Interpretation of wavelength measurements on tape helices

During the course of an investigation of helix propagation by the measurement of the standing-wave pattern in a shielded length of helical line open-circuited at one end, it was found that the minima of the pattern were not equally spaced when the phase-change per turn approached ¿ radians, and furthermore, this phase-change remained nearly stationary at ¿ radians over a finite frequency band. This would indicate a highly dispersive region which is not predicted by theoretical analyses. However, when the results are interpreted, taking into account the presence of space harmonics, an explanation of the phenomenon is obtained which indicates that there will be a region of apparent dispersion for a finite length of helix, although the basic propagation on the infinite helix is dispersionless. Such an effect may be of importance in the operation of travelling-wave devices designed to operate through this band.