The various forms of External Noise in Communication System created outside the receiver are namely
1. Atmospheric Noise,
2. Extraterrestrial Noise,
3. Industrial Noise.
Perhaps the best way to become acquainted with atmospheric noise is to listen to shortwaves on a receiver which is not well equipped to receive them. An astonishing variety of strange sounds will be heard, all tending to interfere with the program. Most of these sounds are the result of spurious radio waves which induce voltages in the antenna. The majority of these radio waves come from natural sources of disturbance. They represent atmospheric noise, generally called static.
Static is caused by lightning discharges in thunderstorms and other natural electric disturbances occurring in the atmosphere. It originates in the form of amplitude-modulated impulses, and because such processes are random in nature, it is spread over most of the RF spectrum normally used for broadcasting. Atmospheric noise consists of spurious radio signals with components distributed over a wide range of frequencies. It is propagated over the earth in the same way as ordinary radio waves of the same frequencies, so that at any point on the ground, static will be received from all thunderstorms, local and distant.
The static is likely to be more severe but less frequent if the storm is local. Field strength is inversely proportional to frequency, so that this noise will interfere more with the reception of radio than that of television. Such External Noise in Communication System consists of impulses, and these nonsinusoidal waves have harmonics whose amplitude falls off with increase in the harmonic. Static from distant sources will vary in intensity according to the variations in propagating conditions. The usual increase in its level takes place at night, at both broadcast and shortwave frequencies.
Atmospheric noise becomes less severe at frequencies above about 30 MHz because of two separate factors. First, the higher frequencies are limited to line-of-sight propagation, i.e., less than 80 kilometers or so. Second, the nature of the mechanism generating this noise is such that very little of it is created in the VHF range and above.
It is safe to say that there are almost as many types of space noise as there are sources. For convenience, a division into two subgroups will suffice.
The sun radiates so many things our way that we should not be too surprised to find that External Noise in Communication System is noticeable among them, again there are two types. Under normal “quiet” conditions, there is a constant noise radiation from the sun, simply because it is a large body at a very high temperature (over 6000°C on the surface). It therefore radiates over a very broad frequency spectrum which includes the frequencies we use for communications. However, the sun is a constantly changing star which undergoes cycles of peak activity from which electrical disturbances erupt, such as corona flares and sunspots. Even though, the additional noise produced comes from a limited portion of the sun’s surface, it may still be orders of magnitude greater than that received during periods of quiet sun.
The solar cycle disturbances repeat themselves approximately every 11 years. In addition, if a line is drawn to join these 11-year peaks, it is seen that a supercycle is in operation, with the peaks reaching an even higher maximum every 100 years or so. Finally, these 100-year peaks appear to be increasing in intensity. Since there is a correlation between peaks in solar disturbance and growth rings in trees, it has been possible to trace them back to the beginning of the eighteenth century. Evidence shows that the year 1957 was not only a peak but the highest such peak on record.
Since distant stars are also suns and have high temperatures, they radiate RF noise in the same manner as our sun, and what they lack in nearness they nearly make up in numbers which in combination can become significant. The noise received is called thermal (or black-body) noise and is distributed fairly uniformly over the entire sky. We also receive noise from the center of our own galaxy (the Milky Way), from other galaxies, and from other virtual point sources such as “quasars” and “pulsars.” This galactic noise is very intense, but it comes from sources which are only points in the sky. Two of the strongest sources, which were also two of the earliest discovered, are Cassiopeia A and Cygnus A. Note that it is inadvisable to refer to the previous statements as “noise” sources when talking with radio astronomers!
Space noise is observable at frequencies in the range from about 8 MHz to somewhat above 1.43 gigahertz (1.43 GHz), the latter frequency corresponding to the 21-cm hydrogen “line.” Apart from man-made noise it is the strongest component over the range of about 20 to 120 MHz. Not very much of it below 20 MHz penetrates down through the ionosphere, while its eventual disappearance at frequencies in excess of 1.5 GHz is probably governed by the mechanisms generating it, and its absorption by hydrogen in interstellar space.
Between the frequencies of 1 to 600 MHz (in urban, suburban and other industrial areas) the intensity of External Noise in Communication System made by humans easily outstrips that created by any other source, internal or external to the receiver. Under this heading, sources such as automobile and aircraft ignition, electric motors and switching equipment, leakage from high-voltage lines and a multitude of other heavy electric machines are all included.
Fluorescent lights are another powerful source of such noise and therefore should not be used where sensitive receiver reception or testing is being conducted. The noise is produced by the arc discharge present in all these operations, and under these circumstances it is not surprising that this noise should be most intense in industrial and densely populated areas. (Under certain conditions, industrial noise due to spark discharge may even span oceans, as demonstrated by Marconi in 1901 at St. John’s, Newfoundland.)
The nature of industrial noise is so variable that it is difficult to analyze it on any basis other than the statistical. It does, however, obey the general principle that received External Noise in Communication System increases as the receiver bandwidth is increased.