Practical_Antenna_Handbook_0071639586

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 47
it a refracting layer. But, as can be seen from Fig. 2.27, the refracting action of the layer
beginning at B-C can be approximated by pretending there is a reflecting surface at altitude
C′.
The mirror analogy is imperfect in another way, as well. If you stand directly in
front of a mirror, you are able to see your image clearly. But the ability of the ionospheric
layer to return a perfectly vertical signal back to earth is frequency dependent.
That is, above a certain frequency, called the critical frequency, or f C , signals sent straight
up continue on into outer space! If, on the other hand, transmitted signals hit the layer
at an angle to the vertical (Fig. 2.28), at some transmitted takeoff angle (a r ) the transmitted
signal no longer escapes from the ionosphere but remains within it. This is called
the critical angle, and it is different for every frequency.
Most important, takeoff angles smaller than the critical angle will be refracted from
the layer and returned to earth. Thus, we see from Fig. 2.28 that there is a range of takeoff
angles for the transmitted signal that will be “reflected” back to earth at some distance
from the source. This is the great magic of the ionosphere.
Conversely, at takeoff angles greater than the critical angle, the transmitted wave
either stays within the ionospheric layer or continues on into space. In other words, for
angles between the critical angle and 90 degrees (pure vertical incidence), the iono-
C'
Lower edge of the
ionosphere
B
C
H V
A
D
Figure 2.27 Finding the virtual height of the ionosphere.