"Syntony" was an early term which meant "tuning", while "aerography" was a word that DeForest briefly favored for referring to radio transmissions. The facility pictures are of an early station built on Block Island, Rhode Island, to provide direct communication with the mainland.
The Electrical Age, February, 1904, pages 80-82:
By LEE DE FOREST, Ph. D.
MUCH has appeared in the technical and popular press concerning the question of interference between a number of wireless telegraph stations when operating simultaneously, and the possibilities of preventing such interference. Yet, notwithstanding, and possibly because of the variety of conflicting opinions and explanations given, much misunderstanding on this subject is in evidence.
It has again and again been shown how it is possible for two wireless transmitters, each sending out Hertzian oscillations of a frequency differing sufficiently from that of the other, to excite two distinct receiving systems, each tuned in syntony with its own transmitter. This is easy to accomplish provided first that the two wave-lengths employed are sufficiently unlike, and, second, that neither transmitter acts too powerfully upon that receiving instrument with which it is not desired to communicate. The old thread-bare analogy explains it very simply. If one lifts the dampers from the strings of a piano and sings loudly one clear, long-sustained note into the instrument, he will hear a faint response from the string attuned to that note, and possibly also from its first, or even second, octave string. Sound similarly another note, and the same effect is noted from another and corresponding string. But let one sing the note very faintly for a fraction of a second only, and he will not find his string responding; conversely, let him shout loudly, even a clear note, and most, if not all, of the strings will take it up to some extent; let him strike the body of the piano a quick, strong blow, and every string vibrates violently.
It is exactly analogous in wireless telegraphy; the simple tuned receiver, as now generally understood, depending for its resonant quality upon the self-inductance of a coil of wire and the capacity of a condenser in circuit with its wave detector, is absolutely at the mercy of a pure tuned wave of whatever frequency, provided this be of sufficient power when it reaches the receiving antenna.
It is, of course, possible to cut in a resistance in this receiving antenna, and to diminish the strength of the impulses received from the offending transmitter so that these are no longer able to excite oscillations in a local resonant circuit which is attuned to quite another frequency. But at the same time it is to be remembered that this inserted resistance must also cut down the effect of the oscillations emitted by the transmitter which is in tune with this receiver, and which it is desired to hear. Consequently, if this latter instrument be less powerful than the hostile transmitter, or further removed, it is impossible by the existing methods to receive its messages free from interference.
The same considerations hold yet more strongly when the source of interference is practically an "a-periodic" transmitter, or one giving out not a succession of uniformly timed impulses, i. e., a train of waves, but simply a single, violent, etheric blow; a "whip-crack," instead of the musical note.
There is, however, a method of protecting a tuned receiver from all influences of this character, and rendering it strictly deaf to any but prolonged wave-trains of its own, or nearly its own, attunement; and I have little fear that the present vulnerability of the so-called protected or syntonic receivers to violent hostile impulses will long exist. Theoretically, this method will make a receiver immune even from the impulses of a powerful transmitter at the same station and utilizing the same antenna wires, but for actual duplex working, additional precautions are necessary.
Before the above method of preventing interference from strong a-periodic impulses is practical for commercial working, a number of details must be refined, and I hope before long to be able to disclose the modus operandum of this invention, realizing that to-day nothing in wireless telegraphy will be more useful, or is awaited with keener interest by technical men.
Another desideratum in syntonic aerography is the production of longer wave-trains, i. e., that each spark shall represent, not as it generally does to-day, a train of three or four impulses rapidly dying away, but one entailing ten or twenty complete oscillations. This effect will not, as some might suppose, necessitate slower word-transmission, for the actual duration of such long wave-trains, even with wavelengths of many hundreds of feet, is to be computed in ten-thousandths of a second. Thus, if the length of a radiated wave is 1,000 feet, a wave-train of twenty complete oscillations, which may be classed as one but slightly damped, is about four miles in length, and 46,500 such wave-trains per second could be sent out, were it possible to obtain so great a spark frequency. But with the spark frequencies actually obtainable and desirable, say 200 per second, it will be seen that intervals of no radiation exist between sparks which represent more than two hundred times the duration of the actual period of radiation. To obtain then an absolutely undamped train of waves would require the generation at each spark of a succession of some two hundred more oscillations than the ordinary oscillating transmitter circuit (containing Leyden jars, inductance helix, and spark gap) will now maintain.
If a hot-wire ammeter, suitably shunted for its own protection, be inserted in the antenna or ground wire of a transmitter taking about two horse power, it will show a reading of three or four amperes, when the inductance in the closed oscillating circuit, to which the antenna and ground are attached, is so regulated that this circuit is "in tune" with the antenna's natural period of vibration. But this same hot-wire instrument, if placed in the oscillating circuit itself, will show a reading twenty or even fifty times as great. This merely goes to prove how small a part of the available energy is actually sent into the antenna and radiated outwards. Were it possible to get a reasonable efficiency of transfer of energy from the oscillating circuit to the aerial we might well expect such a transmitter as the above to operate over the Atlantic Ocean. One fact must be borne in mind here, however; the antenna system is poorly resonant compared with the closed oscillating circuit. It is strongly damped, and therefore a good radiator, and a hot wire inserted there is by no means subjected to the same cumulative heating effect of the current passing to and fro through it as it is by the same number of amperes if the instrument be located in a closed, strongly resonant, oscillating system. The exact quantitative determination, or comparison between the cumulative current effects in antenna and oscillator respectively, is extremely difficult, and theory gives us little data of real value as a basis for such comparisons.
Nothwithstanding all that has been said concerning the great inefficiency of the ordinary spark in air and the large amount of damping of the oscillations thus resulting, with a spark from a battery of, say, ten two-quart Leyden jars in parallel, and not exceeding an inch in length, kept clear of all arc or flame, the oscillations are well prolonged, and if no radiating antenna and earth are attached to the circuit, the latter cannot be styled strongly damped.
Placing the spark-gap under heavy air compression undoubtedly diminishes the damping there resulting; any such method for shortening the spark length, while maintaining the sparking potential, will have this effect. But what is most needed is a closer linking of antenna and oscillating circuits--a more equitable proportioning of the respective amounts of energy in the two.
The current amplitudes in the antenna circuit do not approach those in the exciting and feeding circuit. Here I obtain the best results with the auto-transformer connection, i. e., by making a portion of the helix at the base of the aerial actually a part of the oscillating circuit, so that this same helix comprises the greater part of the inductance in the oscillating circuit. The same principle is used by Professor Slaby. It allows greater current densities in the aerial than I have been able to obtain with any transformer of two distinct and insulated windings. The linking between the resonator and radiator circuits is much more intimate, and the antenna therefore more readily takes up the forced vibrations. This latter is a valuable feature; for transmission is invariably better with long wave-lengths of oscillation, and practical considerations render it generally impossible, where any quantity of energy is to be radiated, to employ antennae whose natural periods of vibration are equal to those of the feeding or oscillating, circuits. In such a case, the best that can be done in attuning the two is to choose an oscillating circuit for which the natural period is a lower harmonic of the natural period of vibration of the antenna circuit, and then to so closely link the two circuits that the antenna circuit is forced to vibrate in pulse with the oscillations of the closed resonator circuit.
The natural quarter-wave-length of the antenna's natural vibration may be made to be very different from that of a simple straight wire of the same length and height. Thus it is common to assume that an antenna consisting of one wire 200 feet in height sends out a wave 800 feet long; but by actual measurement of the natural period of an antenna consisting of ten wires, each 200 feet in length, connected at top and bottom, supported by a mast only 180 feet in height, and spread out widely midway up, in fan shape, a quarter-wave-length of 287 instead of 200 feet was obtained, an increase of 43 per cent. This increase in the natural period of vibration was partially due to the additional capacity, with respect to earth, which such a flare fan-shaped arrangement of wires produced. It was found that this natural period of vibration varied somewhat from day to day, in a peculiar, and as yet unexplainable, manner, due probably to variations in the moisture in the earth and air; and this irregularity--almost invariably found to exist with any antenna--is another weighty reason why it is impossible to depend much on the natural period of the elevated radiating conductors for tuning, but advisable rather to force the antenna always to vibrate strongly to the period of the resonator circuit, where the determinating factors may be kept practically constant.
The same considerations hold for the receiving system; the antenna should serve simply as a collector of neutral periodicity; in other words, be so strongly damped that the period of the received oscillations which it collects and transmits to the resonant receiving circuit at its base shall not be affected by such vibrations as from day to day occur in the natural period of vibration of the antenna.
By this means we virtually obtain a closed resonant circuit at the transmitter operating upon a closed resonant circuit at the receiver through the medium of two neutral antennae, and strong response is obtained only when the resonant circuit of the receiver is in close syntony with that at the transmitter.
It has been abundantly pointed out that syntony is very unsatisfactory unless the resistance and capacity of the receiver itself are always the same upon the receipt of the transmitted impulse. This, unfortunately, is not the case with any coherer, microphone, or auto-coherer. In the filings tube the great variant is the capacity, which is surprisingly large; in the microphone the resistance, which is considerable, varies widely from time to time. As a result, the natural period of the resonant receiving circuit is constantly altering, and close tuning with the same is therefore impossible.
In the form of electrolytic responder which I am now using I find these elements of resistance and capacity surprisingly constant. If the electro-motive force of the local battery as applied to the responder is not too great, a delicate galvanometer in the circuit shows a constant reading, and its deflections, which are exactly proportional to the strength of the received impulses, are not capricious or irregular, as is frequently the case where an auto-coherer is employed. As a result of this regularity it is possible to obtain far better results in electrical attuning than with any form of coherer or any practical wave detector with which I am acquainted, save possibly the magnetic detector; and at present there is no comparison between the sensitiveness of this electrolytic responder and that of the detector.