TC 9-64 _________________________________________________________________________
Natural Interference
2-92. Natural interference refers to the static that you often hear when
listening to a radio. This interference is generated by natural phenomena,
such as thunderstorms, snowstorms, cosmic sources, and the sun. The energy
released by these sources is transmitted to the receiving site in roughly the
same manner as radio waves. As a result, when ionospheric conditions are
favorable for the long distance propagation of radio waves, they are likewise
favorable for the propagation of natural interference. Natural interference is
very erratic, particularly in the HF band, but generally decreases as the
operating frequency is increased and wider bandwidths are used. There is
little natural interference above 30 megahertz.
Control of EMI
2-93. Electromagnetic interference can be reduced or eliminated by using
various suppression techniques. The amount of EMI that is produced by a
radio transmitter can be controlled by cutting transmitting antennas to the
and metallic shielding.
2-94. Radiated EMI during transmission can be controlled by the physical
separation of the transmitting and receiving antennas, the use of directional
antennas, and limiting antenna bandwidth.
VARIATIONS IN THE IONOSPHERE
2-95. Because the existence of the ionosphere is directly related to radiations
emitted from the sun, the movement of the earth about the sun or changes in
the sun's activity will result in variations in the ionosphere. These variations
are of two general types:
Those that are more or less regular and occur in cycles and, therefore,
can be predicted in advance with reasonable accuracy.
Those that are irregular as a result of abnormal behavior of the sun
and, therefore, cannot be predicted in advance. Both regular and
irregular variations have important effects on radio wave
propagation.
Regular Variations
2-96. The regular variations that affect the extent of ionization in the
ionosphere can be divided into four main classes: daily, seasonal, 11-year,
and 27-day variations.
2-97. Daily. Daily variations in the ionosphere are a result of the 24-hour
rotation of the earth about its axis. Daily variations of the different layers
(figure 2-14) are summarized as follows:
The D layer reflects VLF waves, is important for long range VLF
communications, refracts LF and MF waves for short-range
communications, absorbs HF waves, has little effect on VHF and
above, and disappears at night.
In the E layer, ionization depends on the angle of the sun. The
E layer refracts HF waves during the day up to 20 megahertz to
distances of about 1200 miles. Ionization is greatly reduced at night.
2-30
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