Three metamaterial-based gap waveguides between parallel metal plates for mm/submm waves

This paper presents the basic ideas of how local waveguides and transmission lines can be designed to propagate along desired paths in the air gap between two metal surfaces. The principle of operation is related to the performance of artificial magnetic conductors, EBG surfaces and soft and hard surfaces. Three different major types of gap waveguides are described: ridge gap waveguides, microstrip gap lines and groove gap waveguides. Different realizations of the cut-off structures suppressing normal parallel plate modes are described, as well as expected applications, and possible problem areas preferably seen as research challenges.