MAGNETRON AS GENERATOR OF CENTIMETER WAVES 



181 



rotational energ>' has been absorbed. It is possible to maintain the oscilla- 

 tions and extract energy from them because electrons which give energy to 

 the field can do so over many cycles, whereas electrons of opposite phase can 

 gain energy over only one cycle before they are removed. 



Magnetrons oscillating in this manner have been built with split 

 anodes. '"•'! Here the RF field with which the electron interacts is more 

 tangential than radial but the criterion for oscillation is the same, namely, 

 resonance between the field variations and the rotational component of the 

 electron's motion. Operating efficiencies of 10 to 15% have been obtained. 

 It was with a magnetron of this type having an anode diameter of 0.38 mm. 

 that radiation of wavelength as low as 0.64 cm. was generated. •- 



The cyclotron frequency magnetron oscillator has been almost entirely 

 superseded by the traveling wave magnetron oscillator as a generator of 



Fig. 8. — An approximate orbit of an electron which loses energy' to the RF field in a 

 cyclotron frequency or T^'pe II magnetron oscillator, shown for the plane case. If the 

 electron after losing all its rotational energy remains in the interaction space, it gains 

 energy from the RF field, and its orbit builds up cycloidal scallops in a manner directly 

 the reverse of that shown here. The DC electric force on the electron is directed from 

 cathode to anode. 



centimeter waves. In the main this is the result of the impossibility of 

 removing electrons emitted from an extended cathode area from the inter- 

 action region at the proper stage in their orbits. This inherent drawback is 

 not shared by the traveling wave magnetron oscillator which may be oper- 

 ated at higher efficiency without critical adjustment of orientation in the 

 magnetic field or of the potential of auxiliary- electrodes. 



2.4 The Traveling Wave Magnetron Oscillator — Type III: Oscillations 

 have been found to occur in the magnetron which are independent of any 

 static negative resistance characteristic and which can occur at frequen- 

 cies widely different from the cyclotron frequency. In 1935'-^ the elec- 

 tronic mechanism of these oscillations was correctly interpreted as an inter- 



11 H. Yagi, Proc. I.R.E. 16, 715 (1928). 



12 C. E. Cleeton and N. H. Williams, Phys. Rev. 50, 1091 (19361. 

 "K. Posthumus, Wireless Engineer 12. 126 (1935). 



