Coupled helices for use in traveling–wave tubes

Previous theoretical treatments concerning coupled helices in the absence of a beam, while useful for predicting approximate behavior, are not satisfactory for many tube applications because the presence of a beam can substantially modify the behavior. This paper presents a theory of propagation for coupled concentric helices in the presence of a coaxial beam and computes therefrom properties pertaining to coupler and attenuator applications. Included are curves of the propagation constants as functions of coupling coefficient and beam velocity, curves of the inner and outer helix voltages as functions of the physical dimensions, and curves which permit the frequency response for various designs of input and output couplers to be calculated. Sample calculations show that the proper length for input and output couplers is greater than the length for complete energy transfer between helices in the absence of the beam (a criterion commonly used in design work). Calculations also illustrate the related fact that the frequency for maximum transmission through a coupler is lower when the beam is turned on than when off. Only preliminary work has been done to date in computing the properties of a coupled-helix attenuator.