On the transition from radiating to guiding behavior of the half-mode substrate integrated waveguide

The transition from radiative to guiding behavior of a half-mode substrate integrated waveguide (HMSIW) is studied throughout this work. The electric near-field distribution in the substrate plane is first demonstrated at different frequencies over the mono-mode operation band and is then related to far-field radiation patterns. The observation shows that the HMSIW, as a semi-open transmission line, suffers from relatively large radiation loss as it operates at frequencies just above the cut-off. However, the radiated power drops drastically with the increase of frequency. In particular, the attenuation caused by radiation becomes negligible as the frequency increases beyond a critical point. An explanation is given to the phenomena above and an approximate equation is also proposed for estimating the critical frequency point, as low-frequency limit of the HMSIW operational range. These results provide a physical confirmation of the optimal operation frequency range for the use of HMSIW as a low-loss waveguide.