THE RADAR RECEIVER 721 



tion of the similarity of the crystal elements as well as the other elements 

 shown. In practice, no particular difficulty was experienced in maintenance 

 of sufficient balance with the improved production control of crystals during 

 the latter part of the war program. 



The advantages of the balanced radar converter here described are two- 

 fold. First, the signal power dissipation in the beating oscillator branch is 

 reduced to a minimum and conversely the beating oscillator power fed back 

 into the antenna and reradiated is reduced. Second, the noise sidebands, 

 which are associated with the outi)ut frequency of the reflex oscillator, are 

 reduced effectively in the IF output branch by the degree of balance avail- 

 able. This local oscillator noise sideband contribution is normally respon- 

 sible for a definite degradation of the over-all radar receiver noise perform- 

 ance and hence, the use of a balanced converter will contribute to improved 

 l)erformance. An additional advantage of the balanced converter is the 

 minimizing of signal branch impedance variation effects on the beating oscil- 

 lator load impedance and, therefore, its frequency. The variation of the 

 antenna impedance during the scanning cycle has in this design little effect 

 on the tuning of the receiver. 



2.15 The Radar Receiver Beat Oscillator 



Vov microwave radar receiver purposes the selection of the local beat 

 oscillator within the converter assembly has been essentially limited to two 

 types of tubes, both of which were developed during the past war period. 

 I'\)r radar systems operating at frequencies of 1000 mc and lower, the 

 (iL-2C4() lighthouse triode has served quite satisfactorily, while at fre- 

 quencies above 1000 mc, the single-cavity reflex oscillator tube has been 

 extensively employed. Both of these tube types have adequate power 

 output and frequency stability characteristics to meet th© normal radar 

 system requirements. 



Some of the desirable characteristics of a beat oscillator tube for use in 

 military radar receivers can be listed as follows. 



A. At least 20 milliwatts of useful output power is desirable. In the case 

 of the silicon crystal rectifier element, the applied power is limited to 

 approximately 1 mw; however, the availability of additional oscillator 

 power enables the converter designer to effectively isolate the beat 

 oscillator and signal branches by simple inpedance mismatch means. 

 H. The frequency stability of the ideal beat oscillator tube must be in- 

 herently good, or convenient means to automatically contrt)l this 

 frequency must be ])rovided. The maximum allowable radar receiver 

 frequency variation due to all causes is of the order of 1 mc. In terms 

 of the beating oscillator frequencies employed in the radar systems of 

 the past war, this represents an allowable oscillator frequency variation 



