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Ultra-high-frequency field strengths are difficult to predict theoretically because of the inability to separate the effects of diffraction, reflection, and refraction on the measured field. A successful attempt was made to measure the diffraction effect alone. Measurements were made on the diffraction pattern set up by a knife-edge since this simple case lends itself to easy calculation. A natural knife-edge, consisting of a ridge on a peninsula on the Atlantic coast near Bar Harbor, Maine, was utilized, measurements being made on 55 and 110 megacycles. The diffraction was less for the higher frequency, as expected, and the field-strength patterns were fairly well predicted by the simple theory. It was found that horizontally polarized waves were diffracted more than vertically polarized waves over the horizontal knife-edge.