particular layer, it may still be refracted by ahigher layer if its frequency is lower than thehigher layer’s critical frequency.Angle of Incidence and Critical AngleWhen a radio wave encounters a layer of theionosphere, that wave is returned to earth at thesame angle (roughly) as its angle of incidence.Figure 1-6 shows three radio waves of the samefrequency entering a layer at different incidenceangles. The angle at which wave A strikes thelayer is too nearly vertical for the wave to berefracted to earth, However, wave B is refractedback to earth. The angle between wave B and theearth is called the critical angle. Any wave, at agiven frequency, that leaves the antenna at anincidence angle greater than the critical angle willbe lost into space. This is why wave A was notrefracted. Wave C leaves the antenna at thesmallest angle that will allow it to be refracted andstill return to earth. The critical angle for radiowaves depends on the layer density and thewavelength of the signal.Figure 1-6.—Incidence angles of radio waves.As the frequency of a radio wave is increased,the critical angle must be reduced for refraction tooccur. Notice in figure 1-7 that the 2-MHz wavestrikes the ionosphere at the critical angle for thatfrequency and is refracted. Although the 5-MHzline (broken line) strikes the ionosphere at a lesscritical angle, it still penetrates the layer and islost As the angle is lowered, a critical angle isfinally reached for the 5-MHz wave and it isrefracted back to earth.Figure 1-7.—Effect of frequency on the critical angle.1-6
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