A vibrating screen assembly supported by springs is provided which is used to size separate rock and other material. The rearward ends of the foot links are supported for arcuate motion about a pivot axis, and the forward ends of the foot links travel back and forth along a guide. From a force-variation ratio of the baseline force and calibration force an inverse closed loop factor is estimated. Directional couplers couple the first and second signal paths to the third signal path. The first radiating portion is disposed in the slot region and is excited to form at least a resonant mode in the first operating band of the antenna element.