582 THE GENERATION OF MICROWAVE POWER 



emission. But when anode voltage is applied, the back bombardment of 

 the cathode by accelerated electrons will further heat the cathode. To 

 prevent overheating of the cathode, the heater voltage is normally reduced 

 after the anode voltage is applied. It is believed that this electron bombard- 

 ment of the cathode may have a harmful effect upon its life, although the 

 effects of ionic bombardment which depend upon residual gas pressure are 

 probably more severe. 



Resonant System. The anode of a microwave magnetron is normally 

 a heavy metal structure, typically copper, in which is machined or formed 

 the RF circuitry of the magnetron. Typical anode structures consist of 

 a number of slots, wedges, or slots and holes uniformly spaced around the 

 anode, as shown in Fig. 11-4. Each of these slots may be regarded as a 



Fig. 11-4 Typical Anode Structures for Microwave Magnetrons. 



simple cavity resonator, with a strong electric field developed across the 

 opening of the slot facing the cathode. The fields of each resonator extend 

 into the region between cathode and anode, and also into the end spaces 

 at the ends of the anode block. The individual cavity resonators are 

 therefore coupled together, and the entire anode block, together with the 

 end spaces and other parts that contain the electromagnetic field, form 

 the resonant system of the magnetron. 



The complete resonant system presents too complex a problem for 

 accurate, quantitative analysis. Many of its properties can be understood 

 by considering a resonant system with A^ slots or cavities to be a system 

 of A^ coupled resonant circuits. Such a system normally has A^ resonances 

 at different frequencies, although degeneracies exist in the magnetron 

 structure that reduce the independent resonances to A^/2. Normally, it is 

 desired to operate the tube at a particular one of the resonant frequencies, 

 and the other possible resonant modes are troublesome because they may 

 lead to spurious oscillations at undesired frequencies. 



The desired mode of oscillation is usually that in which the phase shift 

 between adjacent segments of the anode is t radians, or 180° (ir mode). 



