Noise, Atmospheric

electrical fluctuations in a receiving antenna caused by the heat radiation of the earth’s atmosphere in the radio spectrum (seeFLUCTUATIONS, ELECTRICAL).

Atmospheric noise worsens the quality of radio reception by engendering acoustical noise and false signals and decreasing measurement accuracy. A quantitative characterization of atmospheric noise is provided by noise temperature, which is equal to the product of the average physical temperature of the atmosphere (≈300°K) and the coefficient of absorption of the atmosphere. The latter depends on the state of the atmosphere (humidity, dust content, and the like), the shape of the antenna radiation pattern, the working length of the wave λ, and the orientation of the antenna in space. Ordinarily, when λ > 10 cm and altitudes of the main lobe of the radiation pattern are more than 20°, atmospheric noise is negligibly small.