RADAR. – This term is commonly used to denote a wide range of criteria, methods, and devices designed to detect, even at night and in fog, through "radio" operations (see RADIOCOMMUNICATIONS), the presence and position of any object, whether fixed or in motion (such as rocks, ships, airplanes, etc.). This technology was developed in secret during the last war; the very name "r." derives from the designation of the specialty "radio detection and ranging" of the British radio institute. The fundamental principle is always that of emitting electromagnetic waves of a given frequency from a small transmitting station and detecting and analyzing, near the same station, any corresponding waves reflected by the object to be discovered and located.
Like all great inventions, this one of radar also had some precursors in certain disturbances in radio transmissions between ships, attributed to the reflection of electromagnetic waves off the ships themselves. But since to clearly realize phenomena of the type now called radar, extremely short electromagnetic waves—i.e., microwaves—are required, it is more accurate to say that radar was conceived by Marconi when, in 1922, he discovered the properties of these microwaves, which had hitherto been largely neglected and which are now precisely utilized in radar; or at least when, in 1932, Marconi himself constructed the first microwave radar device and foreshadowed its use for locating ships and airplanes. It is only fair to add, however, that all these precursors were vastly surpassed when it was recognized as advantageous to associate, in any of the early types of radar, the use of electromagnetic waves with that of the cathode-ray oscilloscope, for it is only by means of this remarkable instrument that the radar's own revelations can be accurately followed and made visible.
de 7. R., Paris 1928; P. P. Trompeo, Col Manzoni, tra Virgilio e R., in Rilegature giameniste, Rome-Milan 1930; J. Giraudoux, R., Paris 1930; H. Bremond, R. et Valéry, Paris 1931. Mario Apollonio
Constructive studies began in many countries around 1935, and soon obstacle detectors were developed for use at night and in fog; but it was only in 1940–41 that England and America produced devices similar to those in use today, capable of detecting airplanes at distances exceeding 150 km. When deployed along all coasts, these devices decisively contributed to the defense of England against night bombings.
THE CATHODE-RAY OSCILLOSCOPE. – In its primitive form, this instrument is quite simple and consists of a glass bulb, cylindrical for part of its length and conical toward the end, with an overall length of about 40 cm, in which a very high vacuum is maintained. At the end of the cylindrical section, a filament heated by an appropriate electric current emits electrons; these, attracted by a perforated disk maintained at a high positive potential, are accelerated, pass partly through the hole in the disk, and continue as a beam or pencil until they reach a so-called "gun," a short metal tube maintained at a high negative potential. This gun, by deflecting the path of the electrons passing through it toward the axis of the beam, reduces the beam's cross-section to a small disk. The thus-condensed electron beam continues naturally to the end of the conical section, where it strikes a screen coated with one of the many substances that emit light when struck by electrons. On the screen, a tiny luminous disk will be observed, which will persist as long as the filament is kept incandescent.
If, however, deflecting forces are applied to the electron beam during its path, the luminous disk will shift on the screen. In particular, if two pairs of parallel plates are introduced into the cylindrical part of the bulb—one pair arranged vertically and the other horizontally, through which the electron beam must pass—horizontal deflections can be imparted to the beam by applying appropriate potential differences to the vertical plates, and vertical deflections by applying similar potential differences to the horizontal plates, with corresponding shifts of the luminous disk. If an alternating potential difference is then applied to one pair of plates, the luminous image will oscillate about the center, and if the alternations are sufficiently rapid, a straight luminous line will be observed, either horizontal or vertical depending on the pair of plates chosen. Finally, it is interesting to note that if both systems of plates are operated by applying suitable alternating differences, the well-known Lissajous figures, described in all physics textbooks, can be produced on the screen.
PULSE RADAR. – The simplest and also the first, chronologically, true radar system is the pulse radar, which is still in use today. It operates as follows. Suppose a sawtooth potential difference, i.e., one that rises uniformly from its lowest to its highest value and then rapidly drops back to the lowest value, and so on, is applied to the vertical plates of the oscilloscope described above, at a frequency of one thousand cycles per second. The luminous disk will then trace a horizontal line and instantly return to the origin, thus drawing a luminous segment that can be diagrammatically taken as a time axis corresponding to one-thousandth of a second, and therefore to the time an electromagnetic wave would take to travel 300 km, or 150 km in one direction. Now suppose a small station capable of producing extremely high-frequency electromagnetic waves emits 300 pulses per second, each pulse being extremely brief—e.g., lasting less than one-millionth of a second, which is technically feasible. Each pulse will consist of thousands of oscillations. In this way, each outgoing pulse, acting on the horizontal plates, will impart a small vertical deflection to the cathode beam, which will be at a certain stage of its periodic horizontal oscillation (determined by the vertical plates), and this deflection will be detectable on the screen, marking the departure time of the pulse. If this pulse, or rather the wave train that constitutes it, encounters an obstacle, it will be reflected and part of it will return to the point of origin, where, after being captured and amplified by a receiving device, it can act on the same horizontal plates and cause a second vertical deflection of the electron beam, thus marking, on the luminous segment, the arrival time of the reflected pulse. In this way, not only will the existence of an obstacle be detected, but a means will also be provided to calculate its approximate distance D. Indeed, the distance D' between the two marks of departure and return of the pulse is to the length l of the entire luminous segment as the distance D of the obstacle is to the aforementioned 150 km. That is: D = l·D'/150 km. Naturally, by varying the base frequency of the oscilloscope, devices can be set up to detect the existence of and locate obstacles at much greater distances or, within certain limits, even shorter ones.
PANORAMIC RADAR. – The above-described pulse radars, with appropriate radio operations, allow the location of objects situated on a surface viewed obliquely, e.g., on the sea or in its vicinity.
But it is of much greater interest to arrive at the revelation of objects situated in space. The radars that can accomplish this more difficult task are called panoramic because they actually provide a panorama of a more or less extensive portion of space, seen from the point where they and the observer are located, and the possibility of following any variations. Once in possession of such panoramas it is not difficult to determine immediately the directions and distances of individual objects with pulse radars or equivalent devices.
The devices of panoramic radars are too complex to be described summarily. It is only recalled that their electronic oscillographs, of various structures, generally operate on the electron beam with actions no longer only of a Coulombian type, but also of an electromagnetic type, and that therefore they are often indicated by the qualification of electromagnetic radars.