RADAR. - This name is generally used to designate a considerable series of criteria, methods and devices designed to detect, even at night and in fog, by means of "radio" operations (v. RADIOCOMUNICAZIONI) the presence and position of some object, fixed or moving (reefs, ships, airplanes, etc.). This technique was developed secretly during the last war; the very name "r." derives from that of the "radio detection and ranging" specialty of the English radio Institute. The fundamental criterion is always that of launching from a small transmitting station electromagnetic waves of a given frequency and of detecting and analyzing, in the vicinity of that same station, the possible presence of corresponding waves reflected by the object that one is interested in discovering and locating.
Like all great inventions, this one of r. also had some precursor in certain disturbances in radio transmissions between ships attributed to the reflection of electromagnetic waves on the ships themselves. But since, in order to clearly bring about phenomena of the type now called r., electromagnetic waves of very short wavelength are required, 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, until then quite neglected, and which precisely in r. are now utilized; or at least when in 1932 the same Marconi built the first microwave r. apparatus and advocated its use for the localization of ships and airplanes. It is fair, however, to add that all those precursors were enormously surpassed when the advisability was recognized of associating in any of the first types of r., with the use of electromagnetic waves, that of the cathode-ray oscillograph, because only by means of this marvelous instrument can the detections proper to r. be followed exactly and made visible.
Construction studies began in many States around 1935 and obstacle detectors to be used at night and in case of fog were soon produced; but only in 1940-41 were there, in England and America, apparatuses similar to present-day ones capable of detecting airplanes up to a distance greater than 150 km., which, applied along all the coasts, contributed decisively to the defense of England from night bombardments.
THE CATHODE-RAY OSCILLOSCOPE
In its primitive and simplest form it consists of a glass bulb, cylindrical for a stretch and then conical, with an overall length of about 40 cm., in which a very high vacuum is created. At the end of the cylindrical part a filament, brought to incandescence by a suitable electric current, emits electrons; these, attracted by a perforated disc maintained at a high positive potential, are accelerated, pass in part through the hole of the disc and continue in the form of a beam or pencil as far as a so-called gun, a short metal tube maintained at a high negative potential, which, bending the path of the electrons that pass through it toward the axis of the beam, reduces its cross-section to a small disc. The electron beam thus condensed will continue naturally as far as the bottom of the conical part, where it will strike a screen coated with one of the numerous substances that emit light when they are struck by electrons. On the screen one will then observe a very small luminous disc that will persist as long as the filament is kept incandescent.
If, however, during the path of the electron beam, deflecting forces are made to act upon it, the luminous dot will move on the screen. In particular, if two pairs of parallel plates are introduced into the cylindrical part of the bulb, one arranged vertically and the other horizontally, between which the electron beam must pass, it will be possible to impart to it horizontal deflections, by applying suitable potential differences to the vertical plates, and vertical deflections, by applying analogous potential differences to the horizontal plates, with the corresponding displacements of the luminous dot. If, then, an alternating potential difference were applied to a pair of plates, the luminous image would oscillate about the center, and, when the alternations were sufficiently rapid, one would observe a luminous straight horizontal or vertical line according to the pair of plates chosen. Finally, it is interesting to note that if the two systems of plates were made to operate by applying suitable alternating differences to them, one could produce on the screen the very well-known Lissajous figures described in all treatises on physics.
PULSE-TYPE RADAR
The simplest and also the first, in order of time, true radar actually built is the so-called pulse type, still in use. It works in the following way. Suppose a periodic potential difference, called sawtooth, is applied to the vertical plates of the oscillograph described, that is, one that passes uniformly from its lowest value to its highest and then plunges very rapidly from this back to the lowest value, and so on; and this with a frequency of a thousand cycles per second. The luminous spot will then travel along a horizontal stretch and return instantly to the origin, thus tracing a luminous segment that can be taken diagrammatically as an axis of time corresponding to one thousandth of a second and therefore to the time an electromagnetic wave would take to travel 300 km, or successively in both directions 150 km. Suppose now that a small station is capable of producing electromagnetic waves of very high frequency, sending out 300 pulses per second, and that these are very brief, e.g. lasting less than a thousandth of a second, which is technically feasible. 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 beam, which will be at a certain stage of its periodic horizontal oscillation (determined by the vertical plates), a small jolt in the perpendicular direction, observable on the screen, and this will mark there the departure time of the pulse. Now if this pulse, or rather the wave train that constitutes it, meets an obstacle, it will be reflected and in part will return to the place of departure, where, picked up and amplified by a receiving apparatus, it can act on the same horizontal plates and cause a second jolt in the vertical direction of the electron beam and therefore, on the luminous segment, the mark of arrival of the reflected pulse. Thus not only will the existence of an obstacle have been detected, but there will also be a way to calculate approximately its distance D. In fact the distance l' 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'/l x 150 km. Naturally, by varying the base frequency of the oscillograph, devices can be set up capable of detecting the existence of and locating obstacles at much greater distances or even, within certain limits, smaller ones.
PANORAMIC RADAR
The previous pulse r. make it possible, by suitable radio operations, to locate objects situated on a surface seen at an angle, for example, on the sea or in its vicinity.
But it is far more important to arrive at the detection of objects situated in space. The r. that can accomplish this more difficult task are called panoramic, since they effectively provide a panorama of a more or less broad portion of space, seen from the point where they and the observer are located, and the possibility of following its eventual variations. Moreover, when one has such panoramas at one's disposal, it is not difficult to determine immediately the directions and distances of the individual objects with pulse r. or equivalent devices.
The devices of panoramic r. are too complex to be described in summary fashion. It is recalled only that their electronic oscillographs, of various structures, generally act on the electron beam with actions no longer only of the Coulomb type, but also of the electromagnetic type, and that for this reason they are often designated by the term electromagnetic r.
Bibliography
Paolo Straneo