Measurement and compensation of frequency-dependent antenna phase center position for microwave holography applications

Microwave imaging technique allows obtaining images of hidden objects in a structure or media using microwaves. This technique is widely used in such applications as: nondestructive testing, medical imaging, concealed weapon detection, through-the-wall imaging, etc. Obtaining radar images in these applications is based on processing phase and amplitude of the reflected signal recorded over an aperture (microwave hologram). The antenna phase center is a virtual point used for spherical approximation of the wavefront surface. For the practical antennas there is no single ideal phase center, it is frequency-dependent. The image reconstruction algorithms available in numerous publications usually omit the problem of antenna calibration. Disregarding the dependence of the phase center position of an antenna on frequency results in poorly reconstructed images. This problem gets more significant as the frequency bandwidth increases. In this paper we propose a calibration procedure in which a one-dimensional wide-band multi-frequency hologram of a point object is registered. Focus plane determination method was used to determine the phase center position on every frequency. This method analyzes the integrated amplitude modulus of a reconstructed image as a function of the reconstruction distance and chooses the distance corresponding to the minimum of the integrated amplitude modulus. Taking into account the phase center position frequency dependence in the reconstruction algorithm gives considerably improved reconstructed images. Experiments with a cylindrical open wave-guide antenna on frequencies from 6 to 12 GHz and different objects have shown the high efficiency of the suggested approach. The largest enhancements of the focused images are observed along the antenna pointing direction.