The diffraction patterns of radar waves about a semi‐infinite copper screen have been measured. The measurements were made by revolving a bisectoral horn in a circle of radius 15″ or 30″ about the diffracting edge. Measurements were made at three positions along the edge, one of which coincided with the corner of the screen so that the pattern obtained was actually that of a quarter‐infinite plane. Measurements were made for two orientations of the screen corresponding to angles of incidence of 0° and −22½°. Theoretical patterns have been calculated for the entire 360° of rotation and these agree well with the experimental patterns. A comparison is made between the theory, measurements, and the equivalent optical measurements.