Fast prediction of high-directivity antenna characterization has been presented. The Cassegrain antenna working at J band frequency has been evaluated using a terahertz (THz) photonic system based on near-field measurement. The generation and detection of THz signal are based on a non-polarimetric electrooptic (EO) frequency down-conversion technique combined with a self-heterodyne system. The synthesis method is introduced to estimate far-field E and H-plane antenna characteristics. The antenna result is calculated using amplitude and phase-synthesized data based on the measured data. A near-field (NF) to far-field (FF) transformation is employed in the antenna radiation pattern. The effect of blocking objects on the antenna (supporting rods and sub-reflector) is discussed. The antenna characteristics of the measured and synthesized data are compared to the ITU F.699 recommendation. The synthesized result agrees with the measured one on the main lobe angle.