e. Operation with high system noise.
(1) Occasionally, traffic channel noise is excessive. This may be caused by a higher noise
level at one or more radio hops than at others. A hop may have relatively high path attenuation due to a
long transmission path or line-of-sight obstructions. A high noise level also may be caused by external
sources such as ignition or radio interference.
(2) Under such conditions, it frequently will be possible to improve the overall system
signal-to-noise ratio. Better reception may be obtained by increasing the signal output of the transmitter
and reducing the gain of the receiver where the high noise level occurs.
(3) There are limits to signal-to-noise improvement; however, changing a multichannel
output of a certain number to a channel output of a lesser number will increase the relative signal-to-
noise level. For example, a 12-channel output may be reduced to a 4-channel output, or a 4-channel
output may be reduced to a 1-channel output, etc. This assumes that all of the transmitter power that
was used in the higher number of channels is applied to the lower number.
(4) When a particular hop is operating with excessive noise, follow the procedure outlined in
the equipment manual.
Learning Event 3:
FADING.
1.
General. Fading may generally be considered a form of interference. It is not normally cause by
man-made interference of sources outside of the transmission medium. Fading is the variation of radio
field strength caused by changes in the transmission medium with time.
a. Violent changes in the ionosphere, known as ionosphere storms, may also cause fading,
especially at frequencies higher than 1500 kHz. These disturbances are caused by vigorous sunspot
activity and may last as long as several weeks.
b. All frequencies used in microwave multichannel links are subject to fading. The most
common method to overcome fading is to increase the power to the transmitter. The use of automatic
gain control in the receiver will compensate for minor changes in signal intensity. Though fading can
occur along any transmission path, the effects of fading are not as pronounced on VHF and UHF paths
as it is on microwave and tropospheric scatter paths. Other aspects of fading, and measures to combat
them, are discussed in the following paragraphs.
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