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It has long been known that the effect of the Kennelly-Heaviside layer during the hours of darkness is such that radio compass-bearings are unreliable unless special antennas are employed. A similar error is introduced when the cross-coil radio-beacon is used for night flying. This error, which manifests itself as a short-period swinging of the on-course zone over angles in excess of 90°, is due to a multiplicity of causes. The predominating sources of error, however, seem to be rotation of the plane of polarization of the original signal and a shift in phase of the frequency-components of the modulated wave.