Biperiodicity as liability and as asset in non-helix linear-beam TWT interaction structures

A biperiodic TWT interaction structure is one having a different geometry imposed on alternate periods, so that the Brillouin diagram shows additional stopbands between those otherwise observed. When copper combs or ladders are used in creating millimeter-wave TWT circuits equivalent to coupled-cavity chains, otherwise-insignificant disparities in the tiny dimensions may accidentally result in biperiodicity and a minimal but troublesome stopband within the amplifying band. On the other hand, some unconventional TWT interaction structures of both early and recent origin deliberately incorporate biperiodicity -- widening such stopbands and modifying the dispersivity and edges of a passband to the benefit of "hot" bandwidth and stability. Modified interdigital lines, and chains of coaxial cavities with their axes transverse to the electron beam, are examples in this category. New variants, and reappraisal of early efforts in contemporary terms, draw attention to properties of potential practical value for present-day centimeter- and millimeter-wave applications.