Before revision: The essential difference between the H waveguide and the NRD guide is that in the latter the spacing between the metallic plates is are less than half the wavelength in a vacuum, whereas in the H waveguide the spacing is greater.
After revision: The essential difference between the H waveguide and the NRD guide is that in the latter the spacing between the metallic plates is less than half the wavelength in a vacuum, whereas in the H waveguide the spacing is greater.