  Using the pre-bunched electron beam of 5-MeV linear accelerator the coherent
Smith-Purcell radiation (CSPR) from a flat periodic target (made of conductive
layers separated by dielectric gaps) in millimeter wavelength region has been
investigated. The angular distribution of this radiation was measured with a
narrow-band detector and experimental data agree with our theoretical
calculations for similar kind of targets. Such properties of Smith-Purcell
radiation as strong dependence of radiation wavelength on the observation angle
overlapping with coherent radiation effect may be used for a noninvasive bunch
length measurement. The possibility of using the room temperature detectors for
single bunch measurements is demonstrated.
