This study investigates the effect of dip brazing on slot geometry and radiation performance in the fabrication of a SAR antenna. Three-dimensional measurements and near-field tests on aluminum prototypes indicate that dip brazing causes dimensional deviations in the slot width and length, thus resulting in performance degradation, particularly in the side-lobe characteristics. Hence, a machining compensation method with an offset of approximately 0.03 mm is proposed. Prototypes fabricated using this approach retained dimensions close to the design values, even after dip brazing. Near-field measurement results confirmed improved side-lobe levels and compliance with key performance requirements. The findings of this study provide experimental evidence that the thermal deformation induced by dip brazing can be effectively mitigated through pre-machining compensation, thereby contributing to enhanced reliability in the fabrication of high-precision SAR antennas.