The radio signal radiated and propagated in a plasma has two modes of the extraordinary component, x and z. When the radio sounding is performed from altitudes more than 500 km, the z component ceases to exist at altitudes greater than the altitude of the F 2 -layer maximum. However, the z component reaches these altitudes during sounding from the vicinity of the main maximum of the electron density. We study the behavior of the z component when the signal source is located at different altitudes in the ionosphere and also consider the capabilities and method of calculation of the plasma-frequency altitude profile by the z-component trace in ionograms.