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Terrestrial Natural and Man-made EM Noise

By Cesidio Bianchi and Antonio Meloni

4. Man-made Electromagnetic Noise

Man-made noise is noise originated by human technologies. It is strongly dependent on the distance from the sources (power lines, radio, TV communication installations and other…) that can be very variable, on frequency and on emitted power. Power and frequency in their time and spatial distribution are the quantities that better describe the man-made radio sources. Other important characteristics are the continuous or impulsive nature of the emitted waves, the modulation and the wave polarization. Man-made sources are mainly located in business, industrial and residential areas; in rural (countryside) and quiet rural the sources become scarce (report CCIR/ITU N.258-5 1990). In spite of its relevance, near the sources, depending on the frequency band, in quiet rural areas the man made noise power is on average 20-30 dB lower than the business and residential area (figure 12).

FIGURE 12 Solid lines indicate median values of man-made radio noise power expressed in terms of Fam (dB above thermal noise at T0 = 288 K). Atmospheric noise (dashed lines) and cosmic background noise (dotted line) are reported for comparison with man-made noise Solid lines indicate median values of man-made radio noise power expressed in terms of Fam (dB above thermal noise at T0 = 288 K). Atmospheric noise (dashed lines) and cosmic background noise (dotted line) are reported for comparison with man-made noise

Also in the case of electromagnetic man-made noise is convenient to discriminate the contributions in the different frequency bands where the main sources are indicated.

ELF band

In this band the strongest source of man made noise comes from the electric power lines which operate ideally at a single frequency, generally 50 Hz (60 Hz in USA). The electric and magnetic field generated at this frequency are practically decoupled and because of power lines multi-polar configuration, decrease dramatically with the distance from the lines. The power lines are now spread over the planet in all continents except Antarctica where natural ELF waves close to 50-60 Hz can be recorded with less disturbances. Also some uninhabited regions in other continents are relatively free from this kind of disturbances but in general in the ELF band the 50-60 Hz electric a magnetic fields are the strongest man-made source of noise both as emitted power and as extension of the source. In figure 13 the electric and magnetic field decay with the distance from the lines are shown.

FIGURE 13 Electric and magnetic induction fields at ground beneath a 380 kV power line Electric and magnetic induction fields at ground beneath a 380 kV power line

VLF - HF band

As in the case of other frequencies, man-made noise emissions are due to electronic devices for indoor use, industrial and radio communication apparatuses, including broadcasting radio, since all these equipments employ electrical oscillators that become unwanted sources of artificial radio emission.
The main localized sources are radio AM broadcasting (figure 14) and some industrial equipment. Among the latter relevant sources are heaters that exploit magnetic induction and dielectric losses in the material. Other weaker but numerous sources such as automotive ignition, heavily contributes to the background noise. We have also inestimable indoor sources such as electrical household appliances, PC monitors TV etc... Other scientific and medical equipment are irrelevant in the background radio noise configuration in the considered range of frequency.

VHF - UHF band

Electromagnetic radiation in these bands are due to the broadcasting FM radio TV and mobile phone service stations (figure 14) and more important automotive ignition.

FIGURE 14 Power emitted by the most common communication systems Power emitted by the most common communication systems

Radar and satellite appliances do not give a very important contribution to background noise level.
Main indoor sources are cordless phone microwave ovens etc.. These equipments do not give significant impact outdoor.

SHF - EHF band

Because of the complexity of the system of radio communications and limitations in industrial applications operating at these frequencies, in this range only particular equipments are employed. These bands include satellite communication systems and survey, radar systems, scientific and medical appliances, all techniques affected by intrinsic technological difficulties in the manufacturing of electrical oscillators, transmission lines, antennas etc... For this reason man-made noise is very low in SHF and EHF bands. Moreover, because of the nature of the physical emission processes to which these phenomena obey and their propagation modes SHF and EHF waves are strictly directional. Other technological applications in this range are related to passive observation and measurement like in radio astronomy.

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