Experiments with double microwave apertures
One of the significant characterizations of microwave imaging is the spatial resolution. The relative long wavelength of microwaves as compared to ultrasonic waves and x-rays necessarily means that aperture arrays (real-time or synthetic) are required for high-resolution microwave imaging. Certain limitations on resolution apply regardless of how the array is realized. Types of antenna arrays fall into two broad categories: Phased arrays which are usually operated in real time by scanning a beam past an object, and aperture synthesis in which data are collected with one or more antennas and later processed to produce an image. In NDE we are concerned with the linear resolution which we define as the product of the angular resolution and the range from the array phase center to the object of interest. For objects imaged in the far field of the array, the angular resolution is inversely proportional to the largest size of the array. We have conducted a series of experiments to study the practical and mathematical aspects of resolution when the object of interest is physically near the microwave apertures.