A novel interaction structure for a TE1,1-mode gyro-travelling-wave tube (gyro-TWT) was proposed in the paper. The interaction structure possesses both the azimuthal and axial periodicities. The interaction structure was designed, modelled and analyzed in simulation for its dispersion characteristics. The sensitivity analyses were carried out for dimensional control on the lower-cutoff-frequency, where the gyro-TWT is being operated. While comparing to the known interaction structures, a dispersion shaping is observed which in turn would be suitable for the wideband performance of a gyro-TWT. The lower-cutoff-frequency of the considered interaction structure increases with the increase in the radius of the projected cylinders, the radial height of the projected cylinders and the number of the projected cylinders in the azimuth, and the decrease of axial periodicity. The radius of projected cylinders and number of projected cylinders in the azimuth control the lower-cutoff-frequency, respectively, the most and least. Thus, the considered structure is a possible interaction structure for a wideband gyro-TWT.