TC 9-64 _________________________________________________________________________
Figure 2-15. Effects of Ionospheric Density on Radio Waves
Frequency
2-67. For any given time, each ionospheric layer has a maximum frequency
at which radio waves can be transmitted vertically and refracted back to
Earth. This frequency is known as the critical frequency. It is a term that you
will hear frequently in any discussion of radio wave propagation. Radio
waves transmitted at frequencies higher than the critical frequency of a given
layer will pass through the layer and be lost in space; but if these same waves
enter an upper layer with a higher critical frequency, they will be refracted
back to Earth. Radio waves of frequencies lower than the critical frequency
will also be refracted back to Earth unless they are absorbed or have been
refracted from a lower layer. The lower the frequency of a radio wave, the
more rapidly the wave is refracted by a given degree of ionization. Figure 2-16
shows three separate waves of different frequencies entering an ionospheric
layer at the same angle. Notice that the 5-megahertz wave is refracted quite
sharply. The 20-megahertz wave is refracted less sharply and returned to
Earth at a greater distance. The 100-megahertz wave is obviously greater
than the critical frequency for that ionized layer and, therefore, is not
refracted but is passed into space.
2-22
Previous Page
