MAGNETRON AS GENERATOR OF CENTIMETER WAVES 307 



circumvented by connecting across the modulator network a condenser in 

 series with a resistance equal to the characteristic impedance of the net- 

 work. The condenser is of low impedance for rapid voltage changes, termi- 

 nating the network properly during the voltage rise. The condenser, 

 however, stores some energy which must be dissipated at the end of the 

 pulse. The discharge of this energy through the magnetron causes very 

 weak oscillations to occur following the main pulse. This condition some- 

 what lengthens the interval of time following the pulse during which the 

 radar receiver may not detect an echo. 



16.2 The 4J45-47 Magnetrons: For the extension of radar range it had 

 been found desirable to use pulses of longer duration with consequent in- 

 crease in total energy radiated per pulse. As indicated in PART I, the 

 longer the pulse, the narrower the frequency spectrum. A long pulse conse- 

 quently makes possible a narrower receiver band width, an improved 

 receiver signal to noise ratio, and greater range. Although it was necessary 

 to reduce the peak power somewhat in operating the 720A-E at a pulse 

 length of 5 microseconds, a substantial increase in range could still be ef- 

 fected. The Radiation Laboratory at M. I. T. proposed to use 720A-E 

 type magnetrons on long pulse operation and performed many tests of these 

 magnetrons under these conditions. Many 720A-E magnetrons performed 

 satisfactorily under these long pulse conditions. However, arcing both 

 internally between cathode and anode and externally across the input leads 

 deteriorated the performance and made the life expectancy of the magnetron 

 questionable. 



To meet the new demands a new series of magnetrons, the 4J45-47 was 

 developed by making two design changes in the 720A-E. The plain, oxide 

 coated cathode was replaced with a cathode of the "mesh" type to be de- 

 scribed in Section 21. M.agnetron Cathodes. The "mesh" cathode proved 

 to be more rugged, involved less arcing, was longer lived, and required a 

 minimum of changes in internal structure. 



External arcing at the input leads was minimized by lengthening the glass 

 section, accomphshed by increasing the over-all lead length and shortening 

 the metal section of the lead at the magnetron body, and by enclosing the 

 leads in housings filled with a silicone anticorona compound. These changes 

 were limited by the requirement of interchangeability with the 720A-E. 



The modified 720B-D magnetrons were coded the 4J45-47. Operating 

 characteristics are giv^n in Table II. These magnetrons were used with 

 long pulses and in the usual service for which the 720.-\-E had been designed. 



17. Magnetrons for Wavelengths near 3 Centimeters — 

 UNDER 100 Kilowatts 



17.1 The 7 25 A and 730 A Magnetrons: As the antenna size of a radar 

 is reduced, it is necessar}- to reduce the operating wavelength to maintain 



