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.