Comment on "Higher order reentrant post modes in cylindrical cavities" [ J. App. Phys. 122, 144501 (2017) ]

A recent article [ B. T. McAllister et al., J. Appl. Phys. 122, 144501 (2017), arXiv:1611.08939] claims to show the existence of “new” higher order reentrant post modes. Indeed, such modes do exist and are known to scientists and engineers for a very long time as modes of a foreshortened quarter-wave coaxial resonator. It is a textbook problem. In this comment, I briefly review the higher order modes in such cavities and provide relevant references.