I 



ULTRA-HIGH-FREQUENCY OSCILLATIONS 285 



the best efficiency in the simple diode of Fig. 1. From these values 

 the negative resistance may be computed from (8). 



As a next step the remainder of the circuit must be proportioned. 

 The tuning relation (1) yields the values of height h for a given diam- 

 eter. The next consideration is to insure that the sum of all positive 

 resistances is less than the negative resistance of the diode. For a 

 circuit with dimensions small compared with the wave-length, approxi- 

 mate formulas for the resistances associated with the losses in the 

 circuit conductors can be readily derived from classical circuit analysis. 



A most important resistance, not so readily computed, is caused by 

 radiation of energy through the gap between the insulating flanges 

 which separate cathode and anode. In most uses of the device, this 

 radiated energy constitutes the useful load on the oscillator but care 

 must be taken that the load is not so heavy as to stop the oscillations 

 altogether. An important distinction must be made as to whether 

 the tube is to radiate into free space or into some enclosure such as a 

 hollow wave guide, for example. In the latter case the radiation may 

 be regulated to a large extent by the geometry of the enclosure. For 

 values of radiation resistance when energy is directed into free space 

 an article by S. A. Schelkunoff ^ may be referred to. 



For oscillation, as pointed out, thesumof all these positive resistances 

 must be less than the negative resistance of the electron discharge and 

 for high efficiency the radiation resistance should be much greater than 

 the sum of all of the other positive resistances. This is usually found 

 to be the case, and in fact the radiation resistance is likely to be so 

 great as to stop oscillations unless the gap is made sufficiently small. 



In designing a hollow wave guide mounting for diodes of the sort 

 pictured in Fig. 1 it was recognized that since the high-frequency wave 

 energy issues from the coaxial resonator as a wave guided along the 

 heater leads, the natural and probably most effective thing to do was 

 to dispose these leads so that the field associated with them would 

 conform as nearly as possible to one of the wave types which can be 

 supported in a hollow wave guide. Of these wave types, the so-called 

 /fi type® is readily generated by high-frequency current in a wire 

 extending across a diameter of the guide, and the wave guide mounting 

 shown in cross section on Fig. 3 is such as to give rise to this type of 

 wave. For mechanical reasons a brass pipe of circular cross sections 

 was chosen for the guide, and its diameter (3J^^ inches) was chosen 

 large enough so that it would freely transmit an IIi wave of the ex- 

 pected frequency. In the mounting, the high frequency circuit is 

 completed from the anode to the wall of the guide through a stopping 

 condenser. 



