An assembly is provided for a transverse electromagnetic (TEM) system. The assembly includes a support frame and at least one resistive sheet. The support frame includes an upper plate and a lower plate. The upper plate defines a first inner surface and an opposed first outer surface. The lower plate defines a second inner surface and an opposed second outer surface. The at least one resistive sheet is coupled to at least one of the upper plate or the lower plate, and extends parallel to the upper plate and lower plate from an exterior of the support frame. The resistive sheet has an inner end disposed proximate the at least one of the upper plate or lower plate and an outer end disposed opposite the inner end, and has a variable resistance that is greater at the outer end than at the inner end.