11-1] THE MAGNETRON 589 



The second form in which noise is important is in the signal-to-noise ratio 

 of the oscillating magnetron. The noise level of the oscillating magnetron 

 is frequently higher than can be explained theoretically, and for CW 

 magnetrons is frequently higher than can be tolerated for CW radar 

 systems. Magnetrons sometimes change in noisiness during life. "Quiet" 

 magnetrons have been built, but there is still much to learn about the design 

 and construction of magnetrons for quiet operation. 



Tuning Means. A variety of means have been employed to tune 

 magnetrons mechanically. In one form, pins are lowered into the resonant 

 cavities of the anode block to change the inductance of these cavities. 

 In another form, a cylinder is inserted between the two rings used for 

 strapping, so that the strap capacitance is increased. In still another 

 technique, a high-^ cavity is coupled to the resonant system of the magne- 

 tron and tunes the magnetron by pulling. These mechanical schemes give 

 varying amounts of tuning up to 10 per cent or more of the center frequency. 

 The magnetron can also be tuned over a narrow frequency range by varying 

 the output load impedance. A number of all-electronic tuning techniques 

 have been investigated. 



Typical Performance of Modern Magnetrons. High-power pulse 

 magnetrons for a variety of frequencies and power levels are currently in 

 production. Typical of X-band magnetrons is the 4J50, which produces a 



To Antenna 



Fig. 11-15 Simplified Block Diagram of a Radar with a Magnetron Oscillator as 

 the Transmitter. 



