SHORT ELECTRIC WAVE RADIATION 551 



At the present time the best method of generating oscillations of the 

 order of 60 megacycles is that of using triode vacuum tubes in regenerative 

 circuits. The physical principles of such circuits are now fairly well 

 understood and the problem of generating ultra-high frequencies resolves 

 itself into a process of designing the equipment in such manner as to 

 eliminate all extraneous capacities and inductances from the main 

 oscillatory circuit. Triode high-frequency oscillators are of two general 

 types. One type of circuit is composed of the usual inductance and 

 capacitance elements, while the second type of oscillatory circuit depends 

 upon the rate of propagation of electrons between the elements of the 

 triode. The latter type is capable of producing the highest frequencies 

 but has the disadvantage that the power obtainable is quite limited. 

 Regenerative oscillators have been found to be the most suitable for 

 producing the frequencies of interest to the biological investigator. 



Regenerative oscillators which will function in the region of 60 

 megacycles are not difficult to set up, provided certain factors are recog- 

 nized and provided for. It is important to recognize that as the fre- 

 quency progressively increases, the values of inductance and capacitance 

 necessary to produce resonance decrease until a point is reached where 

 a straight piece of wire represents a considerable amount of inductance. 

 Also, the smallest of stray capacities at ordinary frequencies often 

 becomes the limitation of the highest frequencies obtainable in vacuum- 

 tube circuits. This is particularly true of the interelectrode capacitance 

 of a vacuum tube and also the distributed capacitance of the leads con- 

 necting the base of the vacuum tube to the elements of the tube. It is 

 therefore necessary in constructing oscillators for ultra-high frequencies 

 to reduce these stray capacities and inductances to the smallest possible 

 value consistent with the mechanical Kmitations involved. 



From the foregoing it can be seen that the proper place to begin in 

 the design of ultra-high-frequency equipment is with such circuits and 

 tubes as will lend themselves to the reduction of all capacities and induc- 

 tances which are external to the main oscillatory circuit. The oscillatory 

 circuits for ultra-high frequencies are similar to those used in low-fre- 

 quency work, the only essential difference being in the values involved. 

 At vdtra-high frequencies a piece of wire a few inches long has sufficient 

 inductance when used in connection with the interelectrode capacities of 

 most commercial vacuum tubes to form an oscillatory circuit, the funda- 

 mental of which is in the neighborhood of 60 megacycles. 



Referring to Fig. 1, it will be noted that the circuit represented is the 

 familiar Hartley oscillator arranged for the operation of two T^^-watt 

 tubes in push-pull. The inductances for the grid and plate circuit are 

 composed of single turns of wire, the diameter of which may be varied 

 in order to vary the fundamental frequency of the oscillator. The 

 limiting frequency of commercial tubes is reached when these inductances 



