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Reflection and transmission coefficients of thin strip gratings are discussed for plane wave incidence from arbitrary direction. Upon use of the low-frequency approximation, simple closed-form expressions are obtained. They are applicable for the analysis of grid reflector antennas. Physical interpretations of the phenomena are also given and interesting characteristics of the gratings are pointed out. The validity and the applicability of our results is examined numerically by making use of the point matching method (PMM). It is shown that the accuracy is excellent provided the period of gratings is smaller than about 0.3 wavelength.