Oscillographic Observations on the Direction of Propagation and Fading of Short Waves

The short-wave transmission path is generally but not always located in the vertical plane through the transmission and receiving points. Direction finding depends upon determining the direction of the wave at the receiving point; it does not give accurate results when the twilight zone is in the way of the wave path. The angle between the earth and the direction of short-wave propagation varies continuously and the changes in this angle are much larger than the changes in angle of propagation in the horizontal plane. The observations are consistent with the view that the fading is mainly caused by wave interference.