  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.
