3.
One percent microwave multichannel fading.
a. One percent fading is taken as the depth of face (in dB) that is exceeded only 1 percent of the
total time in the worst month of the year in the locality concerned. This is approximately 7 hours,
assuming 24 hours a day operation. One percent fading is the sum (total time) of all the separate
intervals (times) that the depth of fade exceeds the stated amount. General fading conditions may
extend over a period of several hours (usually in the early morning hours during worst month
conditions). However, the deep fades that exceed the stated amount for 1 percent fading criteria are of
very short duration, ranging from a few seconds to a minute or so. Thus, out of the total period that
disturbing fades exist, only a total of a few minutes would exceed the stated amount for 1 percent fading
criteria.
b. Microwave fading estimates are based on an expected 1 percent fading depth of 20 dB on a
single 35-mile link. In the month of worst microwave fading, the dB depth of fade, with good siting,
will vary above or below 20 dB based on the square root of the link length. When one link has a deep
fade, the others will seldom have one at the same moment. These figures apply when there is a good
clearance. At, or just above, grazing (on the basis of K = 4/3 true earth radius), the fading probably
would be about 6 dB deeper.
4.
General measures to combat microwave fading.
a. Avoid paths where deep fading is likely.
b. Make the path loss low enough to provide an adequate fade margin.
c. Use space diversity when needed.
d. When fading is of the inverse-bending type (slow fading), raise the antenna higher than just
enough to give free space loss. Clearance (above the grazing line over the surface of the earth or the
most prominent obstacle) approximately twice the clearance necessary for free space loss is
recommended in this case. If fast fading also is prevalent, use space diversity.
5.
Space density.
a. To use space diversity (figure 18), a second receiving antenna is so placed that when the
signal at the first receiving antenna is weak, the signal at the diversity antenna is relatively strong. To
do this, the two antennas must be properly spaced. Each antenna must connect to its own receiver, and
the receiver outputs must be connected through a combining circuit actuated by control voltages taken
from the two receivers.
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