Radio age research, manufacturing, communications, broadcasting, television (1941)

objects which are illuminated by and reflect the radio waves has been made practical. Thus, whereas vision gives accurate angular in- formation and by interpretation, approximate distance information, radar can supply accurate informa- tion in all three dimensions irre- spective of the visibility condition. Although several methods for measuring distance by radio have been proposed, only two of these have come into practical use in present day radar equipment. These have been known as the pulse and frequency modulation (FM) sys- tems. In the pulse system, a short burst of radio-frequency energy, usually between 1/10 and 10 micro- seconds long, is radiated from the transmitter. The interval between pulses lies generally between 1/100 and 1/1000 second, although longer and shorter intervals are used in special equipments. Some time after the pulse is transmitted, it reaches the reflecting object and returns again to the radio receiver. Since the speed of travel of radio waves has been accurately measured and is well known, the time occupied be- tween transmission and reception of the reflected pulse can be used to obtain an accurate measure of the distance of the reflecting object. Spot Moves Across Tube Face In order to measure the time, the spot on a cathode-ray tube is caused to move rapidly across the face, usually starting at the exact instant the pulse is transmitted. The rate of travel of the spot is adjusted to correspond to the scale length and definition desired. It may be as fast as an inch or more per microsecond when great accuracy is desired, or, in the long-range sets, slower than an inch per millisecond. When the signal is received, the trace is brightened or a kink is put in it. The distance of the indication from the start of the trace is proportional to the distance of the reflecting ob- ject. With adequate calibration of the equipment, this characteristic can be used to give a very accurate measure of the distance. Uses Continuous Wave In the FM system, a continuous transmitted wave is used but the radio frequency is swept back and forth rapidly. In the RCA alti- meter, the frequency is swept either 4- or 40-megacycles, depending on the scale used, at approximately a 100-cycle-per-second rate. Since there is a time interval between the transmission and reception of the reflected signal, the frequency of the transmitter has changed during the interval and the received signal will have a different frequency than the transmitted signal. If the rate of change of frequency is linear, the difference will be proportional to the distance of the reflecting ob- ject. The frequency difference can be easily measured by feeding the received signal and a small part of the transmitted signal into a detec- tor. The resultant output has a fre- quency which is equal to the "beat" between the two or the difference in frequency. Most radar sets use the pulse method because of its ability to dis- tinguish easily between reflection from several targets thus fixing the position of each one. The FM method is particularly applicable when accurate information is re- quired as to the position of a single target. In the radio altimeter, where it has been used most suc- PLANE USED BY RCA FOR ITS OBSTACLE DETECTION TESTS IN 1937 - 39. THE V-ANTENNA APPEARS JUST ABOVE THE CENTER OF THE WING-SPAN. cessfully, this single target is the earth's surface. Many methods of presenting the distance and directional informa- tion received by the radar set have been used. Although the informa- tion for forming a complete three- dimensional picture is at hand, three-dimensional indicators are not yet available. For most pur- poses, the information desired can be shown in two dimensions satis- factorily. In cases where horizontal, vertical, and distance information are all wanted at one time, as in night-fighter radar equipment, vari- ous tricks have been used to create the illusion on the two-dimensional indicator of the scene the three- dimensional eye would see if it were at the position of the radar trans- mitter. In the most commonly used form of radar system, the scanning is limited to determining the angular position and the distance of targets without reference to their elevation. This type of limited display is use- ful, for instance, for ground and ship installation to get information about surface targets such as ships on the sea, and also in the search and instrument bombing airborne radar systems. It is also used in ground radars and ship radars which detect aircraft; auxiliary equipment in some cases being em- ployed to determine altitude. RCA Began Research in 1932 In 1932, the RCA organization started research on microwave tubes and components. This apparatus was demonstrated before IRE meet- ings in 1934. At that time, the pos- sibility of reflecting sharp beams of microwaves from metal objects and ionized gas was shown. [RADIO AGE II ]