Industrial interference — also referred to as electrical or man-made noise — manifests as crackles and static in telephones or in radio receiver loudspeakers. Industrial interference typically occurs in areas with operating power plants and various electrical installations, devices, and instruments: electric motors, telecommunication equipment, medical devices, computers, welding machines, electric bells, and ignition systems of internal combustion engines. Interference caused by other radio stations can also be classified as industrial interference.
The radio frequency spectrum of industrial interference and its intensity depend not only on the nature of the sources but also on the degree and effectiveness of localization measures (such as shielding local sources). The highest levels are characteristic of large cities, industrial centers, and moving objects (cars, aircraft, etc.). The level and spectrum of industrial interference are difficult to predict or calculate (except for power line interference) and are usually measured experimentally using special measuring receivers. Some data on interference levels in areas remote from their sources are contained in ITU Report No. 322. There are regulatory limits for industrial radio interference levels from sources located in cities and towns.
Numerous industrial interference sources can be divided into two main groups: devices generating high-frequency signals and devices generating impulse noise. The first group includes broadcast and TV transmitters, cellular base stations and handsets, industrial and medical generators, and local oscillators in TVs and other consumer receivers. Some of these emissions are weak and periodic, radiated at the fundamental frequency or its harmonics.
The second group consists of powerful interference sources such as various electrical mechanisms where operating currents and voltages experience abrupt changes, especially when accompanied by sparking. The simplest examples include electric welding equipment and brushed electric motors. Interference from this group in the frequency range below 20 MHz consists of aperiodic series of pulses of arbitrary shape. Field strength at the reception point depends on the shape, duration, and repetition rate of the pulses. The interference level also depends on distance, propagation environment, and the “radiation contour,” which includes simple supply and switching wires, such as SIP cables of power transmission lines directly connected to the interference source. Faulty ignition systems in internal combustion engine vehicles can produce emissions up to 1 GHz, with the ignition interference spectrum typically in the 25–170 MHz range.
A particular threat to HF reception comes from transformer substations and high-voltage power transmission lines (HVPL). The most typical interference sources are insulators and disconnectors. In the first case, leakage occurs due to aging of insulating surfaces, resulting in weak discharges whose intensity depends on humidity, air pressure, and other factors, periodically developing into strong industrial interference. In the second case, sparking occurs due to contact oxidation, especially under increased load.
Other causes of interference in HVPL include so-called corona discharges, whose intensity increases with the voltage level of the line. One method to mitigate this effect is designing and installing HVPL with three wires per phase. Increased corona discharge not only causes radio reception interference but also reduces the efficiency of the HVPL itself. A partial mechanical defect of the conductor, such as separation of a strand at the end of a phase wire, can also cause corona discharge.
Methods of combating such interference include the use of EH antennas, CCD antennas, shielded antenna systems (oriented loops), loop antennas, noise-canceling Beverage antennas, and coupling transformers with a volumetric turn. Narrowing the bandwidth of the radio receiver and antenna system to the necessary limits also reduces the level of reproduced interference.
Related sections:
Atmospheric interference,
Interference noise.