Traffic Jam Ahead on Short Waves
September 1962 Radio-Electronics

September 1962 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Prior to the advent of wireless communications and distributed electric power distribution networks, the phenomenon of sunspot activity was mostly a curiosity of nature. The average eleven-year cycle of sunspot minimums and maximums had been known since the eighteenth century when astronomers began making recordings of the quantity of observable sunspots (although Galileo was the first to notice sun spots). I specify observable because those on the sun's far side cannot be seen until they rotate with the sun to a place where we can see them. The sun's rotational period varies from about 25 days at the equator to 34 days at the poles. Viewed from above the ecliptic plane, the sun rotates CCW, as does Earth's revolution path around the sun. That causes the apparent rotational period of the sun to be a bit longer... but I digress. The presence and intensity of the aurorae are tied to sunspot activity - more correctly with coronal mass ejection activity - but that was a mere curiosity and entertainment before mankind began controlling the flow of electrons. The story told in this 1962 issue of Radio-Electronics magazine is as relevant today as it was then, with the quality of certain wavelength long distance radio signal communications being profoundly affected by sunspot cycles. We are currently at the peak of the solar cycle 25. 1962 was at the minimum between solar cycles 19 and 20.

Traffic Jam Ahead on Short Waves

Fewer sunspots and more short-wave stations spell trouble for communications men.

By Stanley Leinwoll*

The reliability of high-frequency radio spectrum for long-distance communication is in serious jeopardy. A combination of expanded frequency usage and decreasing spectrum space is increasing interference rapidly in the bands between 3 and 30 mc. Within the next several years, unless remedial action is taken, interference levels will increase fourfold over those that existed several years ago.

Man and nature appear to be conspiring to bring about this deterioration of conditions in the short-wave bands. During the past decade, use of the high-frequency spectrum for international broadcasting has doubled. During the same period, the number of radio amateurs has increased from approximately 90,000 to about a quarter of a million. Use by the military and commercial interests has increased comparably. In the meantime, sunspot activity, which has a direct bearing on the range of frequencies the ionosphere will reflect, has been decreasing steadily toward the low point of the 11-year sunspot cycle.

High-frequency radio signals are propagated over long distances via the ionosphere, a region of ionized gases at a height of from 50 to 250 miles above the surface of the earth. Radio waves striking the ionosphere are reflected and returned to earth the way a mirror reflects light.

The range of frequencies the ionosphere will reflect depends on the intensity of ultraviolet radiation from the sun. The capability of the ionosphere to reflect radio waves, therefore, varies diurnally, from season to season, and geographically, these variations depending upon the relative positions of the earth and the sun.

In addition to these variations, the intensity of ultraviolet light reaching the ionosphere varies over a much longer period. This change is directly related to the number of sunspots on the surface of the sun.

Sunspots are large disturbances on the solar disc. They appear as black spots and are surrounded by whirling masses of hot gas. Although the nature and origin of sunspots are not clearly understood, it is known that they are one of the sources of ultraviolet radiation from the sun.

Sunspots were first recorded by the Chinese more than 2,000 years ago, but it wasn't until the seventeenth century that Galileo started to make records of the sunspots he observed through his telescope. It was another 200 years before Hendrick Schwabe discovered the sunspot cycle. He had observed the sun on every clear day for close to 20 years, and found that the number of sunspots varied over a wide range. During some years he found the sun virtually covered with spots. During other years scarcely a spot was to be seen. He observed that these changes occurred in a regular manner.

Fig. 1 shows how sunspot activity has varied over the last 100 years. The number of years for a complete cycle - from minimum to maximum and back to minimum again - averages a little over 11 years, and for this reason is called the 11-year sunspot cycle. Over the past 200 years the length of sun-spot cycles has varied from 9 to 14 years, the average about 11.1 years.

Influence on the Ionosphere

Because the level of solar activity is in constant flux, ultraviolet radiation from the sun is also changing constantly. As a result, the 11-year sun-spot cycle is probably the most important factor influencing the way the ionosphere affects long-distance high-frequency radio communication.

Fig. 2 shows a comparison between sunspot number and average noontime critical frequency in the vicinity of Washington, D. C., for the past 20 years. The critical frequency is the highest frequency for which an echo is received when a pulse of radio energy is sent straight up. Since a direct relationship exists between the critical frequency and the range of frequencies the ionosphere can reflect over long-distance circuits, the importance of variations in sunspot activity is further highlighted.

From 1957 through 1959 more sunspots appeared on the sun than ever before recorded. As a result, the range of frequencies the ionosphere could reflect was greater than had ever been observed, and conditions in the 10-, 11-, 13-, 15- and 16-meter bands were the best in the history of radio.

Fig. 3 shows variations in F-layer critical frequencies at Washington, D. C., in December during sunspot maximum and minimum conditions. Note that during maximum conditions the noontime critical frequency is nearly twice the minimum value. During nighttime hours the variation from high to low sunspot activity is not as influential on critical frequencies as during the day, although it certainly is significant.

Since 1958, the number of spots on the sun has been declining steadily. As a result, conditions in the bands that were best during sunspot maximum have been deteriorating. The 11-meter band is now "dead" and, except for occasional openings, will remain that way for many years to come. With sun-spot activity expected to continue moving downward (Fig. 4) for the next several years, conditions in the higher bands will continue to worsen. Although there will be an improvement in the lower bands-49, 75, and 90 meters - the loss in spectrum space in the higher bands will be considerably greater than any corresponding gain in the lower bands because more spectrum space will be lost at the upper end than will be gained at the lower. Also, low-frequency bands double as local broadcasts in tropical areas. During minimums, these local signals travel to the US and cause additional interference.

Fig. 5 shows how changes in sun-spot activity affects a particular circuit. The New York City-London, England, path is a good example. During the winter months of sunspot minimum, significant portions of the 9-, 11-, 15-, 17-, 21- and 26-mc international broadcast bands fall below the maximum frequencies usable during sunspot maximum.

Since 1954, when the last sunspot minimum occurred, international broadcasting has grown rapidly. In the European area, which was already heavily saturated with competitive broadcasts in 1954, the number of transmitters as well as total transmitter power has increased significantly, as shown on Fig. 6. This increasing trend shows no sign of abating. Additional transmitters mean more competition for available channels and higher power means increased interference from co- and adjacent-channel signals.

The situation becomes ever more serious when we consider Africa where many of the new and developing countries are rapidly adding high-power transmitters, most of which will be in direct competition with European allocations.

To make matters even worse for the listener or the radio amateur, there is the ever present problem of Communist jamming. It is estimated that since 1954 over 1,000 jammers have been added to an already massive system which is now estimated to consist of over 2,000 transmitters.

These jammers, operating against the broadcasts of the Free World, add to the problem, not only by directly causing interference, but also by forcing the broadcasters to transmit the same programs on many frequencies simultaneously in an effort to get their message through. Note that the western administrations do not jam.

Possible Solutions

In painting this rather grim picture we do not mean to imply that chaos in the high-frequency bands is inevitable, nor that engineers and broadcast technicians are not fully aware of the growing threat to short-wave communications.

Great strides in the field of spectrum conservation on a national as well as an international level have been made. Scientific research is constantly seeking new means of using the ionosphere and ionospheric phenomena more efficiently and significant improvements in equipment design, as well as anticipated breakthroughs in the field of space communications, offer considerable hope. (See the author's article "Sporadic-E Opens New Horizons," Radio-Electronics, October, 1961).

In the field of equipment design, interference has been effectively reduced by improvements in frequency and band width control, the reduction of spurious and harmonic radiation and advances in antenna design. Advances in the fields of information theory, noise filtering and compression and modulation techniques enable more information per kilocycle to be transmitted than ever before.

Some progress has been made by the International Telecommunications Union, working under the United Nations. Last fall, an 11-member panel of experts met in Geneva and made recommendations for alleviating congestion in the high-frequency bands, including replacement of double-sideband transmission systems in the fixed bands with single-sideband, use of directional antenna systems on all long-distance short-wave fixed and broadcast services, reduction of the number of frequencies used simultaneously for high. frequency broadcasting, and transfer of relatively short-range internal broadcast services to the medium-wave or vhf portions of the radio spectrum.

Communications satellites which can provide multi-channel systems in the microwave region are being developed. Such systems would be able to handle levels of communication traffic many times the present levels in the entire short-wave spectrum.

Another approach to solving the problem involves expanding multi-channel long-distance transmission cables such as the trans-Atlantic cables now in operation. The California-Hawaii cable could be extended to the Far East, new cables to Latin America could be laid and eventually US cables could be linked with other systems such as the world-wide British Commonwealth system now being constructed.

Finally, expanding vhf and microwave systems such as those being used in Western Europe would do much to alleviate congestion in the hf bands. Plans for linking the entire Western Hemisphere through such a system were discussed at an international conference held in Mexico City last year, and even the possibility of linking it with Europe and Asia is under serious consideration.

The use of frequencies in the vhf, uhf and microwave ranges, expanded cable facilities, and space communications systems would do much to free desperately needed spectrum space for services which can only utilize the hf bands. Once military and commercial interests were assured of an adequate number of channels outside the short-wave bands, spectrum space for international broadcasting and radio amateurs could be expanded to meet the needs of these rapidly growing services.

Other techniques to maximize efficient use of the hf spectrum are under investigation by those of us engaged in international broadcasting. These include sporadic-E propagation, use of backscatter techniques to determine whether a particular frequency is being heard with sufficient strength in the reception area, use of optimum radiation angles by employing vertically slewable antenna systems in conjunction with backscatter equipment to determine optimum propagation modes and, finally, paralleling transmitters to conserve spectrum space and increase transmitted power.

Radio amateurs also have been doing their part. Increased use of directional antenna systems and SSB equipment, as well as round-robins in which a group of amateurs use a single channel to communicate among themselves, has helped conserve precious spectrum space.

Despite the measures that have been taken to alleviate growing congestion in the short-wave bands, interference levels continue to rise at an alarming rate. We believe that one of the greatest challenges of the 60's will be the solution of the problem of radio spectrum conservation. The problem has been thought out and many possible solutions exist. If we are to avoid communications chaos in the next 5 years, we must take action. We have the tools, but they must be used.

* Radio frequency and propagation manager, Radio Free Europe.

Posted August 9, 2024