Study of Passband and Stopband Properties of Sheet-Beam Folded Waveguide Structures

In this paper, the passband and stopband characteristics of a sheet-beam folded waveguide structure (FWS) were studied using the equivalent circuit analysis, which is based on the transmission line representation of a rectangular waveguide (WR) and its derived structures. The dispersion and characteristic impedance results obtained for the FWS show the space harmonic nature of the periodic structure. The standard expression of interaction impedance was modified by replacing the characteristic impedance of the waveguide with that of the FWS to better understand the performance of the device near the band edges. The dispersion and the characteristic impedance which were obtained from the transfer matrix of a single period of FWS were analyzed and compared with that of the simple WR. The dependence of the bandgap (stopband) on the size of the beam tunnel was also studied in this analysis. An FWS with a sheet-beam tunnel was fabricated using the micromachining techniques with optimized cutting parameters and well-sharpened cutting tools under minimum quantity lubrication (MQL) conditions. The dimensional and electrical parameters of the structure were measured. The dispersion measurements were carried out. The deviations of the measured dispersion from the values obtained by equivalent circuit analysis are between +0.4%/-0.47% in the entire cold passband and +0.4%/-0.26% in the operating passband 210-250 GHz and are within permissible limits. The simulated measurements of interaction impedance match with the values obtained by the equivalent circuit analysis to be within 10% in the operating passband.