38 BELL SYSTEM TECHNICAL JOURNAL 



to the middle electrode. Input and output connections were tapped on 

 this half-wave line near the short circuited ends. During reception this 

 assembly introduces negligible loss in the receiving line. However, during 

 the transmitted pulse a small amount of the transmitted power ionizes the 

 gas in the switching tube and effectively short-circuits the receiver line. 

 This device, which in later forms came to be called a "T-R Box", is located 

 near the receiver input and the length of line between it and the junction 

 with the transmitter line can be adjusted to an odd multiple of quarter 

 wavelengths to present the desired high impedance at that point during 

 transmission. 



Receiver 



The re:eiver delivered with early Mark 3 equipments was identical to 

 that used in Radar Mark 1. It was of the superheterodyne type em- 

 ploying one stage of RF amplification (doorknob tube), 316A oscillator tube, 

 and doorknob first detector. The intermediate frequency amplifier had a 

 bandwidth of about 1 megacycle at a midband frequency of about 30 mega- 

 cycles. The second detector and video stages were located in the indicating 

 equipment. A photograph of this receiver is shown in Fig. 11. 



Since in microwave w^ork the controlling noise is that produced in the 

 receiver, it is desirable to reduce this noise to the theoretical limit of thermal 

 agitation in the input circuit. However, in 1939 tube limitations and 

 circuit design techniques at these frequencies resulted in performance far 

 short of this goal. The amount by which the receiver noise exceeds the 

 theoretical minimum has been termed the receiver "noise figure" and in 

 this early receiver the noise figure was about 24 db. It was recognized that 

 considerable improvement in maximum range could be obtained by reducing 

 this receiver noise. 



Shortly after first deliveries of Radar Mark 3 a new tube (GL-446 or 

 "lighthouse" tube) was made available by the General Electric Company 

 which showed promise of providing a substantial improvement in the 

 receiver noise figure. An amplifier using this tube was accordingly de- 

 signed by Bell Laboratories in which coaxial cavities were used for tuning 

 elements. Two stages of amplification were used to replace the single 

 "doorknob" tube stage previously employed. The new amplifier resulted 

 in a reduction of the receiver noise figure to about 9 db and provided a 

 marked improvement in maximum range capability of the radar. These 

 amplifiers were manufactured and shipped to the Fleet for field installations 

 on early equipments and were included in productions on equipments 

 shipped subsequently to availability of the amplifiers. A photograph of 

 the receiver with the two amplifiers installed is shown in Fig. 32. 



Another field modification provided automatic gain control of the signal 



