Electrical Noise

Transmission Lines.⁴⁸ High-voltage transmission lines are a serious source of radio interference and cause excessive noise unless the lines are designed, constructed, operated, and maintained properly. The main sources of disturbance are corona that forms on the wires if they are too small for the voltage used, corona that may form on insulator tie wires and insulator pins and hardware, breakdown of the air adjacent to the insulator because of faulty insulator design, and small sparks, arcs, and corona that may occur on line hardware such as bolts, etc. Much progress has been made in controlling radio interference⁴⁸ of this type, and transmission lines are constructed and operated so that they are essentially noise free.

Distribution Systems,⁴⁹ Distribution systems cause radio interference resulting in noise much as for transmission lines, except that the voltages are lower and hence corona and certain other types of interference that occurs on transmission lines may not be so bothersome. Distribution systems are within city areas close to many receiving antennas, however, and interference from them may be serious. Here again, good design, construction, operation, and maintenance will greatly reduce interference. A distribution system may cause noise in a variety of ways, including sparks and arcs at fuses, cutouts, bushings, hardware, trees, etc.

Electric Equipment.⁴⁸ In general, electric equipment of good quality is constructed so that radiation and noise interference is not serious. Unless such apparatus is installed and maintained properly, satisfactory design is of no avail. Home appliances, in particular, are often poorly maintained and may cause noise because of sparking and arcing contacts. Methods of shielding electric equipment were considered on page 82 (see also reference 48).

Electric Trolley Systems.⁵⁰ For street railways and other direct-current traction systems the voltages are commonly 600 volts. The trolleys themselves cause sparks. Other causes of interference are sparking motor commutators, sparking generator commutators, and harmonics caused by mercury-arc rectifiers often used to provide the direct-current power. Good design, construction, and maintenance are essential, and the design may include the installation of filters at the substations, particularly when rectifiers are used.

Interference from Vehicles. Engine ignition noise is a source of radio interference. This can be mitigated by the use of shielding and filtering, following the general principles outlined on page 82. Special automotive equipment is available for this purpose. Rubber-tired vehicles accumulate electric charges⁵¹ which may cause explosions in addition to radio noise interference.