This paper presents an analytical model to characterize the radiation pattern of focused aperture antennas. The model is based on the classic parabolic on pedestal distribution for amplitude, but in this work the focusing phase term is considered and applied in the Fresnel region. The model is useful for millimeter and submillimeter wave imaging radar systems that usually work in the Fresnel region of the antenna. Analytical closed expressions are developed to predict the available resolution (transversal beamwidth) and operating range (axial beamwidth) of such systems. The effects of the first- and second-order phase distributions on the aperture have also been also studied in order to show the scanning effect, the axial refocusing, and the astigmatic beam degradation.
