A Novel 24 GHz One-Shot, Rapid and Portable Microwave Imaging System

Abstract

I. INRODUCTIOIMicrowave imaging is based on measuring the relative scattered field from an object over a known two-dimensional (2D) space and then using various imaging algorithms (i.e., synthetic aperture, holographical, reconstruction techniques. etc.) to obtain geometrical and dielectric distribution of the object (or essentially its image) 11-21. To this end, an individual scanning and non-perturbing probe or an array of probes capable of measuring the magnitude and phase of the scattered electric field component of interest is used to obtain the coherent electric field distribution over the 2D space (i.e., mapping the field) at certain prescribed spatial intervals. The basic probe design requires that its deployment must not significantly perturb the scattered field to be measured. Furthermore. the probe should be robust against parasi tic reflections and coupling between probe array elements 12-31. A probe, which meets the above design requirements, is one employing the modulated scatterer technique (MST). The use of MST for electric field measurements was first introduced in 131. Since its introduction. MST has been incorporated into \ arious linear scatterers. such as small dipole antennas, and has been extensively used for electric field mapping and imaging applications.MST-based 2D imaging systems have been designed and built with an array of sub-resonant dipoles at frequencies up to 12 GHz (X-band) [2,41. Although such designs use minimally perturbing elements and result in a compact array for field sampling and measurement, they suffer from several drawbacks. The inefficiency of the sub-resonant dipoles, places their scattered signal very close to the noise floor of most detection systems. Furthermore. the mutual coupling among the dipoles can significantly limit system dynamic range. These problems become even more significant and challenging at higher frequencies such as those in the millimeter wave region (30-300 GHz) where currently there is a great need for rapid imaging systems at these frequencies.In this paper, a novel MST-based 2D imaging system capable of rapid electric field mapping, operating at 24 GHz (K-band), is discussed. The design is versatile enough to be directly implemented at higher frequencies. Modulated scatterer technique allows for spatial tagging of the desired electric field facilitating the evaluation of magnitude and phase of a scattered field at distinct and localized locations within a 2D spatial domain (i.e., the probe array). In other words, when the receiver demodulates the MST signal, it gives an output proportional to the magnitude and phase of the incident (or scattered) electric field at the location of the modulated probe. Therefore. an array of modulated probes provides for coherent electric field distribution over the desired 2D space. Another important feature of MST is that the effect of other non-coherent signals such as clutter and noise, adversely influencing the measurement. is significantly reduced through various means including coherent averaging 121...

Portable Real-Time Microwave Camera at 24 GHz

“…The proof-of-concept design described in [17] was founded on efficient spatial multiplexing of scattered electric field over the 2-D space of slot array. The analysis showed the possibility of effective and rapid "electrical scanning" of the array.…”

“…The utilization of a rectangular waveguide network in the collector, instead of the lossy free-space collection system used in [17] is one of the main features of this imaging system. Rectangular waveguides are widely used low-loss transmission lines, and can therefore improve the efficiency of the overall system by reducing attenuation and radiation losses of the desired electric field picked up by the resonant slot array elements.…”

“…Another limitation associated with MST is the signal transfer from the scatterer to the receiver. Spatial collection schemes or passive combiners are lossy solutions, lowering the overall system dynamic range [14], [17]. Furthermore, the mutual coupling among the array elements (e.g., dipoles) can significantly reduce the system dynamic range.…”

“…This design overcomes many of the mentioned limitations associated with the current imaging systems. It is a combination of a switched RF multiplexed system and MST measurement approaches, utilizing a PIN diode-loaded switchable resonant slot antenna as its array elements [17], [18] to perform the switching function directly at the array element locations rather than using an RF switching network. Subsequently, the isolation among the array elements, and consequently the dynamic range is markedly improved.…”

Development of an Optically Modulated Scatterer Probe for a Near-Field Measurement System

“…Both electrically and optically modulated probe arrays have been demonstrated [12,13]. Each probe could be modulated at a different frequency to measure the field at each probe simultaneously to further reduce measurement time.…”