4-4.
SEASONAL
a. In any given area, the change from spring to summer or from any season to another will result in
changes in both atmospheric noises and interference caused by solar radiation.
b. The atmospheric seasonal changes are primarily due to changes in temperature and humidity. As the
temperature or humidity gradually increases, the interfering noises will increase in direct proportion.
c. During the night, when the portion of the earth in which you are located is not facing the sun, you will
not receive the same amplitude of solar noises. The electron bands in the upper atmosphere will lift and be a
greater distance from the earth. Interfering noise from the sun will diminish because the sun is not primarily
directed toward the night side of the earth.
4-5.
TERRESTRIAL
a. Interference caused by geographical conditions is associated with the metallic or chemical content of
the earth surrounding the location of the ground station.
b. In specific areas or points, the earth may have a high metallic content which will effectively introduce
a magnetic field. This magnetic field may be coupled into the transmitting or receiving equipment by way of the
desired signal, it may couple the desired signal to ground, or it may reduce the power of the signal. This metallic
interference normally remains constant.
c. The interfering noise signals that accompany volcanic eruptions are normally effective only in the
local region. The noise is caused by particles that have been electrostatically charged by the movement of gas and
lava up through the earth's surface, by the heat of the lava, and by the precipitation of dust or smoke particles.
4-6.
FADING
a. Signal fading is not a noise-producing type of interference. It is classed as interference only because
it makes reception of a desired signal difficult and thus interferes with the efficiency and accuracy of electrical or
electronic equipment.
b. Fading, or fluctuation, of a desired signal may be due to disturbances in the medium through which
the signal is propagated. The tropospheric, stratospheric, and ionospheric layers above the earth's surface
constitute the medium. An incoming signal may lose or gain strength; either condition is called fading. However,
it is only when the normal signal strength weakens that reception becomes difficult. The signal strength may drop
so low that the signal fades or disappears in the background noise. While the background noise level may remain
constant, the desired signal may rise or fall below that level. The frequency of the fading cycle may be slow or
rapid and may result in an instantaneous complete loss of the signal.
c. For all practical purposes, the effect of fading is a function of the signal-plus-noise-to-noise ratio
of the particular equipment involved.
344 L4
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