Rob's web

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - N.B.S. equatorial region V.H.F. scatter research program for the I.G.Y.


50 Mc. men to have south American beacon stations for DX monitoring.

Ealey in 1951 amateurs throughout the eastern half of the United States began hearing a station operated by the Collins Radio Company at a frequency just below the 50-Mc. !land. 'l'he amazing thing was that the station could be heard practically all of the time even when the band could not be described as "open."

More recently, articles published in the October, 1055, "scatter issue" of the Proceedings of the IRE and elsewhere have revealed that the mysterious station was and is a part of a large program of research investigating a new form of long distance transmission. It had been found that practical communications over paths from 700 to 1400 miles in length could be carried on with complete reliability at frequencies as high as (10 Me. 'l'he term "v.h.f. ionospheric forward scatter" is applied to this type of propagation. The word scatter describes the fact that very small but useful amounts of radio energy in this frequency range are returned toward the earth when high-powered transmissions are beamed at the ionosphere. It is now known that this kind of propagation is due to a combination of the effects of turbulence and meteors in the lower ionosphere. Small irregular changes in the normal variation of atmospheric density with height, called "irregularities" or "blobs," cause the scattering.

Experiments performed at various locations have shown that there is a definite variation in ionospheric scatter effects with latitude. At arctic and subarctic latitudes, such as in Alaska, the median power transmitted over a standard path is some ten times stronger than that propagated over a comparable path at temperate latitudes, such as in the United States. Other aspects of the scatter phenomenon, including the way the signal varies with time of clay and time of year, also change with latitude. Up to the present time, no thorough experiments of a similar nature have been performed near the equator.

In the study of the ionosphere, as in other fields of geophysics, much of what is learned depends upon the observation of two or more effects which take place simultaneously. For instance, when a sudden ionosphere disturbance (SID) occurs, bringing severe absorption of signals over standard h.f. circuits, the power transmitted over a 50 Mc. scatter circuit generally increases slightly! Because the h.f. absorption is known to be due to an increase of ionization in the lowest region of the ionosphere, the U region, this effect tends to show that scatter is also partly present in that region. Another example is the occurrence, at arctic latitudes, of scatter signals that are somewhat stronger during periods of magnetic storms and auroral displays. The need for information of all sorts which may correlate with other data taken simultaneously, even in other parts of the world, is the underlying reason for the International Geophysical Year.

By reverse reasoning, the experiment to complete the latitude information on regular ionosphere scatter was included in the United States ICY effort because so much other data will be taken at the same time. Another justification was that it was expected that the transmissions used in such a scatter project could be received in other parts of the world as a means of increasing propagation information.

It is hoped that one of the long-unexplained effects in v.h.f. propagation may be better understood in this way. This is the transequaturial transmission observed during the equinox months over such paths as Buenos Aires to Mexico City. Amateurs in these places c.rmmtnicate amazingly often on the 50 Mc. band during March. April, September, and October in years of sunspot maximum. They do so generally at hours in the evening after the regular F-layer transmission should have subsided. The distinguishing feature of the signals propagated in this way is their rapid flutter-fade which may make speech only partially intelligible. A similar effect can also be observed over paths between the United States and Argentina on the 28 Mc. band. Some form of ionospheric scattering may play a part in this type of propagation.

Propagation ideas leading to design of experiment

1 Elongated blobs and spread-F

The ICY experiment explained in detail in later paragraphs is not merely a repetition, at a different latitude, of the original test over the Cedar Rapids, Iowa, to Sterling, Virginia, path. In addition, an attempt will he made to observe regular scattering via the F layer at a frequency near 50 Mc. Another investigation is designed to study the unique localized effects which occur within about two degrees of latitude of the magnetic equator - the line along which the earth's magnetic field is exactly horizontal.

One concept, that of elongated scattering centers or blobs, has emerged in recent thinking as an important feature of practically all ionospheric scatter phenomena.(1) Except for the obvious example of meteor trails, the earth's magnetic field is responsible for the irregularities being cigar-shaped rather than spherical, as they would be in the absence of a field. This is because the free electrons, which cause most ionospheric radio effects, tend to move more easily parallel to the magnetic lines of force of the earth's field than they do transverse to the field. This effect is more pronounced at elevations in and above the E layer (about 100 kilometers above the earth's surface) since collisions of the electrons with other larger atmospheric particles tend to suppress the directional effects of the earth's field at lower elevations.

Amateurs can most easily observe the effects of elongated blobs in the scatter phenomenon that accompanies auroral displays, often called simply "auroral propagation." The fact that antennas at both ends of a circuit must be pointed in a northerly direction in the northern hemisphere during an auroral opening is time result of the directional pattern, called "aspect sensitivity," of the elongated blobs. The best scattering is observed when the blobs lie in time sane geometrical plane in which a large mirror would have to lie to allow reflections from the same point.

Most present-day theories accept the idea that small sections along the length of elongated scattering centers reflect radio waves independently of the others. The amount of energy reflected by any one small section depends upon the number of free electrons it contains in comparison to the surrounding volume. The amount of energy scattered from any given blob is approximately proportional to the square of its length contained in what is called the "first Fresnel zone." The first Fresnel zone is defined by that part of the blob within which the total length of the path from transmitter to receiver is limited to being less than one-half wave length longer than its minimum value for the blob. This is illustrated in Fig. 1.

Fig 1
Fig. 1. Blob orientation for maximum forward scattering.

Outside the first Fresnel zone the phase of the transmitted signal changes so rapidly that contributions from adjacent parts of the blob tend to cancel, leaving the contribution of the first Fresnel zone predominant. Although the illustration of Fig. 1 shows the blob in the same plane as the transmitter and receiver and the two ray paths, a similar picture also holds for the axis of the blob lying in other directions, including the one perpendicular to that plane. The only requirement is that the blob lie in the plane which is perpendicular to the bisector of the angle between R1 and R2, shown in the figure as a dotted line.

Geometry now allows one to formulate rules for describing the length of the first Fresnel zone for any elongated scattering center. If the angle between R1 and R2 is held constant and the direction of the blob in the plane perpendicular to the bisector is varied, the longest reflecting zone occurs when the blob is in the plane of R1 and R2, as in the illustration. The shortest occurs when the blob is perpendicular to this plane. Intermediate lengths occur at intermediate angles. Likewise, the smaller the angle between R1 and R2, the shorter is the reflecting zone, except when the blob is perpendicular to the R1R2 plane, as in east-west auroral communication.

It may now be seen that the magnetic equator of the earth is an ideal place to study the effects of elongated scattering centers parallel to the lines of force of the earth's magnetic field. There the lines of force are horizontal and are oriented north-south. A north-south forward scatter path, in which the angle between the R1 and R2 of Fig. 1 is as large as possible, should make the best use of the blobs existing at any given height. A comparison between an east-west path and a north-south path, in which the scattering occurs at the same midpoint, would allow one to determine just how important the effect of elongation is.

Within recent years, the radio propagation group at Stanford University has been observing radar echoes in the h.f. band which appear to be due to elongated blobs.(2) These echoes are similar to auroral echoes in that they fade rapidly, but they are observed at times when there is no auroral disturbance, and at latitudes at which auroral effects do not usually occur. They observe echoes from the E region - about 100 km. high and the height at which most v.h.f. auroral echoes occur - and also from the higher F region. This evidence is a good reason for believing that significant amounts of elongated irregularities in the F region should be observable at the equator.

Another reason is that the phenomenon known as "spread-F" is very prevalent in equatorial regions. Spread-F is a term applied to a phenomenon giving rise to a special kind of record found using vertically-pointing "ionosphere sounders," which sweep the frequency range from 2 to 25 Mc. Normally these radar-type equipments receive one, two or more echoes from the F region which are "clean" - that is, the echo pulse is about as wide as the transmitted pulse and the height of the echo is easily measured. During spread-F conditions, the echo pulse is broadened out, with various parts of the echo fading in and out with respect to the others. This has long been thought to be due to the presence of some kind of scattering. The prevalence of spread-F near the equator suggests that the conditions necessary for forward scattering in the F region should be particularly good in that area.

2 Sporadic-E

One of the interesting by-products of an experimental forward scatter circuit is the ability to observe the occurence of sporadic-E ionization. Sporadic-E has long been known by amateurs as the source of "short skip " openings on the 10-and 6-meter bands. On a scatter recording, sporadic-E (or E,) appears as a rapid increase in signal strength, some 20 to 80 dB above the scatter level, which is maintained over a period of ten minutes to as much as hours. Shorter enhancements having this appearance are usually due to exceptionally large meteor trails. E. can also be observed on ionosphere sounders of the kind mentioned in the section above. From the maximum frequency at which the sounder observes echoes, it is possible to estimate the maximum frequency at which strong transmission will be found on an oblique path, such as a scatter circuit.

Amateurs in North America well know the statistical characteristics of sporadic-E. It occurs most often there during the months of May. June, and July, and is most prevalent around noon and during the evening. A secondary peak in activity occurs in December and January. At the magnetic equator - for example, at the Huancayo, Peru, sounder station - E, is extremely prevalent year round. The maximum echo frequencies suggest that short skip on a 50 Mc. circuit should be possible during much of the same time. At Huancayo, the maximum in E. activity occurs during the daylight hours, with hardly any occurring at night. The activity is spread much more evenly over the entire year than in North America.

The E. observed at Huancayo and other equatorial stations must he of a peculiar variety, since it is found to be so prevalent only very near the magnetic equator. The sounder station at Talara, Peru, only 8 degrees of latitude north of Huancayo, shows hardly any trace of this unusual activity. In fact, there are strong indications that the Huancayo variety of E. is confined to a very narrow band of latitude near the magnetic equator. There is a strong likelihood that the high E, activity is associated with a dense stream of current, called the "equatorial electrojet," which circles the earth near the magnetic equator at E-region levels. Because of these interesting equatorial effects, a special experiment involving the cooperation of several countries, including the United States, will be performed during the IGY. It will utilize data obtained from a close chain of four ionospheric sounding stations extending from Huancayo to Talara.

Evidently there is a good possibility that the special conditions which cause the E. at Huancayo may also make the phenomena of v.h.f. forward scatter there rather special, and not necessarily typical of the equatorial region. A second scatter path, with the same orientation, but having a midpoint some six degrees of latitude separated from Huancayo, would permit the observers to separate those effects which are peculiar to a band of latitude enclosing the magnetic equator from those which are common to the entire low-latitude region.

The experimental arrangement

An experimental program with v.h.f. scatter, organized for the United States IGY effort by the National Bureau of Standards, will be carried out in South America. The seven stations planned and the paths involved are shown on the map of Fig. 2. The dashed lines describe regions with the magnetic dip angles indicated, and are called "isoclines." The station at Clorinda, Argentina, will be equipped by NBS and operated by the Argentine Navy as part of the 1G1 participation of Argentina. Likewise, the station at Sao Paulo, Brazil, will he equipped by NBS and operated by the University of Sao Paulo as part of the Brazilian IGY program. The station at Huancayo, Peru, will he operated by the Institute Géofisieo de Huancayo as part of their work supported by the U. S. The other stations will be operated by the National Bureau of Standards.

Fig 2
Fig. 2. Locations of experimental stations with respect to geomagnetic latitude.

A schematic illustration of the experimental arrangement of the paths on the west coast of South America is shown in Fig. 3. The east-west path across the continent has similar but more limited objectives.

Fig 3
Fig. 3. Schematic arrangement of the stations showing expected scattering media at the midpoints.

High-power transmissions will originate at points near Antofagasta, Chile, and Arequipa, Peru. Both stations will use rhombic antennas, 1000 feet long, pointed northwestward along the west coast of South America. The Antofagasta transmitter will simultaneously feed a pair of stacked five-element Yagi antennas pointed eastward toward Clorinda and Sao Paulo. Regular v.h.f. scatter signals from the lower E layer will be received by the Trujillo station from Arequipa, and by the Huancayo station from Antofagasta, thus providing the latitude comparison mentioned earlier. The receiving station near Guayaquil will receive signals from the Antofagasta transmitter, it is hoped, by F-layer scatter. A small transmitter placed at Huancayo, using a Yagi antenna, will also be monitored at Guayaquil. By checking for the presence of strong Ea transmissions over the Antofagasta-Huancayo path and the Huancayo-Guayaquil path it will be possible to tell when strong signals from Antofagasta received at Guayaquil are really due to double-hop E-layer effects such as double-hop Ea or one-hop E. and one-hop lower E-layer scatter. The Antofagasta-Guayaquil circuit and the one from Arequipa to Trujillo will have the advantage that observations can be compared with the ionosphere sounder records of the Huancayo sounder, thereby aiding interpretation of both.

Transmission from all three stations will be simply an unmodulated carrier most of the time. On the hour and half hour, the transmission will be interrupted for approximately 2 minutes so that the receiving stations can measure background noise levels and detect any possible interference. Identification in code will be made just before the transmitter goes off for the two-minute break. A short period of pulse transmission will at times follow the two-minute break, during a period about three minutes long. The pulses will primarily be used to check whether high signal levels at the receiver are due to scattering at the mid-point of the great circle path, or due to F-layer-propagated ground back scatter.(3) Table 1 gives the frequencies, call signs, and powers to be used.

Table 1
Station LocationCallPower Radiated*Frequency
Antofagasta, ChileCE8AE3 kW or 20 kW49.960 Mc.
Arequipa, PeruOA3AAE3 kW or 20 kW49.920 Mc.
Huancayo, PeruOA3AAF50 W49.880 Mc.
*Transmitters for both 3 kW and 20 kW are being taken to Antofagasta and Arequipa. One or the other will be used depending upon experimental requirements.

Coverage of the antenna beams beyond the area of the experimental paths is shown in Figs. 4 and 5. These indicate that the beams of the transmitters are aimed at points in North and Central America where amateurs in Canada, the United States, and Mexico may from time to time be able to hear the transmissions. The frequencies were chosen specifically with this possibility in mind. Amateurs receiving any of these transmissions are urged to include reports of such reception with their regular reports to the ARRL IGY Propagation Research Project.

Fig 4
Fig. 4. Coverage of the Antofagasta station includes most of central America, the gulf and plains states.

Fig 5
Fig. 5. The main lobe of the Arequipa antenna Bill cover most of Mexico and California. The signals will undoubtedly be heard over much of the United States under certain conditions.

Acknowledgements

The success of this experimental program depends greatly upon the excellent cooperation and aid given to the NBS personnel by the IGY National Committees of Argentina, Brazil, Chile, Ecuador, and Peru. It would he impossible to acknowledge separately the help of the many individuals in those countries, and in the United States, which has made this program possible.

Notes

  1. Booker, Jour. Al. & Terr. Phys. 8, 201 (1956); Jour. Geophysical Res. 61, 673 (1956).
  2. Leadabrand, Stanford Univ. Radio Propagation Laboratory Technical Report No. 98, Dec. 9, 1955.
  3. Villard and Peterson, QST, March, 1952, p. 11.

Kenneth Bowles, K0CIQ
Robert Cohen.