Practical_Antenna_Handbook_0071639586

20 p a r t I I : F u n d a m e n t a l s
water, which is much denser than air, the speed of radio signals is about one-ninth that
of the free-space speed. (Contrast this with sound waves, which travel almost four
times faster in water than in air!) This same phenomenon shows up in the basis for the
velocity factor (v F ) of transmission lines. In foam dielectric coaxial cable, for example, the
value of v F is 0.80, which means a signal propagates along the line at a speed of 0.80c, or
80 percent of the speed of light.
The Earth’s Atmosphere
Electromagnetic waves do not need an atmosphere in order to propagate, as Marconi
and his contemporaries would have immediately realized if they could have witnessed
today’s data communications between NASA’s ground stations and their interplanetary
probes in the near vacuum of outer space. But when a radio wave does propagate
in the earth’s atmosphere, it interacts with it. In general, the radio wave will be subject
to three potential effects as a result:
• It will suffer increased attenuation (relative to normal free-space path loss over
a comparable distance).
• Its path may be bent or redirected.
• Its polarization may be altered.
All of these effects vary with frequency. In addition, different regions of the atmosphere,
which consists largely of oxygen (O 2 ) and nitrogen (N) gases, play differing roles:
The atmosphere (Fig. 2.7) consists of three major regions: troposphere, stratosphere,
and ionosphere. The boundaries between these regions are not very well defined, and
they change both diurnally (over the course of a day) and seasonally. Keep in mind, also,
that the distinctions between these regions are a matter of definition of gas densities
and behaviors by groups of scientists, not the result of some fundamentally profound
differences in the gases that inhabit each.
The troposphere occupies the space closest to the earth’s surface, extending upward
to an altitude of 6 to 11 km (4 to 7 mi—roughly the cruising altitude of most jet airliners).
The temperature of the air in the troposphere varies with altitude, becoming considerably
lower than temperatures at the earth’s surface as altitude increases. For
example, a +10°C surface temperature could exist simultaneously with –55°C at the
upper edges of the troposphere.
The stratosphere begins at the upper boundary of the troposphere (6 to 11 km) and
continues upward to the beginning of the ionosphere (≈50 km). The stratosphere is
called an isothermal region because the temperature in this region is relatively constant
across a wide range of altitudes.
The ionosphere occupies the region between altitudes of about 50 km (31 mi) and
300 km (186 mi). It is a region of very low gas density because it is the portion of our
atmosphere where the earth’s gravity has the least “pull” on individual molecules. Beyond
the ionosphere, our gravity system is so weak that any air molecules that stray
there ultimately drift off into space, never to be seen again.
Of particular interest to our study of high-frequency propagation is the fact that a
mixture of cosmic rays, electromagnetic radiation (including ultraviolet light from the
sun), and atomic particle radiation from space (most of these from the sun also)—all
impinging on the outermost edges of our atmosphere—has sufficient energy to strip