The focal region of an offset reflector exhibits highly intricate field components, including co-polar and cross-polar components. Efficient design of a primary feed to mitigate cross-polarization in offset geometries necessitates a precise understanding of these field characteristics. This paper presents a comprehensive analysis of the fields in the focal region of a single offset parabolic reflector. The formulation involves discretizing the reflecting surface into numerous small finite surfaces. Subsequently, the reflector is illuminated by a plane wave with linear polarization, resulting in a current distribution on its surface. Both cases of linearly polarized waves are examined. By utilizing the current distribution as the source, the fields in the focal region are determined. The total field in the focal region, encompassing both co-polarized and cross-polarized fields, is computed through double integration over the illuminated reflecting surface. These field components serve to predict the magnitude and phase of asymmetrical higher-order modes generated by a primary feed, thus significantly reducing the cross-polarization component in the secondary pattern of the offset reflector.