A three stage superconducting undulator (modulator, dispersive section, and radiator) is under construction at Brookhaven National Laboratory. Sections of the radiator, consisting of 25 cm long steel yokes, each with 18 mm period, 0.54 T field, and 8.6 mm gap are under test. The magnetic measurements and operational characteristics of the magnet are discussed. Measurement results and analyses are presented, with emphasis on the integrated field quality. The magnet winding and the effects of the various trims are discussed.