2-29
Natural Interference
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 will decrease as the operating
frequency is increased and wider bandwidths are used. There is little natural interference above 30
megahertz.
Control of EMI
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 correct frequency, limiting bandwidth, and using electronic filtering networks and metallic
shielding.
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.
Q27. What are the two main sources of emi with which radio waves must compete?
Q28. Thunderstorms, snowstorms, cosmic sources, the sun, etc., are a few examples of emi sources.
What type of emi comes from these sources?
Q29. Motors, switches, voltage regulators, generators, etc., are a few examples of emi sources. What
type of emi comes from these sources?
Q30. What are three ways of controlling the amount of transmitter-generated emi?
Q31. What are three ways of controlling radiated emi during transmission?
VARIATIONS IN THE IONOSPHERE
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: (1) those which are more or less regular and occur
in cycles and, therefore, can be predicted in advance with reasonable accuracy, and (2) those which 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
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.
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 (fig. 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.
