DESIGN FACTORS OF THE 1553 TRIODE 497 



how the figure of merit depends on the internal parameters of the tube, 

 such as spacings, current density, and so on. 



3. Optimize the internal parameters to make the figure of mer t as good 

 as possible, with due regard to practical limitations like cathode activity, 

 life, cost, etc. 



4. Use enough experimental checks to make sure the estimates are 

 sound. Build then the type of tube which appears to fill the require- 

 ments best, including the practical as well as the technical limitations. The 

 figures of merit serve now as quantitative checks both of how well the 

 tube satisfies the application, and also of how accurate is the theory. 



Given a good accurate design theory, the whole process could in prin- 

 ciple be calculated in advance. Such a theory would permit great savings 

 in effort, since spot checks of relatively few parameters are sufficient to 

 insure accuracy even when the theory is used to predict a wide range of 

 phenomena. The extent to which presently available microwave tube 

 theory meets this need is considerable, as will appear from some of the 

 results below. 



The degree of accuracy required of a theory increases as the develop- 

 ment process continues. For preliminary estimates, such as deciding what 

 tube type to develop, the theory can be rather rough and still be satis- 

 factory. For complete predictions of final performance, only experimental 

 construction can suffice. By this means the theory can be checked, so that 

 it can serve future designs with improved accuracy. 



The method outlined here is not new, but rather follows standard 

 practice fairly closely. It does, however, give more than usual quantita- 

 tive emphasis to the figures of merit, using them to codify the procedure; 

 and it incorporates a certain amount of quantitative calculation at micro- 

 wave frequencies. It will be seen that the theory of Llewellyn and Peter- 

 son needs only some semi-empirical supplementation in the low-voltage 

 input space, as has already been pointed out by Peterson.^ 



Preliminary Estimates — Choice of Tube Type 



For the New York to Boston microwave relay, an output amplifier 



was developed using already available velocity-modulation tubes. ^ With 



four stagger-tuned stages, the amplifier proved satisfactory for this 



service, and in fact tests indicated that this system could be extended to 



considerably greater distances and still give good performance. It was 



apparent, however, that these amplifiers would not be satisfactory for a 



coast-to-coast system. 



* L. C. Peterson, "Signal and Noise in Microwave Tetrodes," /. R. E. Proc. (Nov. 

 1947). 



» H. T. Friis, "Microwave Repeater Research," B. S. T. J. 27, 183-246 (April 1948). 



