  The intensity of Smith-Purcell radiation from metallic and dielectric
gratings (silicon, silica) is compared in a frequency-domain simulation. The
numerical model is discussed and verified with the Frank-Tamm formula for
Cherenkov radiation. For 30 keV electrons, rectangular dielectric gratings are
less efficient than their metallic counterpart, by an order of magnitude for
silicon, and two orders of magnitude for silica. For all gratings studied,
radiation intensity oscillates with grating tooth height due to electromagnetic
resonances in the grating. 3D and 2D numerical models are compared.
