Composite right/left-handed substrate integrated waveguide and half-mode substrate integrated waveguide

Composite right/left-handed (CRLH) substrate integrated waveguide (SIW) and half mode substrate integrated waveguide (HMSIW) are investigated and presented in this paper. By introducing series interdigital capacitors, which are realized by etching slots on the waveguide surface, both balanced and unbalanced CRLH waveguide transmission lines (TLs) are implemented. These proposed guided-wave structures support the propagation of backward waves below the characteristic waveguide cutoff frequency. Waveguide miniaturization is thus achieved. Dispersion diagram and transmission responses are discussed. In particular, a 3-dB backward wave directional coupler is investigated using CRLH HMSIW TLs. Measured results are presented to verify the analysis.