D.E. Edemskij - Ph.D. (Phys.-Math.), Leading Engineer, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN)
V.V. Kopeikin - Ph.D. (Phys.-Math.), Leading Engineer, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN)
A.V. Popov - Dr.Sc. (Phys.-Math.), Head of Department, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN). E-mail: popov@izmiran.ru
I.V. Prokopovich - Post-graduated Student, Junior Researcher, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN)

Analytical approach and numerical algorithm allowing one to explain and quantitatively describe image formation of a complicated object in microwave holography. It is shown that the main role in the image distortion plays losing a part of the scattered radiation angular spectrum. We derive a quantitative estimate of the holographic radar spectral window and point out a way to increasing spatial resolution of the radar. In order to clarify the nature of an extended object image formation and to find a physically based method of pattern recognition we performed a series of experiments with a near-field holographic radar and a planar test object. Their numerical simulation and theoretical analysis have been done. A good agreement between experimental and simulated microwave images confirms the applicability of the Kirchhoff diffraction theory for qualitative description of the scattering and object reconstruction process in realistic experimental conditions. However, conformity of the microwave image to the object shape can not be considered satisfactory. Our experiments and their theoretical analysis show that the image quality is determined mainly by the mutual location of the object, radiation source and recever antenna array. Our spectral approach explains the peculiarities of experimental and numerical results and allows one to choose the problem parameters as to obtain a good microwave image. With given wavelength, antenna array size and probing depth, the microwave hologram represents a part of the object spatial spectrum determined by the illumination incidence angle and the spectral window width. If a significant part of the spatial spectrum falls into the device spectral window one can recon on a good image quality, otherwise one has to expect a considerable image distortion. Coherent superposition of several holograms obtained with different positions ot the radiation source essentially improves the microwave image.