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The amplitude modulation waveforms of linearly polarized electromagnetic waves due to the scattering by slowly rotating metal plates are investigated theoretically and experimentally. The nature of the modulation waveform depends on the orientation and dimensions of the object, its rotation speed, and very strongly on the incident and receiving (or scattering) directions. For a rectangular metal plate rotating in its own plane, the modulation is independent of time in the specular and forward directions. Near the specular and forward directions the modulation waveform is a sinusoidal function having a frequency equal to twice the rotation frequency of the plate. Further away from these directions, the waveform is a sinc-pulse type repeating at twice the rotation frequency. Similar results are also discussed for a rotating skewed metal plate. The observed waveforms are found to be in good agreement with theoretical results.