel, metallic rods. The phase and intensity of 1.69 cm wavelengthmicrowaves reflected from the object were measured with a bistatic system that had a fixed transmitting antenna and a receiving antenna thatscanned a linear path. Range was approximately 20 wavelengths, and the scanning path length was about twice the range. Images were com-puted from the sampled data by a backward propagation algorithm that was based on the angular spectrum concept. This algorithm has beenpreviously utilized to study buried dielectric anomalies,4 antenna nearfields, and wind -generated water waves.5 All these objects were com-plicated so we utilized the rods to study two -point resolution. In addition we sought to study enhancement by image superposition, or moreprecisely by multiplying the transmittance of two image transparencies for distinct object orientations. This work extends some earliermicrowave work on non -coherent addition of the intensities of images formed for several object aspects.6The calculations reconstructed the intensity near the objects and gave the spatial distribution of the scattered field. The scattered fieldsmight be considered artifacts, but they appear real, for they show loci of maxima and minima predicted by elementary physical optics ofscalar diffraction theory.MeasurementThe measurement apparatus included a continuous wave generator of 1.69 cm wavelength microwaves, two antennas, and a networkanalyzer that measured phase and intensity. The antennas were arranged as in Figure 1 with the electric field perpendicular to the X -Z plane.The transmitting antenna was fixed; it was an open -end waveguide, 1 cm by 2.3 cm aperture, of height 35.6 cm above the plane Z