Relationship between h.f. absorption at vertical and oblique incidence

Calculations are made of the absorption, in model ionospheres, by a phase-integral method, at vertical incidence and by 3-dimensional ray tracing at oblique incidence. The results are used to derive an empirical relationship for the variation of the absorption with path obliqueness, to an accuracy considered adequate for the prediction of the performance of h.f. communication circuits. The relationship takes into account both nondeviative and deviative absorption, and includes the effects of the Earth´s magnetic field and is applicable to both E and F modes. It is shown to be more accurate than three other expressions currently in use.