The attenuation of radio waves reflected from the E-region of the ionosphere

Measurements of the absorption of waves travelling between Slough and Inverness (740 km), and reflected once from the E-region during the process, are compared with similar measurements made simultaneously for vertical incidence at Slough. The absorption over the oblique path, calculated by Martyn´s absorption theorem from the value obtained at vertical incidence, was found to be much too low. Approximately correct values are obtained from the formula for non-deviative absorption, provided that the lowest-frequency (2 Mc/s) vertical-incidence results are used: otherwise the oblique-incidence value is too high. It is therefore considered tentatively that the absorption measurements which have been made for many years past at 2 Mc/s, using vertical-incidence reflections at Slough, might prove to be a useful guide to the absorption likely to be obtained over other oblique E-region paths during any part of the sunspot cycle, and they have been formulated accordingly. The anomalous variation of absorption which occurs during winter in high latitudes is also considered. The additional transmitter power required, as a consequence of fading, to enable a desired signal level at the receiver input to be exceeded for given percentages of the time is discussed, and a method of calculating losses due to polarization changes brought about by magneto-ionic splitting is indicated.