436 BELL SYSTEM TECHMCAL JOURNAL 



the testing of microwave radars. However, similar methods and instru- 

 ments have also been employed in the testing of microwave communication 

 systems and such applications can be expected to increase. In this situation 

 the developers and users of microwave communication systems are fortunate 

 in that almost all of the techniques and devices developed for radar testing 

 are equally applicable to communication systems. This is even true of the 

 video units, which are useful in connection with pulse modulated telephone 

 systems and AM or FM television systems. 



So many persons, both within and outside Bell Laboratories, have con- 

 tributed to the developments described that the authors have reluctantly 

 reached the conclusion that the assignment of individual credit should not 

 be attempted. 



Requirements 

 Operation of Typical Radar 



Subsequent discussion m.ay be simplified by first reviewing briefly the 

 operation of a somewhat typical radar, as shown in Fig. 1. Under the con- 

 trol of d.c. or so-called video pulses from the miodulator, short pulses of radio 

 frequency energy are delivered by the magnetron transmitter to a highly 

 directive antenna, ordinarily arranged to scan a section of space. Energ>- 

 reflected from an object or "target" in the path of the beam is intercepted 

 by the same antenna. The received pulses or echoes are converted to an 

 intermediate frequency by heterodyning against a local oscillator, the 

 frequency of which may be automatically controlled. 



To enable the same antenna to serve for both transmitting and receiving, 

 a TR tube or transmit-receive switch is usually provided. This consists 

 of a partially evacuated resonant cavity containing a spark gap which breaks 

 down during the transmitted pulse, thus preventing the transmitted power 

 from injuring the sensitive receiver. An RT tube, consisting of a sunilar 

 resonant cavity and spark gap, may be provided to prevent absorption of 

 the received signal by the transmitter. After amplification and detection 

 of the received signal, the resultant video pulses are applied to an indicator 

 which may present information in any of several different ways. Cus- 

 tomarily the direction of the target (determined by antenna orientation) 

 and its range (determined by reflection interval, 10.7 microseconds per mile) 

 are shown. The system may be used merely for searching, or for fire con- 

 trol, bomb direction, or other functions, with additional equipment as re- 

 quired. 



Types of Tests and Test Sets 



Figure 2 shows an early assemblage of radar test equipment for the 10 

 cm range, initially produced in 1942, which has seen wide usage. 



