Virtual Height

Figure 6. The ionized layers of the ionosphere cause refraction and resulting reflection of radio waves as indicated for a typical daytime condition. The ionization density is shown at the right. Note that the several "layers" E, F1, and F2 are not distinct from each other and that the ionization density is greatest for the F2 layer. The ionization density is measured in number of ions per cubic centimeter. (Adapted from Reference 8.)

If the frequency is below about 4.2 megacycles, some of the energy will be absorbed in the E layer, and the rest will be reflected back toward the earth. By properly displaying the initial pulse and the reflected pulse on a cathode-ray tube and by the use of suitable time-measuring equipment, the virtual height of the reflecting layer can be determined.¹⁵ By virtual height is meant¹ the "height at which reflection from a definite boundary surface would cause the same time of travel as the actual reflection, for a wave transmitted from the ground to the ionosphere and reflected back." The virtual height is usually specified for the lowest frequency at which reflection occurs,¹ This is sometimes called the equivalent height, or the effective height, of the layer.