Self-Cohering a Large, Distorted Antenna for Microwave Imaging
A large phased array designed for microwave imaging consists of many independent antenna elements each weighted appropriately to provide the desired radiation pattern, and the weighted outputs are summed to form the system output. The arguments of the weights determine beam direction. The argument is a sensitive function of element location in the array and phase shifts in the circuits of each element receiver channel. Uncompensated position and/or phase errors result in severe image distortion.
The Valley Forge Radio Camera is a large imaging microwave radar. Its 83 m antenna is very flimsy, highly distorted and nonrigid. The distortions are severe, bordering on 1 wavelength rms in the three position coordinates at the operating wavelength of 3 cm. An adaptive subsystem dynamically calibrates the array based upon measurements made of the radiation field at the array due to reflections from targets. Described is one of the algorithms used, the nature of the experiments conducted and the evidence of the validity of the adaptive beamforming method. High resolution microwave images of a town in the neighborhood of the laboratory are shown.
KeywordsAntenna Array Antenna Element Microwave Image Corner Reflector Phase Antenna Array
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