Abstract :
A simple, rapid, graphical method is given for obtaining maximum usable frequencies and effective reflection heights of radio waves, from vertical-incidence measurements of the critical frequencies and virtual heights of the various layers in the ionosphere. The method consists of the use of "transmission curves," which are superimposed on the curve of frequency against virtual height, observed at vertical incidence. The intersection of the curves gives the level of reflection in the ionosphere. The factors considered in deriving the transmission curves are variation of virtual height with frequency, effect of the curvature of the ionosphere and earth, influence of the earth\´s magnetic field, and absorption by or reflection from lower layers in the ionosphere. A chart is included for rapid calculation of the factor sec ϕ0, used in plotting the transmission curves.
