2.
Microwave multichannel fading aspects.
a. For paths above the horizon, two general kinds of microwave fading exist; inverse-bending
and multipath. They differ in origin, character, locations where they are most prevalent, and effects on
(1) Inverse-bending fading is typically found over water. It is rare over land, except on some
foggy paths near large bodies of water. The individual fades may last for minutes or hours, until the
climatic phenomena no longer exists.
(2) Multipath fading may occur over many kinds of paths. It may result from reflections
from water or highly reflective earth. At the longer distances, it may be a result of refraction from an
elevated layer of the toposphere. It may also result from adjacent multiple paths caused by local
irregularities in the troposphere. The duration of individual multipath fades is from about a second to a
minute.
(3) Both kinds of fading may occur at the same time on the same path. When the signal is
weakened by inverse-bending, it is more subject to multipath effects.
b. The maximum depth of fading occurs when the air is still and humid, and the ground is
warmer than the air. The minimum depth of fading occurs when the air is dry and well-stirred by winds.
Therefore, deep fading is ordinarily seasonal.
c. The depth of fading tends to be greater at longer distances, but climatic conditions are fully
as important as distance. The amount of fading, or its time occurrence, cannot be accurately predicted;
and therefore, must be estimated.
d. Fading results in a poorer signal-to-noise ratio. When the audio signal-to-noise ratio is
seriously impaired by fading, telephone communications becomes difficult, errors appear in the
telephone signaling and in VF teletypewriter traffic, and facsimile reproductions show spots, blotches, or
blackout. If the fade is deep enough, the signal is entirely lost in the noise.
e. In pulse modulation systems, when the RF signal weakens, the audio signal-to-noise ratio is
about proportional to the RF signal-to-noise ratio until the latter reaches a low value called the breaking
point. If the RF signals are weaker than this breaking point, the audio noise increases much faster than
the RF signal decreases. The margin between the steady-state conditions and the breaking point is
sometimes called the fade margin. When the breaking point is reached on any system, the noise has
already reached a level above the 0 dB transmission level. This seriously impairs reception.
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