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 59
Ionospheric Sky-Wave Propagation
Sky-wave propagation occurs because signals in the ionosphere are refracted sufficiently
to be bent back toward the earth’s surface. To observers on the ground, the signal appears
to have been reflected from a radio “mirror” at the virtual height somewhere
above the junction of the stratosphere with the lower boundary of the ionosphere. The
skip distance is the shortest distance along the surface of the earth from the transmitter
site (A in Fig. 2.32) to the nearest of all points on the earth (point C in Fig. 2.32) for which
the ionosphere is capable of refracting the transmitted signal back to earth. At shorter
distances, the higher-angle radiation from the transmitted signal either remains in the
ionospheric layer or continues on out into outer space. The ground-wave zone is the distance
from the transmitter site (A in Fig. 2.32) to the locus of points where the ground
wave fades to an unusably low level (point B in Fig. 2.32). The skip zone is the distance
from the outer edge of the ground-wave zone to the skip distance, or the distance from
B to C in Fig. 2.32. Note carefully that only one of these distances, the ground-wave zone,
is strongly dependent upon transmitter power level (or, more precisely, effective radiated
power). The other distances are a function primarily of transmitter frequency and
the state of the ionosphere at any given time.
At some frequencies and power levels the sky wave and the ground wave may interfere
with each other. When this happens, the sky wave arrives at the receiver with a
signal whose amplitude and phase relative to the ground-wave signal depend on the
specifics of its path up to the ionosphere and back down. Since the ionosphere is in
constant motion (think slow undulations, like a snake), the sky wave will arrive with
randomly varying amplitude and phase relative to the ground wave. Thus, the sky
wave can selectively strengthen or cancel the ground wave, giving rise to a class of fading
mechanisms discussed further under “Ionospheric Fading”.
Angle of Incidence and Radiation Angle
One of the factors determining skip distance is the angle at which the radio wave enters
the ionosphere. If the transmit antenna is relatively omnidirectional, it “sprays” the
Ionosphere
Sky wave not
effective
Sky wave
Ground wave
A
Ground-wave zone
B
Skip distance
Skip zone
C
Figure 2.32 Ground-wave zone and skip zone.