COUPLED HELICES 167 



would wish to have as Httle attenuation as practicable associated with 

 the inner helix (inside the TWT). This requires the attenuating element 

 to be associated with the outer helix. Miller has shown that the maxi- 

 mum total power reduction in coupled transmission systems is obtained 

 when 



ai — 0:2 



where ai and 012 are the attenuation constants in the respective systems, 

 and ^b the beat phase constant. If the inner helix is assumed to be loss- 

 less, the attenuation constant of the outer helix has to be effectively equal 

 to the beat wave phase constant. It turns out that 60 db of attenuation 

 requires about 3 beat wavelengths (in practice 10 to 20 helix wave- 

 lengths). The total length of a typical TWT is only 3 or 4 times that, 

 and it will be seen, therefore, that this scheme may not be practical as 

 the only means of providing loss. 



Experiments carried out Avith outer helices of various resistivities and 

 thicknesses by K. M. Poole (then at the Clarendon Laboratory, Oxford, 

 England) tend to confirm this conclusion. P. D. Lacy" has described a 

 coupled helix attenuator which uses a multifilar helix of resistance 

 material together with a resistive sheath between the helices. 



Experiments were performed at Bell Telephone Laboratories with a 

 TWT using a resistive sheath (graphite on paper) placed between the 

 outer helix and the quartz tube enclosing the inner helix. The attenua- 

 tions were found to be somewhat less than estimated theoretically. The 

 attenuator helix was movable in the axial direction and it w^as instructive 

 to observe the influence of attenuator position on the power output from 

 the tube, particularly at the highest attainable power level. As one might 

 expect, as the power level is raised, the attenuator has to be moA-ed nearer 

 to the input end of the tube in order to obtain maximum gain and power 

 output. In the limit, the attenuator helix has to be placed right close to 

 the input end, a position which does not coincide with that for maximum 

 low-level signal gain. Thus, the potential usefulness of the feature of 

 mobility of coupled-helix elements has been demonstrated. 



4. CONCLUSION 



In this paper we have made an attempt to develop and collect together 

 a considerable body of information, partly in the form of equations, 

 partl}^ in the form of graphs, which should be of some help to workers 

 in the field of microwave tubes and devices. Because of the crudity of the 

 assumptions, precise agreement between theory and experiment has not 



