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.
48 p a r t I I : F u n d a m e n t a l s Escape wave Critical angle wave High angle wave Low angle wave r Figure 2.28 Sky-wave propagation as a function of antenna radiation angle. sphere does not return the transmitted signal to earth. In short, it does not support earth-to-earth communications links at this frequency. The distance on the earth’s surface from the transmitting antenna out to the closest point of returned signal from the ionosphere is called the skip distance, and all locations closer to the transmitter than that distance are said to be inside the skip zone. The Sun’s Effect on the Ionosphere Several sources of energy contribute to ionization of the upper atmosphere. Cosmic radiation from outer space creates some degree of ionization, but the majority is caused by solar energy. Events on the surface of the sun sometimes cause the radio mirror to seem to be almost perfect, and this situation makes spectacular propagation possible. At other times, however, solar disturbances (Fig. 2.29A) can disrupt radio communications for days at a time. There are two principal forms of solar energy that affect shortwave communications: electromagnetic radiation and charged solar particles. Most of the radiation is above the visible spectrum, in the ultraviolet and x-ray/gamma-ray region of the spectrum. Because electromagnetic radiation travels at the speed of light, solar events that release radiation cause changes to the ionosphere about eight minutes later. Charged particles, on the other hand, have a finite mass and thus travel at a considerably slower velocity, requiring two or three days to reach earth. Multiple sources of both radiation and particles exist on the sun. Solar flares, for instance, can release huge amounts of both radiation and particles. These events are unpredictable and sporadic. Some layers of the sun rotate with a period of approximately 27 days, causing specific sources of radiation to face the earth once every 27 days. Thus, many components of solar radiation levels and the corresponding ionospheric activity tend to repeat every 27 days, as well.
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48 p a r t I I : F u n d a m e n t a l s<br />
Escape wave<br />
Critical angle wave<br />
High angle wave<br />
Low angle wave<br />
r<br />
Figure 2.28 Sky-wave propagation as a function of antenna radiation angle.<br />
sphere does not return the transmitted signal to earth. In short, it does not support<br />
earth-to-earth communications links at this frequency. The distance on the earth’s surface<br />
from the transmitting antenna out to the closest point of returned signal from the<br />
ionosphere is called the skip distance, and all locations closer to the transmitter than that<br />
distance are said to be inside the skip zone.<br />
The Sun’s Effect on the Ionosphere<br />
Several sources of energy contribute to ionization of the upper atmosphere. Cosmic<br />
radiation from outer space creates some degree of ionization, but the majority is caused<br />
by solar energy. Events on the surface of the sun sometimes cause the radio mirror to<br />
seem to be almost perfect, and this situation makes spectacular propagation possible.<br />
At other times, however, solar disturbances (Fig. 2.29A) can disrupt radio communications<br />
for days at a time.<br />
There are two principal forms of solar energy that affect shortwave communications:<br />
electromagnetic radiation and charged solar particles. Most of the radiation is above<br />
the visible spectrum, in the ultraviolet and x-ray/gamma-ray region of the spectrum.<br />
Because electromagnetic radiation travels at the speed of light, solar events that release<br />
radiation cause changes to the ionosphere about eight minutes later. Charged particles,<br />
on the other hand, have a finite mass and thus travel at a considerably slower velocity,<br />
requiring two or three days to reach earth.<br />
Multiple sources of both radiation and particles exist on the sun. Solar flares, for instance,<br />
can release huge amounts of both radiation and particles. These events are unpredictable<br />
and sporadic. Some layers of the sun rotate with a period of approximately<br />
27 days, causing specific sources of radiation to face the earth once every 27 days. Thus,<br />
many components of solar radiation levels and the corresponding ionospheric activity<br />
tend to repeat every 27 days, as well.
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