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¶A receiver with a "klystron," "lighthouse" or other oscillating tube, used to convert the microwave echo to a lower radio frequency so that it can be amplified.
¶An "oscilloscope" ("scope" for short), radar's screen, which is a cathode-ray tube such as is used in television. The most common type, the "Plan Position Indicator," is a circular dial with an electronic beam like a minute hand, which sweeps around the dial in synchronization with the scanning antenna, painting in its fluorescent wake a picture of what radar sees.
Radar's ability to report what it sees depends on differences in its targets' reflecting power (which engineers call the "dielectric constant"). Metal is an excellent reflector; earth, an indifferent one. Water also is a good reflector, but because of its flat surface, the radar beam caroms off at an angle and no echo reaches the receiver (except from a spot in the center of the beam); hence water appears black on the scope.
Echoes from the earth are affected by the angle at which the radar beam strikes its irregularities, and by "shadows" cast by raised objects. Thus mountain tops and ridges are easily distinguishable from the surrounding terrain.
One of radar's toughest identification problems—and one of the most fascinating sidelights on the great supergadget—was how to tell whether detected planes or ships were friendly or hostile. It was solved by an ingenious instrument called I.F.F. ("Identification, Friend or Foe"). When an I.F.F.-equipped plane is hit by a friendly radar beam, the instrument automatically flashes back a coded identifying signal.
Privacy in the Bathtub. Radar still has many limitations. Since it travels only in a straight line, it cannot "see" beyond the horizon. Because it cannot see through water or most solid obstructions, there is little chance that it will ever invade the privacy of four walls.
At ground level, a radar beam is scattered by ground irregularities. For this reason, contrary to Sunday-supplement predictions, it is not practical as an anticollision device on autos or railroads.
Many of radar's wartime jobs, based on locating a noncooperating target, in peacetime could be performed just as well by ordinary radio. Nonetheless, engineers predict a great postwar future for it. For one thing, they expect it to be required equipment on ships and possibly on commercial planes.
On a ship, radar is insurance against collision with icebergs, rocks or other ships; it can take a vessel at full speed through a crowded harbor and dock it in the foggiest weather. In the air, radar, supplemented by a map of the terrain, would keep a pilot as well oriented as if he were flying over his living-room rug, would ward off collisions with mountains and other planes. It would, of course, prevent such accidents as the Army bomber's crash into the Empire State Building last month.
Phenomenon in a Pipe. Radar enthusiasts have suggested many other uses, from controlling traffic at airports to studying the speed of high-flying birds. But to physicists, radar is only an item in the vast possibilities opened by the discovery of microwaves.