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.
3.
For all practical purposes, the effect of fading is a function of the signal-plus-noise-to-
noise ratio
of the particular equipment involved.
If the background noise level remains constant and the signal diminishes, the reception of a weak signal
becomes difficult. The application of automatic gain control is useless because increasing the gain to amplify
the fading signal to the normal level will cause the noise level to be amplified an equal amount, and the result is
the same poor signal-plus-noise-to-noise ratio. The peculiar atmospheric conditions that cause fading may last
for hours or only a few minutes. Generally fading is more prevalent during the summer months and during
daylight hours.
LEARNING EVENT 7: INTERNAL AND MANMADE
1. Internal interference is present to some extent in every electrical or electronic receiver. This noise arises
from the natural action of electrons in transit within electron tubes and in other circuit components. Even if the
receiving equipment is perfectly aligned and all of the internal components are in the best condition, the internal
interference will still exist.
a. Thermal noise is caused by the thermal agitation of electrons in conductors. Thermally agitated
electrons generate minute voltages which add to or subtract from the circuit voltage and thereby cause electric
noise.
b. Short-effect noise is caused by the inconsistency of electrical currents. Electrical current is composed of
minute electrical impulses which are the result of electrons changing energy states. This lack of continuity
creates noise. Short-effect and thermal noise are closely related in their causes and effects.
c. Spontaneous emission is created by electrons giving up energy when they revert to a lower energy state.
This energy induces noise voltages into the conductors.
2. Many kinds and types of equipment produce undesirable radio frequency impulses which are transmitted
and travel out through the air exactly as if they were deliberately prepared for broadcast. In addition, some
equipments radiate back through the power line to other equipments unless the radiation is stopped by an
impedance or absorbed by a reactance.
SS0031
3-4
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