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[References: p. 156 (10 tit.)]

Grating transition radiation (GTR) emitted by electrons interacting with a diffraction grating was experimentally confirmed. This radiation appears when a charge intersects the grating at some inclination angle θ between the main grating plane and the charge velocity vector. If the condition θ=0 is fulfilled the GTR tends to the conventional Smith-Purcell radiation. In this report we present the experimental investigation of spectral and orientation dependencies of a coherent GTR in sub-THz wavelength region emitted by 8 MeV electron at KEK LUCX facility. The coherent GTR spectral line shapes up to 5th diffraction order were measured for different inclination angles (θ = 0-25 deg) with the grating period 4 mm. A pair of room-temperature Schottky barrier diode (SBD) detectors with sensitivity bands 60-90 GHz and 320-460 GHz were used. The estimation of radiation frequency dependence on the inclination angle using the dispersion relation justifies the usage of different narrow-band SBD detectors to investigate different diffraction orders of coherent GTR spectral lines. It was shown that the bandwidth of the GTR spectral lines decreases with the diffraction order. The comparison with coherent transition radiation in the same frequency range showed that the brightness of both radiation mechanisms are comparable.