The dispersion equation of a dielectric loaded anisotropically conducting tape helix slow wave structure for planar TWTs is derived. By assuming the current density behavior on the rectangular tape helix, the dispersion relation is obtained through accurate boundary conditions that restrict the field only on the tape surface and not in the gap regions. Substitution of the field equations in the boundary conditions results in the six complex constants of the field equations. The complex constants are re-substituted in the last boundary condition that consists of the restricting function or the indicator function to arrive at the dispersion equation. The derived dispersion equation can be used to obtain the dispersion characteristics for a more practically relevant planar TWT interaction structure. © 2019 IEEE.