even a combination of the direct and earth-reflected components.
Furthermore, beyond the horizon, the measured field cannot be totally
accounted for by any combination of free-space waves that decrease in
signal strength with increasing range from the source.
In terms of
intensity and properties, the difference is attributed to a surface
wave that propagates along the air-ground interface. These waves are
important only at frequencies below about 50 megahertz and especially
below 3 megahertz where they are used for commercial broadcast and
navigation purposes.
At higher frequencies, the surface wave
attenuates rapidly.
b. Signals propagated in the surface wave mode are among the most
stable and reliable signals in all modes utilizing natural
transmission paths. However, the surface wave mode is vulnerable to
atmospheric noise that has a strong, vertically polarized component
and
that
interferes
with
similarly
polarized
surface
wave
transmission.
43. Waveguide and Mixed Mode.
a. The space between the earth's surface and the ionosphere takes
form of a spherical shell or cavity capable of supporting propagation
for distances far beyond those at which the ground wave alone is
effective. This propagation mode, which occurs below 300 kilohertz,
is called the waveguide mode.
Thus, depending on distance, the
transmitted signal in this frequency range can be accounted for by
the surface wave, the waveguide mode, or a combination of the two.
The distances involved can range from a few to thousands of miles.
Since both the ionosphere and the ground are utilized in this mode,
the properties and changes of these boundaries are reflected in its
characteristics.
However, perturbing effects are significantly
smaller than those in the ionospheric refraction mode.
Propagation
is extremely stable and particularly suitable for transmission of
frequency and time reference signals and for navigation.
Also,
because the wave penetrates sea water to a considerable extent at
these low frequencies, this mode is used for communicating with
submerged submarines.
b. Because propagation in this mode is by vertical polarization,
it is vulnerable to atmospheric noise.
The emission bandwidth
obtainable is dependent on both distance and frequency and is, as a
rule, very narrow (on the order of 20 to 200 hertz). Large antenna
installations and high transmitted power are characteristic of
communications systems and employ this mode of propagation.
c. Summary.
Table 1 summarizes the propagation modes discussed
in this lesson. This table should be viewed not as source of precise
information, but rather as a panoramic presentation of the general
characteristics of the propagation modes discussed.
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