Noisefield . 30 min .1991
External (radiated, airborne)
radio frequency noise can be classified according to its origin as being
either natural or man made. Natural radio-noise sources include thunderstorms,
lightning, cyclones, electron acceleration and emission processes in the
magnetosphere, the sun and radiostars (such as the supernova remnant Cassiopeia
A) Man made radio-noise sources include gap discharge and high voltage
corona discharges of electric power lines; ignition systems; electric
motors; ultrasonic equipment; fluorescent lighting; mercury-arc rectifiers;
electric buses and trains; industrial heaters; electric fences; standard
time, standard frequency, radio telemetry, and control signals; meteorological
monitoring and observation;the out- of- band emissions of medical diathermy
equipment and coherent transmitters; and incidental, electrical or electomechanical,
radio-noise sources. Man-made radio-noise has come to be recognized as
the collective radiation emanating from a large range of electrical devices
that pulse, oscillate, arc, spark, or switch on and off (interrupters).
These collective incoherent radiations contribute to form a noise-field
intensity (also known as incidental metropolitan noise, the composite
noise environment, or the electromagnetic ambient) which achieves a maximum
value at or near the centre of an urban area and decreases as distance
from the urban core becomes greater. Much of the radiated energy is propagated
above the surface and in this manner an ambient noise envelope is created
about a city. (radiated, airborne) radio frequency noise can be classified
according to its origin as being either natural or man made. Natural radio-noise
sources include thunderstorms, lightning, cyclones, electron acceleration
and emission processes in the atmosphere. |