In the last blog posting we looked at probably the biggest long term threat to the future usefulness of the radiofrequency spectrum, the accelerating large scale consumption of a scarce natural resource for short term convenience and profit (see, “Why is the US Strip Mining the Radio Spectrum?”). This time we will look at a less obvious but possibly more insidious threat, the ever-increasing world-wide pollution of the spectrum.
Such pollution is, of course, known as radio frequency (RF) noise, which is defined (for our purpose) as “unwanted signals.” Such signals are not intentionally generated for communications purposes, hence when their frequencies occur within a communications channel they tend to compete with and to fully or partially obliterate the meaning of the message carried in that channel. (The field of Information Theory deals, in considerable part, with the problems of conveying messages in the presence of noise.)
Noise is a fundamental process, which arises from the fact that physical objects possess energy. This energy results in their constituent parts (atoms and free electrons) being set into constant motion by the distribution of energy within the object. Thus a resistor sitting on a table will produce a tiny noise voltage just by virtue that it has absorbed enough thermal energy to bring itself to room temperature. Thermal noise generally is of very low level and it does not usually radiate into surrounding space. But it does play a role in the design of receivers.
Noise processes of larger magnitude that do radiate RF noise into space can also be naturally-occurring phenomena, but not all radiated noise occurs naturally. There are two broad classes of radiated noise sources: natural and man-made. Natural sources are further sub-divided into cosmic and terrestrial.
Cosmic noise sources include nuclear processes within stars, processes occurring on planets and asteroids, and the background radiation remaining from the Big Bang at the creation of the universe. Cosmic noise is important in setting the ambient noise level for signals above about 15 MHz, but with increasing frequency it diminishes in magnitude from that point. One of the few major disruptions that cosmic noise produces occurs when the sun aligns directly with the bore of a ground based receiving dish, which is in use with a geostationary radio satellite link, and solar radiofrequency radiation overwhelms the satellite signal. These disruptions occur only for a few minutes twice a year. Other intense (non-noise) reception problems occur periodically from large-scale solar eruptions.
Natural terrestrial noise sources include, in major part, the continuous series of lightning strikes associated with worldwide thunderstorms primarily in the tropics and, to a smaller extent, processes involving charged particles trapped in the upper atmosphere by the earth’s magnetic field. The radio frequency noise produced by natural terrestrial sources tends to peak at the lowest frequencies and to diminish in strength with increasing frequency. Thus the background noise level from tropical storms may be sufficiently high at the low end of the HF band of frequencies (3 MHz) to prevent reception of weak communications signals, but of little consequence at the high end (30 MHz) where cosmic noise sources become more important.
And then there is man-made noise. This noise arises from the fact that humans have harnessed electricity (the flow of free electrons in conductors, driven by the “pressure” produced by differences of electrical potential [voltage] along the conductors) to do useful work. Moving electrons produce surrounding electromagnetic fields that can, and often do, propagate through free space. Noise fields can also be created from electrical discharges, when breakdown of dielectric materials leads to arc discharges. With good engineering design these manufactured fields can be prevented from radiating and thus producing noise. However, “good engineering design” is too often the polar opposite of “good quarterly ‘bottom line profits’,“ and this is almost always an unequal contest.
Regulatory enforcement is intended to redress the balance in this contest, but as we have previously seen the magnitude of the problems and the application of counter-pressure on the regulators from sources of vested (financial) interests have been effective in blunting regulatory enforcement. Thus the world is now awash in man-made noise. Let’s see how large the increase in RF noise has become during the time while all of us have been devoting our attentions to other matters.
Since blog postings on Amateur Radio Service topics seem to draw the largest sets of reader responses, we’ll structure this discussion in ARS terms. In this case the ARS is the “canary in the coal mine,” but the effects on all users of the RF spectrum are similar.
The following “unplanned experiment” will illustrate the problem of man-made noise pollution in a graphic manner. The Curmudgeon lives in a reasonably large metropolitan area. Several months ago, in the middle of an afternoon, the commercial power suddenly failed at his house. Outages are uncommon but not unknown there, and the Curmudgeon is ready for them. Most of the on- board communications gear is powered directly from a battery plant, with the batteries under constant trickle charge (not unlike almost every fixed microwave installation and telephone central office). The electronics are not even aware when/if commercial power fails. And a gasoline-powered generator in the back yard will provide 240 VAC power to the house during extended outages.
As soon as it was evident that the outage was not due to a household or neighborhood fault and that power would not be quickly restored by power company circuit switching, it became clear that no more useful work could be done that afternoon. Thus it was a good occasion to turn to the Amateur (and other) radios for some monitoring to see what might be happening.
The Curmudgeon’s station equipment (radios and antennas) is modern and meets currently accepted performance levels. And, of course, many signals were available for monitoring that afternoon, as the hams in 99.99% of the US were not suffering power blackouts.
But one thing had radically changed. Normally on 20 meters (14 MHz) the background ambient noise level indicated by the radios would vary, under changing daily conditions, between S-5 and S-9 (signal strength units). A change of one “signal strength unit” represents, ideally, a doubling or halving of signal source field strength voltage. But that afternoon the background noise level was …….virtually undetectable! The radio signal strength readout indicated an ambient noise level during the power outage ranging between undetectable and S-1.
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver’s antenna input assuming the input impedance of the receiver is 50 ohms.
For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.
The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.
Signals stronger than S9 are given with an additional dB rating, thus “S9 + 20dB”, or, verbally, “20 decibels over S9″.
At least 99% of the routine background ambient noise power had been silenced when commercial power stopped flowing. The radio noise that was heard that day consisted largely of the thermal noise produced by receiver and antenna. And signals from other Amateurs across the country stood out in bold relief from the minuscule noise background. Signals that would ordinarily have been buried under the prevailing noise level now could be easily received. It was as though access to the rest of the world had been “opened up.” And a similar situation existed on AM broadcast band and all the other Amateur HF bands that were also checked.
In the days shortly after power had been restored, some planned testing indicated that the amount of everyday background noise contributed from inside the Curmudgeon’s own house was negligible. Likewise, removal of the antenna transmission line from the receiver reduced the indicated ambient noise level back to that of the thermal receiver noise. Thus no noise was being conducted into the receiver through the power or other wiring. All of the noise was being radiated through space from external source(s) to the receiving antenna.
Thus the “unplanned” field testing made possible by the power outage and the later confirmatory planned tests showed conclusively that a large radiated noise field exists in a metropolitan environment. More importantly, virtually all of the ambient noise is being created through the use of electricity, both in its distribution through the electrical grid and in the operation of electrical devices.
This is not exactly “breaking news” to most of us, but we do tend to forget how prevalent and how large the noise floor problem really is. Until, of course, these rare “experiments” come along.
Moreover, a few fortunate Amateurs live in remote, sunny locations and do not have commercial electrical service. In some cases their nearest commercial electrical service may be several miles away, and thus these folks live in homes and operate Amateur stations powered by their own solar photovoltaic plants. In speaking with some of these folks over the years, they have said that their ambient noise levels are very low every day (unless electrical storms are in their area) and do not become troublesome. This is the matching, mirror image picture of the “accidental experiment” that was reported above.
If the rapid strip mining of the remaining spectrum for short-term profit and convenience is analogous in human biology to a heart attack, then the slow and insidious invasion of man-made noise into the spectrum would be analogous to a growing cancer. Noise systematically chokes off the ability to operate reliable wireless communications circuits, and the cumulative effects of its attack mimic those of metastasizing cancer cells.
Remember that we’ve introduced this problem of man-made noise in terms of the Amateur Radio Service. But the discussion applies equally as well to military, maritime, broadcast, and utility users of the radio spectrum. All are plagued with these background noise problems.
In the next part we will look at the effects the increasing RF noise pollution and what might be done about it.
What do you think?
Let’s save the universe for RF!
The Old RF Curmudgeon