Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

3.III. Natural LF/MF/HF noise

Also in this frequency range the natural electromagnetic noise has its main source in the atmospheric electric discharges; noise is generally decreasing in amplitude with the frequency and is affected by ionospheric conditions. Since LF/MF/HF frequencies were very soon used in radio communications and broadcasting, in this frequency range the natural radio noise was the object of scientific studies in the radio engineers community and natural LF/MF/HF noise radio measurements started very early as the radio devices developed. The electromagnetic waves radiated by impulsive lightening discharges cannot escape from the ionosphere border. Waves penetrate through the lowest ionospheric layers were they are, depending on the frequency, variously absorbed. Radio waves are reflected by the upper layers of the ionosphere up to a particular frequency, named ‘critical frequency’, that is dependent on local ionospheric condition. Moreover the incidence angle between the wave and the ionospheric layers plays an important role. In fact depending on the path geometry several propagating mode can be established. The results are summarized in figure 10 where the atmospheric noise vs. frequency is plotted. In the same figure the comic noise, that starts at frequencies greater than the of the ionosphere plasma frequency begins to appear. The cosmic noise lowest frequency depends on the ionospheric conditions i.e. on electron plasma frequency about (15-30 MHz). The amplitude of the cosmic noise decrease with the frequency even if lower frequencies are more attenuated than higher. In the 1960th a campaign of atmospheric noise measurement started by Consultative Committee International Radio (CCIR) gave the main contribution to the knowledge of background radio noise in this frequency range. 16 radiometers were installed worldwide and the results were summarized on report CCIR N.322 in 1964 and N.322-3 in 1988. The planetary atmospheric noise distribution is reported in figure 9 where cosmic noise is also reported. In the figure the strongest level are associated to the equatorial region and the weakest in Antarctica.

FIGURE 9 Electric field strength vs. frequency for atmospheric and cosmic noise Electric field strength vs. frequency for atmospheric and cosmic noise