Page 121 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecrafi Applications
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Figure 5-3. Electromagnetic wave propagation. Certain frequencies of
electromagnetic energy propagate differently.

The ranges achieved depend on a number of factors including frequen-
cy, angle of incidence, and transmitted power (to be discussed shortly), as
well as time of day (or night) and other factors which may affect the prop-
agation of the signal. A powerful enough signal may, after refraction by
the ionosphere, reflect off the earth’s surface back toward the ionosphere
to be refracted again. These multiple-hop transmissions can achieve
tremendous ranges and allow essentially worldwide communications. Fre-
quencies used for this type of communication make up the shortwave
band which has been used for long-range communications since Marconi
explored the possibility of such use at the beginning of this century.
Space Waves. Above the critical frequency (generally above 30 MHz),
electromagnetic waves are not affected by the ionosphere enough to be
deflected back down to the surface. Communications using these fre-
quencies are all line-of-sight (LOS) communications because the trans-
mitting station and receiver must have a direct path between the two.

Antenna Theory

Electromagnetic waves that propagate through space originate from,
and are received by, some sort of antenna structure. Power is delivered to
an antenna at the desired frequency, causing electron movement in the
structure. The movement of electrons causes an associated electromag-