Coupling Analysis of the Surface States in Periodic Microwave Transmission Line

The spectrum of electromagnetic waves in periodic linear structures, such as periodic waveguides or chains of microelements i.e. spheres, cavities, exhibit the sequence of stop bands for propagating waves. Breaking the translational symmetry of the periodic microstrip can also lead to the localization of the microwaves at the microstrip edge. In this paper, we investigated periodic microstrip transmission line represented as 1D photonic crystal operating at the GHz frequencies. On the ground of topology, we explain the condition of surface state existence. The transmission measurements and numerical calculations support our theoretical predictions. Moreover, we show that in the symmetric microstrip the surface states split into symmetric and antisymmetric modes due to evanescent wave coupling between the modes localized on the opposite sides of the microstrip. Interestingly, both modes offer significant microwave transmission inside the frequency gap, which is promising for applications.