386 



BELL SVSTFJr TECHNICAL JOURNA L 



standardize on a mid-band frequency of 30 mc or 60 mc. The latter fre- 

 quency has been used in most radars developed at the Bell Telephone 

 Laboratories. 



In pulsed radar systems the transmitter is turned on and ot^ by the 

 modulator in such a way that radio frequency energy is generated for a 

 pulse duration r seconds at a repetition rate which is usually several hundred 

 per second. When the transmitter is modulated by a pulse which approaches 

 that shown in Fig. 1(a), the energy-frequency distribution in the transmitted 

 signal is as shown in Fig. 1(b). Although the maximum of the distribution 



TIME 



(a) 



FREQUENCY 



(b) 



Fig. lb- 



Fig, la — Modulating pulse 

 -Amplitude — frequency distribution in transmitted RF pulse. 



curve hes at the transmitter frequency F, there is considerable energy in 



1 . ^ 1 . 



the range F zL tt and the receiver usually has a bandwidth of at least ~ in 



It t 



order to make as efficient use as possible of the energ}^ in the echoes reflected 

 by the target. For many radar applications, this requirement and the 

 problems of transmitter and beating oscillator frequency stability result 

 in the use of a bandwidth of as much as 10 mc for the IF amplifier. 



The Western Electric 386A tube, Fig. 2(a), had characteristics which 

 approached those needed to meet the IF amplifier requirements discussed 

 above. It was therefore slightly modified in physical form for convenience 

 of use and recoded the Western Electric 71 7A tube, Fig. 2(b). The 71 7A 

 tube was used extensively in IF amplifiers in several radar systems. As the 

 emphasis on small size and light weight for airborne radars increased, and, 

 as the need for better characteristics became more pressing, further develop- 

 ment was undertaken which resulted in the ^^'estern Electric 6AK5 tube. 

 Fig. 3. 



The importance of size and weight for radar systems to be used in air- 

 planes is obvious. The use of miniature tubes in airborne equipment has 



