How Are Shortwaves Propagated?, December 1931/January 1932 Short Wave Craft

Prior to atmospheric sounding rockets and orbiting satellites, all information gained and theories developed on the nature of Earth's upper atmosphere and its interaction with electromagnetic waves (radio in particular) were purely academic, not the result of empirical data. That is not to say the theories were wrong (although some were), just that they were incomplete. For that matter, even today there is still much to be learned and, according to an excellent article in the October 2015 issue of the ARRL's QST magazine titled "Five Myths of Propagation Dispelled" (by Carl Luetzelschwab, K9LA), there is still a lot of misinformation being believed and promulgated about shortwaves and how they travel in the atmosphere. This work (very much worth your time) is a great testament to the level of expertise that exists in the realm of Amateur Radio, and the contributions made by it to the science world.

The rest of this article appeared in the February/March 1932 edition of Short Wave Craft.

Fig. 1 - This diagram shows the direction of maximum radiation from a vertical antenna excited by harmonics.

Fig. 2 - The radiation from a horizontal antenna is evenly distributed over almost 180 degrees, as shown.

Fig. 3 - The space radiation is bent downward; more exactly it is refracted and totally reflected (at certain frequencies).

Fig. 4 - Radiation at various frequencies, 1 to 4, some of which are partly bent downward, some passing through the Heaviside layer with parallel deflection.

The author gives high credit to short wave amateurs who have contributed greatly to the data here presented on short wave phenomena. The question of whether short waves penetrate the Heaviside layer, thus making possible radio communication with other planets, is here considered.

Before the extraordinary range of short waves was discovered by amateurs, it was held as incontrovertible that the electric waves followed the surface of the earth, and that the strength of the field decreased in proportion to the distance. It was assumed as simply natural without its causing any more surprise and attention, that for communication at a very great distance only long waves were service-able, with the expenditure of correspond-ingly great energies. Operation was carried on with wavelengths of 2 to 3 kilometers (that is, with frequencies from 150,000 down to 100,000 cycles) and with energies of many hundred kilowatts.

The shorter the wave, the less suitable it seemed for distant communication. Waves of a few thousand meters were used in continental communication, but not in transoceanic. Waves of about 1,000 meters and less were intended for internal communication and for neighboring states. Finally came the waves of 600 and 300 meters for communication of ships with one another and with coast stations; that is, mostly for very short distances.

Waves of less than 300 meters were considered entirely useless, because they actually proved very unreliable in communication at short distances; for which at any rate, they appeared in question. It did not even cause thought that, during the war, weak German ship and field stations in Turkey were occasionally heard on the 300-meter wave by crystal receivers located in Germany. Likewise, the fact that the ships with their resounding transmitters disturbed or drowned out the first 300-meter radio stations at night from "impossibly" great distances, received no consideration. The fact was established: waves of 300 meters and less are absorbed by the influence of the sun's rays in their course along the surface of the earth. That they were more serviceable at night and, under certain circumstances, audible at very great distances, was attributed to the absence of the solar radiation.

Now, against considerable resistance, these views have fundamentally changed. The pioneers of the new conception were the amateurs, who even today have at their disposal the greatest experience and in part stand preeminent in the clarifying of still doubtful problems. Below is a brief outline of the now familiar laws for short waves, which touch on the new problems of propagation foremost in interest. The general laws here given rest on the personal investigations of the writer in the years 1926 and 1927; but, with reference to their general physical basis, on previously known facts or theories. The special data regarding the influence of the weather are based on independent researches performed by Dr. Karl Stoye and the writer, who have had occasional interchanges of ideas. These investigations are still going on.

(1) The maximum radiation from a vertical antenna, especially if it is stimulated by harmonics, projects obliquely upward at an angle. (See Fig. 1.)

(2) A horizontal antenna radiates evenly, over an angle of nearly 180 degrees (Fig. 2),

(3) At a height of 50 to 100 kilometers (30 to 60 miles) above the surface of the earth, there is, according to Heaviside's theory, a stratum of atmosphere which, because of the sunlight and the electron radiation of the sun, is distinguished by a very large number of free negative electrons per unit of space and, because of the slight atmospheric density, by a very great number of heavy ions or positive particles. In view of the great open stretch, there takes place, by impact ionization, a further increase in the number of free electrons. The electron density gradually increases in a vertical direction and again decreases. The dielectric constant of the Heaviside layer is smallest where, in consequence of very great electron density, the electrical conductivity of the layer is greatest. This gradual change in the dielectric constant effects a refraction similar to astronomical refraction (also analogous to the formation of the "Fata Morgana" and mirages) and finally total reflection of the electromagnetic radiation (see Fig. 3). The space radiation is thus bent downward.

Fig. 5 - Assumed diffusion of energy by strata of high relative moisture; normal course of radiation in dotted lines. For the sake of simplicity, a straight course of radiation and reflection was drawn, instead of indicating refraction.

(4) The refraction is, as in the case of light, dependent on the frequency. High frequencies (short waves) are less strongly refracted than low frequencies (long waves). A pencil of electric waves of different frequency, increasing from I-IV (cf. white light) would behave as in Fig. 4. (This is similar to the production of rainbow colors in the refraction of white light.) The range is smaller in the case of long waves than in the case of short ones. Very high frequencies (ultra-short waves) are no longer refracted, but pass, with a parallel deflection, through the Heaviside layer; since, in consequence of the slight refraction, the limiting angle for total reflection is not reached. Rays striking the Heaviside layer perpendicularly pass through it unrefracted.

(5) The energy of ground radiation, whose proportion of the total radiation is great (especially with horizontal antennas ) is quickly absorbed in consequence of the ion density being high near the ground, and because of other sources of loss. On the contrary, the space radiation moves along in the Heaviside layer almost without loss, because of the slight ionic density.

(6) The absorption in consequence of the greater ionic density near the ground is less, with high frequencies, than with the lower ones. The fact that the ground wave is nevertheless (as a rule) more quickly dissipated, with high frequencies, than with lower, is attributable to other sources of loss.

(7) Since the ground radiation is used up after a few miles, while the space radiation descends again to the earth only after a greater distance, there results a silent zone, in which there is no reception or only weak signals are heard.

(8) The height or make-up of the Heaviside layer, or perhaps both factors, changes with the time of day and of year and with the changing activity of the sun spots. Therefore these factors have a great influence on the propagation of the short waves.

With equal frequencies, the range is greater at night or in the winter than by day or in the summer; hence, for example, for these wavelengths:

20 meters by day in the summer: European communication, by day in the winter: DX (distance) communication;

40 meters by night in the summer: still European communication, by night in the winter: DX (distance) communication;

80 meters by day in the summer: almost useless, by day in the winter: places very near at hand; by night in the summer: European communication, by night in the winter: also DX (distance) communication.

(9) The shorter the wave (the higher the frequency), the better it is suited for communication by day and in the summer; but the less it is suited for communication at night and in the winter.

(10) Ultra-short waves are not deflected downward; with regard to their usefulness for communication, they behave almost like light waves. (In so far as communication with other heavenly bodies might be considered, then ultra-short waves would, be the most suitable.) The limit between the ultra-short waves and those still serviceable for "DX" (distance) is not sharp, but varies with the time of day and of year. It lies at about 10 meters, as calculation and practical experiments have shown. The present experiments with 10-meter waves therefore lead toward "DX" communication in summer and by day, which should be noted.

Fig. 6 - Vertical antenna (a) excited into harmonic oscillation and horizontal antenna (b) with its characteristic radiation.

(11) Considerable influence seems to be exerted according to investigations not yet completed, by the weather or, more correctly, the condition of the atmosphere at the edge of the stratosphere. In fact, there evidently is a considerable significance in the "moisture content" in the higher strata of air; shorter waves show themselves most sensitive to these influences. The influence of the weather is therefore stronger on 20-meter waves than on those of 40 or 80 meters length.

(12) Uniformly dry air over transmitter and receiver seems to be the best condition for good "DX" (distance) radiation (by day there is strong interference by increased absorption).

(13) Meteorological conditions, and probably also the Heaviside layer, are subject to marked changes (particularly the Heaviside layer) at twilight, and at times of disturbances in the earth's magnetism. The results are more or less rapid displacements of the zones and, therefore, changes in signal strength. This gives an explanation for "fading" which, according to the current explanation that it is caused by the difference in phase between space wave and ground wave, would be inexplicable in the case of short waves.

(14) At places in the middle of the zone of maximum sound intensity, the power of the transmitter received plays a small part. With favorable atmospheric conditions, one hears very slight energies (weak signals) with the sound intensity R9.

(15) The form of antenna, vertical or horizontal, is of distinct significance. From Fig. 6 it is evident that the horizontal antenna is more favorable for close communication (Europe); the vertical antenna, excited on a harmonic, is better for "DX" communication, though to be sure over a relatively narrow zone.

(16) From the viewpoint of short waves, it is also possible for us to look differently at long waves. Here too the ground wave is far from playing the part still assigned to it today. It does not reach far; with our chief German stations, in the autumn of 1930, not even 200 kilometers (125 miles).

Reception improvement in the local zone is a question of the antenna, likewise a question also of frequency! This effect should be studied carefully by those who are seeking the salvation of long-wave radio by utilizing tremendous transmitting powers. -Funk Bastler.

* The following is a section from the book "Radioamateurstation für kurze Wellen," by F. Bödigheimer. This should be of great interest to all short wave amateurs.

How Are Shortwaves Propagated?*December 1931/January 1932 Short Wave Craft

How Are Shortwaves Propagated?*
December 1931/January 1932 Short Wave Craft