Since the development of high-energy product Sm-Co magnets, ring type Sin-Co magnets have been producted for TWTs at Raytheon. A direct correlation between hysteresis loop measurements and the functional measurements made on the periodic permanent magnet (PPM) stacks of TWT ring magnets is highly desirable. A vibrating sample magnetometer which is capable of measuring theB-Hloop directly on ring magnets has been developed. A correlation between the measuredB-Hvalues as compared to the measured values in PPM stacks is presented. The PPM stacks are normally given a stabilizing thermal treatment at a temperature as required by the operating conditions of the TWT. It is shown that for a given thermal treatment, the higher the intrinsic maximum energy product -(4π MH)max the higher the axial field as measured in PPM array.