Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

3.IV. Natural UHF/SHF/EHF noise

In this range of frequencies the natural predominant noise is the background cosmic (or galactic) noise. In many characteristics cosmic noise is similar to the terrestrial natural noise, like distant lightning strikes, or man-made radio noise; for this reason it was difficult at the beginning of radio measurements to identify the sources. The discovery of the origin of this background noise is usually also marked as the birth of radio astronomy. In 1931 Karl Jansky built an antenna operating at 20.5 MHz and after months of careful observations he concluded that the source of the recorded noise was outside the solar system; since then those frequency bands have been carefully investigated by radio astronomers (Krauss 1988). In 1940 Grote Reber made the first radio map of our Milky Way galaxy at a frequency of 160 MHz. After that the radio emission from the Sun, Jupiter and other celestial bodies have been also identified by radio astronomers. In figure 10 the solar and other main sources are reported. Some of these sources are very strong or show a particular behavior (figure 11). The synchrotron radiation has a characteristic wide spectrum like other impulsive radio sources (Bekefi et al. 1977). Moreover galactic or stellar systems give place to strong and most common noise source: the thermal noise. Thermal noise is arising from the electrons and ions in motion in a dissipative media. Others sources are the bremsstrahlung radiation mainly due to electron proton collision (solar cromosphere, Orion nebula etc.).

FIGURE 10 Solar radio emission Solar radio emission

FIGURE 11 Some sources of radio emission and cosmic background Some sources of radio emission and cosmic background

The Earth is not the only planet that shows a large variety of radio signal. Almost all planets have an electromagnetic background, for instance Venus is quite similar to Earth, and the four gas giants, Jupiter, Saturn, Uranus and Neptune are natural emitters of HF radio signals. Due to its huge atmospheric disharges Jupiter is one of the major radio emitter. The sun is the most powerful electromagnetic emitter in our solar system able to generate a very broadband radio emissions. In the solar radio emission, several phenomena like flares, magnetic waves, and storms of electrically charged nuclear particles and ions directly cause and/or influence the propagation of electromagnetic waves throughout the known solar spectrum.