From the History of Physics

S. I. Vavilov’s Unknown Work on Radio Detection

P. P. Feofilov and I. A. Shlyakhter

In the many-sided scientific biography of S. I. Vavilov there is one little-known page. It is his work in the field of radio.

In 1914 S. I. Vavilov brilliantly graduated from Moscow University. Declining the offer to remain “to prepare for a professorship” at the university, where, in his words, “instead of professors police officers began to appear,” S. I. Vavilov had to enter military service. He enlisted as a volunteer in one of the engineer battalions of the Moscow Military District. It was in this status that the First World War found him. S. I. Vavilov was sent to the active army, where he spent almost four years, first as a private and then as an ensign (the first officer rank in the old army, corresponding in our time to second lieutenant), in engineer and radio units, passing through the fighting in Galicia, Poland, and Lithuania.

Having been initiated into science in the first Russian school of physicists, created by P. N. Lebedev (by that time he had already published two scientific articles), S. I. Vavilov managed, even in the difficult conditions at the front, in a field radio laboratory, to carry on scientific work and to seek new possibilities for applying the radio apparatus then available at the front. Unfortunately, we know very little about this period of S. I. Vavilov’s activity. Until recently, only his article “The Frequency of Oscillations of a Loaded Antenna,” published in 1919 in the Proceedings of the Physical Institute at the Moscow Scientific Institute, and a number of abstracts in the Bulletin of Military Radiotelegraphy and Electrical Engineering for 1917, were known. From S. I. himself it was known that during the war he had proposed and tested at the front a new method for determining the position of enemy radio stations; however, the essence of this method remained unknown.

Fig. 1.

Recently it proved possible to discover the lost manuscript by S. I. Vavilov, “A Method for Determining the Location of a Radio Station by the Strength of Reception of Its Operation,” written by him in October 1916.

In the years of the First World War, radio—the invention of our great compatriot A. S. Popov—was undergoing a serious examination

Fig. 2.

as a new and powerful means of communication. Radiotelegraphic communication, or, as it was called, communication by wireless telegraph, was successfully used for communication between separate military subdivisions. Typical of that time were spark transmitters operating on damped oscillations, and receivers with crystal detectors. Electronic tubes were at the stage of intensive laboratory development and did not yet have wide application in military radio communications.

The few transmitting stations were usually located at headquarters; therefore, the detection of enemy radio stations made it possible to determine almost without error the location of the headquarters. In this connection, the detection of enemy radio stations constituted an important tactical problem from the very beginning of the use of radiotelegraphy in military affairs. However, at that time the methods of radio detection had not yet become established, not only from the point of view of the design of direction finders, but also from the point of view of the choice of the physical principle underlying the method. The existing direction-finding stations did not make it possible to obtain sufficient accuracy of fixes, since their number was small and, when the transmitting station being located was situated near the front line, the angles between the lines connecting the direction finders with the station often differed substantially from right angles, which greatly increased the error in determining the location of the radio station. Moreover, the sensitivity of the direction finders existing at that time was far from always sufficient.

S. I. Vavilov proposes in his work an extremely simple and ingenious method for determining the location of radio stations without any direction finders, by means of standard receiving stations. The essence of the method proposed by him consists in the fact that, at several receiving stations located at known points, the strength of reception of the signals of the station whose location is to be determined is measured in conventional units. Using the data on the relative strength of reception, the posed problem can be solved by means of a simple geometrical construction.

In this formulation, S. I. Vavilov’s work may be regarded as one of the first attempts at a range-finding solution of the direction-finding problem, a problem that was later solved by the remarkable works of L. I. Mandelstam and N. D. Papaleksi on radio interferometry. It is known that range-finding methods make it possible to obtain greater accuracy than azimuthal methods, i.e. methods based on the determination of directions.

The reception strength of a radio-station transmission, i.e. its “audibility” \((A)\), is proportional to the square of the current strength in the receiving antenna \((I)\):

\[ A = kI^2. \]

S. I. Vavilov takes the current strength \(I\) to be inversely proportional to the square of the distance to the radio station. S. I. Vavilov points out that this relation can be accepted only approximately; the insufficient study of the laws of radio-wave propagation compelled him to confine himself to this approximation*). Under these assumptions the audibility proves to be inversely proportional to the fourth power of the distance. If the radio station is received simultaneously by two identical—

*) As further investigation of the laws of propagation of radio waves has shown, in some cases this relation is indeed satisfied.

with receiving stations located at distances \(l_1\) and \(l_2\) from it, then

\[ \frac{l_1}{l_2}=\left\{\frac{A_2}{A_1}\right\}^{1/4}. \]

To determine the strength of reception, S. I. Vavilov uses not the direct method of measuring the current strength in the antenna—an extremely complicated method and practically inaccessible under the front-line conditions of that time—but the method of bringing audibility down to the threshold of auditory perception. In the choice of this method one cannot fail to see the influence of the school of P. N. Lebedev, from which S. I. Vavilov came, and in particular of his teacher P. P. Lazarev, who devoted a number of works to the study of thresholds of auditory and visual perception. On the other hand, it is known how widely S. I. later applied the method of determining the strength of light by weakening the latter to the threshold of perception. The application of this method (the “extinction method”) made it possible to solve a number of important optical problems and led to the discovery of a new, remarkable type of radiation—the radiation of an electron moving at “superluminal” speed—the so-called Vavilov–Cherenkov phenomenon. Thus the first scientific impressions leave a trace on the whole subsequent activity of a scientist. It is not without interest to note that this method was used more than once later for determining the strength of a radio receiver.

To bring audibility down to the threshold value, S. I. Vavilov used a rheostat shunting the telephone. If, with a shunting resistance \(r\), audibility in the telephone disappears, then it may be considered that the initial audibility is

\[ A=\frac{m}{R}\,\frac{R+r}{r}, \]

where \(R\) is the resistance of the telephone, and \(m\) is a proportionality factor.

When the receiving radio stations are identical, i.e. when \(R\) and \(m\) are equal,

\[ \frac{l_1}{l_2}=\left\{\frac{r_1}{r_2}\,\frac{(R+r_2)}{(R+r_1)}\right\}^{1/4}. \]

Thus, knowing the resistance of the telephones of the receiving stations and the shunt resistance necessary to extinguish the sound in the telephone, one can find the ratio of the distances of the receivers from the station sought. Bringing audibility down to the threshold value can be carried out, after some training, with sufficient accuracy.

The geometrical constructions carried out by S. I. Vavilov showed that, to determine the location of a radio station, it is necessary to have no fewer than three receivers. Two receivers make it possible to determine only the circle on which the station sought is located.

station. Three receivers make it possible to construct two such circles, intersecting at two points. To determine which of these points corresponds to the true position of the station is, as S. I. Vavilov indicates, not difficult under front-line conditions. Four receiving stations make it possible to solve the problem quite unambiguously. The problem can also be solved from data on the strength of reception by two stations if, in addition, the direction found by the direction finder is known.

S. I. Vavilov considers in detail the ways of implementing the method he proposes. He regards “ground aviation stations of the R.O.B.T. and T. (Russian Society of Wireless Telegraphy and Telephony) with an elongated mast and network” as the most suitable.

Fig. 3.

and network.” Special attention is naturally paid to uniformity, identical adjustment, and stability of detector operation. The detector need not necessarily be especially sensitive; much more important is the stability of its operation and the possibility of equipping a number of stations with detectors of approximately equal sensitivity. The crystal detectors with which most radio stations were equipped at that time were considerably inferior, in constancy of sensitivity, to other existing detectors (for example, magnetic ones); however, obtaining a sufficient number of such detectors, as S. I. Vavilov points out, was hardly possible. In order to introduce corrections for detector sensitivity, it is proposed to make a daily measurement of the audibility strength of signals given at specified hours by one’s own transmitting station.

Clearly recognizing the approximate character of the relations on which the method is based, S. I. Vavilov nevertheless considers them admissible and refers to a quite satisfactory

Scheme of antenna distribution

Fig. 4.

(at that time) the accuracy of distance determination established as a result of experimental verification.

The experimental verification of the method was carried out by S. I. Vavilov on October 12–13, 1916, in the area of the city of Lutsk.* At a receiving station located on the outskirts of Lutsk, the reception strength of nine stations situated at various distances and operating, in accordance with the order of the commander of the radio division of the Special Army, in a definite regime was determined. The ratio of the distances to these stations, found by the formula given above, was compared with the true ratio. The experiments showed that, with sufficient training of the listener, the formula underlying the method is justified with an accuracy of up to 5%. S. I. Vavilov considered the principal possible sources of error to be: a certain inaccuracy of the formula itself, the relative non-equipower of the transmitting stations, and errors in the observations of the listener.

In an appendix to the work, S. I. Vavilov gives the “Plan for Testing the Accuracy of the Method,” developed by him, in which, with his characteristic concreteness and precision, he provides instructions on how the tests should be conducted in order finally to establish the accuracy of determining the location of a transmitting station by the proposed method.

The plan indicates what type of stations should be used, what detectors, rheostats, and telephones they should be equipped with, where they should be located, and so on. Even such “minor details” as checking watches, measuring the resistance of telephones, etc., are not overlooked.

The successes of modern radiophysics and radio engineering have made it possible to create methods that considerably surpass, in sensitivity and accuracy, the method proposed at the dawn of the use of radio for detection purposes by the 25-year-old ensign Vavilov. Nevertheless, the manuscript that has been found is of great value both as a document characterizing the development of radio methods of detection in Russia during the years of the First World War, and as evidence of the active participation of physicists in the creation of domestic military technology, and, finally, as a new page in the multifaceted creative biography of the outstanding Soviet scientist Sergei Ivanovich Vavilov.

* It is extremely characteristic of S. I. Vavilov that the manuscript presenting the method and containing the results of its experimental verification is dated October 17, 1916, i.e., it was written immediately after the tests had been carried out.