RADAR. — This designation is used generically to indicate a considerable series of criteria, methods, and devices capable of detecting, even at night and in fog, by means of “radio” operations (v. RADIOCOMUNICAZIONI), the presence and position of some object, whether stationary or moving (reefs, ships, aeroplanes, etc.). This technique was developed secretly during the last war; the designation itself, “r.,” derives from that of the specialty “radio detection and ranging” of the English radio institute. The fundamental principle is always that of emitting from a small transmitting station electromagnetic waves of a given frequency and detecting and analysing, near that same station, the possible presence of corresponding waves reflected by the object one wishes to discover and locate.
Like all great inventions, that of r. too had certain forerunners in particular disturbances in radio transmissions between ships, attributed to the reflection of electromagnetic waves from the ships themselves. But since the clear production of phenomena of the type now called r. requires electromagnetic waves of extremely short wavelength, that is, microwaves, it is more logical to say that r. was conceived by Marconi when, in 1922, he discovered the properties of these microwaves, hitherto greatly neglected, and which are now used precisely in r.; or at least when, in 1932, Marconi himself constructed the first microwave r. apparatus and anticipated its use for locating ships and aeroplanes. It is nevertheless right to add that all those forerunners were enormously surpassed when the advisability was recognized of associating, in any of the earliest types of r., with the use of electromagnetic waves, that of the cathode-ray oscillograph, since it is only by means of this marvellous instrument that the detections peculiar to r. can be followed accurately and rendered visible.
Constructional studies began in many countries around 1935, and obstacle detectors for use at night and in fog were soon produced; but only in 1940–41 were apparatuses analogous to those in use today obtained, in England and America, capable of detecting aeroplanes at distances exceeding 150 km. When installed along all the coasts, they made a decisive contribution to the defence of England against night bombing.
L'OSCILOGRAFO A BAGGI CATODICI. - In its primitive and simplest form, it consists of a glass bulb, cylindrical for one section and then conical, with an overall length of approximately 40 cm., in which a very high vacuum has been created. At the end of the cylindrical section, a filament, brought to incandescence by a suitable electric current, emits electrons; these, attracted by a perforated disk maintained at a high positive potential, are accelerated, pass partly through the disk’s aperture, and continue in the form of a beam or pencil toward a so-called gun, a short metal tube maintained at a high negative potential which, bending the path of the electrons passing through it toward the axis of the beam, reduces its cross-section to a small disk. The electron beam, thus concentrated, naturally continues to the end of the conical section, where it strikes a screen coated with one of the numerous substances that emit light when bombarded by electrons. A very small luminous disk will then be visible on the screen, remaining there as long as the filament is kept incandescent.
If, however, deflecting forces are brought to bear on the electron beam during its passage, the luminous spot will move across 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, with the electron beam passing between them, horizontal deflections can be imparted to the beam by applying suitable potential differences to the vertical plates, and vertical deflections by applying analogous potential differences to the horizontal plates, with corresponding movements of the luminous spot. If an alternating potential difference is then applied to one pair of plates, the luminous image will oscillate around the centre; and, when the alternations are sufficiently rapid, a horizontal or vertical luminous straight line will be observed, depending on which pair of plates has been selected. Finally, it is interesting to note that, by operating the two systems of plates and applying suitable alternating potential differences to them, the very familiar Lissajous figures described in all treatises on physics can be produced on the screen.
R. A IMPULSI. — The simplest and also the first genuine r. to be developed, chronologically speaking, is the so-called pulse type, which is still in use. It operates as follows. Suppose that a periodic potential difference is applied to the vertical plates of the oscillograph described above, one of the so-called sawtooth type—that is, one which passes uniformly from its lowest value to its highest and then drops from the latter extremely rapidly to the lowest value, and so on—with a frequency of one thousand cycles per second. The luminous spot will then traverse a horizontal segment and instantly return to the origin, thereby tracing a luminous line that may be taken diagrammatically as a time axis corresponding to one-thousandth of a second and thus to the time an electromagnetic wave would require to travel 300 km., or subsequently, in both directions, 150 km. Now suppose that a small station capable of producing extremely high-frequency electromagnetic waves launches 300 pulses per second, and that these are very brief—for example, lasting less than one-thousandth of a second—a technically feasible arrangement. Each of these pulses will consist of thousands of oscillations. In this way, each outgoing pulse, acting on the horizontal plates, will impart to the cathode ray, which will be at a certain stage of its periodic horizontal oscillation (determined by the vertical plates), a small deflection in the perpendicular direction, visible on the screen; this will mark there the time at which the pulse was sent. Now, if the pulse—or rather the train of waves of which it consists—in-
counter an obstacle, it will be reflected, and part of it will return to the point of departure, where, having been captured and amplified by a receiving apparatus, it may act upon the same horizontal plates and cause a second vertical deflection of the electron beam and consequently, on the luminous segment, mark the arrival of the reflected pulse. Thus not only will the existence of an obstacle have been detected, but it will also be possible to calculate approximately its distance D. In fact, the distance l′ between the two marks indicating the 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′/l × 150 km.
Naturally, by varying the basic frequency of the oscillograph, devices can be arranged capable of detecting the existence and locating obstacles at much greater distances or, within certain limits, at shorter ones.
PANORAMIC RADARS. – The preceding pulse radars, by means of suitable radio operations, make it possible to locate objects situated on a surface viewed obliquely, for example, on the sea or in its vicinity.
It is much more important, however, to achieve the detection of objects situated in space. The radars capable of performing this more difficult task are called panoramic because they actually provide a panorama of a more or less extensive portion of space, viewed from the point at which they and the observer are located, together with the possibility of following any changes therein. Once such panoramas are available, it is not difficult to determine immediately the directions and distances of the individual objects by means of pulse radars or equivalent devices.
The devices of panoramic radars are too complex to be described even summarily. It need only be recalled that their electronic oscillographs, generally of various designs, operate on the electron beam not only through actions of a Coulombian type, but also through electromagnetic actions, and are therefore often designated as electromagnetic radars.