VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 125 



permits the construction of rigid grid structures capable of high energy- 

 dissipation for the higher frequency range. 



The external tuned circuit for the higher frequency mode of oscilla- 

 tion takes the form of a Lecher system connected between the two 

 grid terminals. When so connected the dependence of frequency upon 

 circuit tuning is pronounced, as contrasted with the negligible de- 

 pendence observed if the Lecher system is connected between the 

 plate and the grid. When oscillating in the higher-frequency mode 

 the spiral grid tube shows only a comparatively small dependence of 

 frequency on grid potential and this may be compensated by a pro- 

 portional change in plate potential. This, coupled with the fact that 

 the output increases rapidly with increasing grid potential, makes it 

 possible to apply various schemes of amplitude modulation. Charac- 

 teristics of the type shown in Figs. 21, 22, and 23 for the straight-wire- 

 grid tube cannot be taken except for a limited portion of the range 

 due to the inability of the grid to dissipate the energy required in the 

 upper portion of the grid voltage or grid current ranges. 



While the spiral grid tube will also oscillate in the lower-frequency 

 mode, its efficiency and output are considerably lower than the corre- 

 sponding values for the straight-wire-grid tube previously discussed. 

 Its field of usefulness is, therefore, largely limited to the higher fre- 

 quency mode of oscillation in the frequency range above 600 mega- 

 cycles. 



The "Magnetron" Oscillator 



The "magnetron" in its simplest form consists of a cylindrical 

 diode or 2-electrode tube, with a uniform magnetic field in the direction 

 of the electrode axis. The original type of tube has been largely 

 superseded for ultra-high-frequency generation by the so-called split- 

 plate magnetron, first used by Okabe,^^ in which the cylindrical anode 

 is divided longitudinally into two (or more) segments to the terminals 

 of which is connected the tuned circuit. Such a tube is shown in 

 Fig. 25. 



In the frequency range from 300 to 600 megacycles the split-plate 

 magnetron compares favorably with the negative grid tube both in 

 output and in anode efficiency. Its use has been limited because of 

 the complicating factor of the magnetic field, and the attending 

 modulation difficulties. For frequencies higher than 600 megacycles 

 the magnetron provides larger outputs than those so far reported by 

 other means. It has been used at frequencies up to 30,000 mega- 

 cycles, a value well above that so far reported for any other type of 

 vacuum tube. 



