A calibration fixture for an MWD logging tool consists of a plurality of concentric co-planar conductive calibration loops (22, 24), mounted on a carrier disk of insulate material (16). The disk positions the calibration loops around the logging tool mandrel coaxially. The logging tool supports conventional transmitter(s) and receiver(s). The calibration loops are terminated with a multiple-tap impedance quantizer (34) for discretely varying the impedance of the calibration loops. When a transmitter is activated to emit an electromagnetic signal, the calibration loops are excited to re-radiate a portion of that signal into the receivers with an intensity that is a function of the loop impedance thereby to simulate the effects of formations having known resistivities.