Radiocommunications

RADIOCOMMUNICATIONS. - I. EARLY DEVELOPMENTS. - The scientific basis of this means of various transmissions lies, as is well known, in Maxwell's prediction of the possibility of producing electromagnetic waves that propagate at the speed of light (v. ELECTROLOGY; MAXWELL, JAMES CLARK) and in their experimental realization by H. Hertz. But the technique for the passage from theory to practical requirements gave reason to foresee such difficulties that no one dared venture forth before certain conclusive experiments by the twenty-one-year-old G. Marconi had decisively demonstrated that those difficulties were, at least to a large extent, surmountable. In fact, after Hertz's results, in all the physics laboratories of the world small devices had been built to produce electromagnetic waves and observe their propagation, generally limited to the space of a schoolroom or a corridor; but no one had been imaginative enough to think of founding systematic criteria of telegraphy at a distance on that weak phenomenon. Indeed, even during Marconi's first experiments, illustrious experts on electromagnetic waves spoke out without reservation against that possibility when it was put before them.

The major difficulties that in general stood in the way of the idea of passing from delicate laboratory experiments to practical applications consisted: 1) in the fact that the energy density of the radiation propagating in spherical waves, having at their center the emitting device, attenuates very rapidly, that is, proportionally to the square of the distance; 2) in the fact that the waves tend to propagate in a straight line and therefore cannot follow the curvature of the earth.

But already in the experiments of 1895, with the ingenious insertion of the oscillating electrical device between an antenna, consisting of a long straight vertical conductor, on one side, and the earth on the other, and the analogous insertion in the receiving device, Marconi had reduced those difficulties to a minimum. The waves, by then no longer spherical but predominantly cylindrical around the antenna, attenuated their energy density only proportionally to the distance; moreover they propagated while remaining, in a certain sense, with their base anchored to the earth's surfaces; therefore they followed its curvature and its unevennesses. It was thus that Marconi was able to demonstrate that with that arrangement signals could be received even behind a hill and at distances of a few kilometers. Radiotelegraphy was therefore possible.

II. THE ERA OF THE GREAT LONG WAVES

In the experiments recalled above, the adoption of the antenna and of the connection with the earth in the transmitting and receiving systems, by increasing their capacitance, had automatically brought the wavelengths from the usual values of the order of a meter to values of the order of a hundred meters. Progress in this direction led at the end of 1901 to the well-known first transmission of the letter a of the Morse telegraph alphabet (three dots) from the station of Poldhu (England) to St. John's, Newfoundland. It demonstrated beyond doubt that, with waves of suitable length, the curvature of the earth did not constitute an insurmountable obstacle to transmissions.

From that time on, everywhere, besides perfecting the individual parts of the transmitting and receiving systems, attention was given to the construction of installations with ever greater wavelengths (up to 20,000 km.) and with ever taller antennas (up to more than 300 meters), to which naturally had to be associated corresponding radiation powers. But the practical results, however remarkable already, never corresponded to the hopes, because, in transmissions over very great distances, there still existed mysterious uncertainties that prevented the regularity of radio services.

It was again Marconi who brought a little light into the grave question. As early as 1902 he had observed, while experimenting, that no differences in intensity were found in the signals received by day and by night when they came from distances of less than 800 km., but that by day the signals from distances greater than 1000 km. failed completely, whereas by night they arrived normally from distances of 2500 km. and still decipherable from distances of 3500.

It was therefore the light of the sun that brought the disturbances. At that same time he had observed that over a distance of 244 km. nighttime signals could be emitted with an antenna of 12 m., whereas to obtain daytime signals of equal intensity an antenna of fully 185 m. was needed. Finally, noting that often a ship could not communicate with a nearby station, while it communicated regularly with a distant one, he discovered the existence of what are now called zones of silence.

And it was precisely on these experimental bases, applying the Galilean criterion, that it became possible to arrive at an at least approximate understanding of what happens in the upper layers of the atmosphere. Some of these, at a height of about 100 km., under the action of the ultraviolet rays coming from the sun and of cosmic rays, become strongly ionized and become conductors; therefore they strongly absorb long waves, as does every other conductor, while they absorb little and therefore reflect shorter waves. The mysterious facts were thus clarified, but while the majority of technicians thought that in the face of the natural fact pointed out nothing could be done but take it into account in assuming the commitments of transmitting telegrams, Marconi took the most decisive step for the future of radio transmissions: that is, he returned to experimenting with transmissions using short waves, which he and the technicians had neglected, and in a short time it was certain that only with them could all the difficulties be solved. Announcing the new discovery he said verbatim: "I was mistaken, and with me all the others who have followed me. I shall nevertheless be the first to retrace my steps, abandoning long waves for short waves, on which the future of r. will be founded."

III. MEDIUM WAVES AND RADIOTELEPHONY

Before going on to give some brief notes on the characteristics of short-wave transmissions, it is worth briefly considering the technique of medium waves, by which the usual radio broadcasts predominantly take place everywhere. In the first place, it should be noted that with these it is no longer a matter of telegraphy, that is, of a simple transmission of conventional signals, but of telephony, that is, of the transmission of sounds, a far more difficult matter. But the already highly advanced technique succeeded in rapidly overcoming the difficulties. The transmitting station is tuned to emit electromagnetic waves of a given wavelength λ, for example 300 m. Naturally its frequency ν will be such that the product λν equals the speed of light; therefore ν will be equal to 1,000,000 periods or alternations per second, that is, so rapid that, unless one has very special devices, they give the impression of a continuous flow of electromagnetic energy. The emission of a Morse telegraph signal corresponds to a simple interruption of that radiation, as brief as possible for the dots, a little longer for the dashes. To transmit a certain sound, for example an A sharp of 435 periods, it is necessary instead that the said electromagnetic flow undergo 435 alternations per second, or, in other words, that upon the very high electromagnetic frequency there be superimposed the acoustic frequency of the sound one wishes to transmit, something that is achieved through suitable technical devices. Thus the waves that propagate through the ether always have the fundamental frequency ν of the emitting station, but modulated with the acoustic frequency of the note, and can be picked up only by a receiver tuned to the frequency ν and give rise to the sound corresponding to the modulation either in a telephone or in a loudspeaker. All these transformations take place while preserving not only the frequency (the note) but also the timbre of the sound being transmitted, which makes possible the transmission not only of music, but also that, far more difficult, of speech.

IV. SHORT WAVES

This is now the designation for waves with a wavelength between 100 and 15 m, reserving the name of ultra-short waves, or that of microwaves, respectively for those with a wavelength between 15 and 1 m, and less than 1 m.

Among the useful properties of short waves, besides that of being almost insensitive to the harmful effects of sunlight, and that — perhaps even more useful — of being equally insensitive to bad weather, there is the very valuable and by now widely exploited one of being fairly easily directional, a property on which modern radio applications are essentially based.

Short waves, even apart from the fact that they are generally produced hertzianly, that is, with a transmitting device not connected on one side to the earth, already have a marked tendency to propagate in a straight line, like light waves, which are extremely shorter still. This property of theirs makes it possible to produce them, as Marconi said, in beams, even of small cross-section, which do not tend appreciably to spread either during propagation or in reflections, and which can be directed from the moment of emission in a predetermined direction. And this is chosen in such a way that, reflecting off the already mentioned ionized and therefore conducting and reflecting spherical layer, the beam itself strikes exactly the desired place. Naturally, if the distance is very great, for example when one wishes to telegraph to the antipodes, a single reflection will not suffice; but then one can arrange the initial direction so that the reflected beam strikes a place (generally the sea) from which it is reflected again toward the upper layer, so that with another reflection, or several more, it reaches exactly its destination. It is understood that this theoretical procedure must be supported by suitable technical aids and controls; but the fact is that in the relatively short time of a few years it has been possible not only to telegraph but also to telephone as far as the antipodes. It is interesting to note that beneath the reflections of the upper layer there are large zones of absolute silence analogous to the less absolute ones observed also in the case of long waves, very useful for avoiding the excessive publicity of transmissions. Equally interesting and useful is the fact that the whole transmission process takes place exclusively through successive electronic phenomena and motions, without material mechanisms, which would inevitably introduce delays due to their inertia. It is precisely this absence of delays that made it possible to achieve in telegraphy a speed of transmission even greater than the already very high one reached by telegraphy with conductors. Economically there is also the great advantage of an enormous saving of energy, because with short-wave and beam transmissions installations of quite limited power suffice, which have the merit of not interfering with one another.

Ultra-short waves do not present sufficient differences with respect to the preceding ones to have particular applications. Microwaves, on the other hand, still less dissimilar — though still very far off — from their optical sisters, are at present already of great scientific and technical interest. In particular they are now in use in radar application (v. RADAR) and in growing use in the various visual applications (v. TELEVISION).

Paolo Straneo

V. RADIO AND CATHOLICS

The Church could not remain indifferent before the problems that arise for consciences from the good or bad use of radio as a means of propaganda, and before the need to use radio as a weapon of the apostolate.

1. Radio and the Holy See

The Holy See established the first radio station in Vatican City in 1931, under the guidance of the inventor of radio himself, Guglielmo Marconi, and in that year the pontiff Pius XI began those radio messages which, under his pontificate and especially under that of his successor, would carry to every part of the Catholic world, with a speed and breadth hitherto unknown, the thought of the Church and the exhortations of the Supreme Pastor, also transmitting news, celebrations and religious instruction in the principal languages.

A decree of the Sacred Penitentiary, De Benedictione Papali ope radiophonica accepta, of 15 June 1933 (AAS, 23 [1931], p. 277), sanctions the liturgical validity of the radio-transmitted papal blessing and the possibility of gaining the indulgences attached to it even for radio listeners; indeed, in the same Latin formula that follows the blessing, there is the reference "cunctis Christi fidelibus qui adsunt et qui ubique terrarum ope radiophonica... benedicentis vocem pie devoteque accipiant". On the other hand, Mass heard over the radio is not recognized as valid for the purpose of fulfilling the obligation of a holy day of obligation.

2. Catholic initiatives

In Europe the broadcasting stations almost always belong to the State or to parastatal bodies. Nevertheless Catholics seek to influence them, sometimes obtaining their own special broadcasts. In many countries there is the broadcasting of Mass and of the Sunday Gospel, and at times also of Catholic catechism. Before the Second World War the broadcasts of the R.R.O. (Dutch Catholic Radio) were particularly interesting, as it carried out a direct part of the radio program, and the German Catholic radio organization was also notable. In Italy too, in May 1940, the Centro cattolico radiofonico arose with the aim of influencing, morally and spiritually, by direct and indirect means, the programs of the national radio.

Raimondo Spiazzi