The authors present the results of an experimental investigation of microwave transmission through thin plates of sodium and potassium metal carried out at 80 GHz and 1.3 K in the perpendicular-field geometry. The characteristics of Gantmakher-Kaner oscillations and cyclotron phase resonance are presented. Although the physical causes of these signals are well understood, a number of details do not appear to be adequately described by current theory. We also describe two other classes of signals that are superimposed on the first two, but whose origins are still uncertain.