Figure 2-1. Typical interferences sources.

TM 11-490-5

Figure 2-1. Typical interferences sources.

(2) Atmospheric noise. The primary source of

periods of high sunspot or solar flare activity and

decreases during periods of low activity. The periods of

atmospheric

noise

discharge

high activity may last several days at a time, and noise

thunderstorms, which may result in a type of radio

levels may be 10 to 20 dB greater than the levels

interference commonly referred to as static.

associated with low activity. During a sunspot or solar

additional source of atmospheric noise is precipitation

flare activity, noise will first be noticed at the higher end

static, produced by rain, hail, snow, or dust storms.

of the high frequency (HF) region, then at the lower end

Atmospheric effects are erratic in character, consisting

as the disturbance matures. At times, the RF spectrum

of short, randomly recurring bursts of noise which lack

of these outbursts is quite wide and involves a major

any regularity in phase or amplitude and which are

portion of the radio frequencies now in use.

spread throughout the RF spectrum. The average

power level of atmospheric noise is relatively constant

(4) Galactic noise. Galactic noise originates

over short periods of time, but may vary considerably

over longer periods of time. Atmospheric noise levels

beyond the solar system from a number of sources

depend upon such factors as frequency, time of day,

within the heart of the galaxy; that is, the Milky Way.

Galactic noise intensity varies with frequency and

season, equipment location, and weather conditions.

position within the galaxy, being strongest in the

Atmospheric noise becomes significant at frequencies

direction of the galactic center (the constellation

below 50 MHz, and dominates all other natural

interference sources below 30 MHz. This noise is

Sagittarius). On the surface of the earth, galactic noise

usually the limiting factor in communications below'30

is normally confined to ultrahigh frequency (UHF) and

MHz.

very high frequency (VHF) regions.

c. Manmade Interference Sources. Manmade

(3) Solar noise. Solar noise increases during

2-2