Practical_Antenna_Handbook_0071639586

22 p a r t I I : F u n d a m e n t a l s
electrons away from the gas molecules of the ionosphere. These freed electrons are negative
ions, while the O 2 and N molecules that lost the electrons become positive ions. The
density of the air is quite low at those altitudes, so each free electron can travel a long
distance before bumping into a positive ion, at which point they neutralize each other’s
electrical charge by recombining.
Ionization does not occur at lower altitudes—i.e., in the troposphere and stratosphere—partly
because much of the incoming radiation is blocked before it can reach
that far and partly because the air density is so much greater at lower altitudes that the
positive and negative ions are more numerous and closer together, and recombination
occurs rapidly.
Because a large percentage of the total radiation “raining” on our atmosphere comes
from the sun, ionization levels in the ionosphere vary with the time of day, with the
season (since the earth is farther from the sun during the northern hemisphere’s summer),
and with the solar radiation levels, both short term and long term.
Ionization of the upper level of the earth’s atmosphere by radiation from space
causes the ionosphere to have electrical characteristics not shared by the lower levels.
While the details are beyond the scope of this book, the net effect is to set up the possibility
of electrical interaction between the ionosphere and radio waves that reach it. Not
surprisingly, the effects of the interactions are frequency dependent.
A second effect of radiation from space is to alter the characteristics of bands of
magnetism (so-called magnetic belts) encircling our globe. When these bands are disturbed
by excessively high cosmic radiation, they alter the magnetic fields surrounding
the earth, causing disruption of normal EM propagation. As with ionization effects,
some frequencies are affected more than others.
As we shall see, the ionization of our upper atmosphere is a major factor in the extreme
variability of medium- and high-frequency radio-wave propagation. It is, quite
simply, the “stuff” of magic for those of us who have been mesmerized by the unpredictability
of long-distance terrestrial radio communications on those bands.
EM Wave Propagation Phenomena
If you have ever studied optics, you know that the path and polarization of visible light
can be modified through reflection, refraction, diffraction, and dispersion. Radio waves
(which, like visible light, are electromagnetic waves) can be affected the same way. Figures
2.8A and 2.8B illustrate some of the wave behavior phenomena associated with
both light and radio waves. All four effects listed here play important roles in radio
propagation.
Reflection and refraction are shown in Fig. 2.8A. Reflection occurs when a wave
strikes a denser reflective medium, such as when a light wave in air strikes a glass mirror.
The incident wave (shown as a single ray) strikes the interface between less dense
and more dense media at a certain angle of incidence (a i ), and is reflected at exactly the
same angle, called the angle of reflection (a r ). Because these angles are equal, a light beam
or radio signal undergoing pure reflection can often be traced back to its origin.
If the incoming light wave arrives at certain other angles of incidence at the boundary
between media of two different densities, refraction is the result. The amount and
direction of the change are determined by the ratio of the densities between the two
media. If Zone B is much different from Zone A, the bending is pronounced. In radio
systems, the two media might be different layers of air with different densities. It is pos-