Ionospheric absorption at vertical and oblique incidence

Measurements of the ionospheric absorption of radio waves of frequency 9.15 Mc/s over an 800-km path, and of frequency 5.455 Mc/s at vertical incidence at the mid-point of this path, have been made simultaneously over a period of 14 months. A discrepancy between the observed results and those predicted by Martyn´s absorption theorem is found, which varies diurnally. The oblique-incidence results are examined in detail, and empirical formulae for diurnal and seasonal variation of absorption are derived.