Calculation of the Characteristic Impedance of Transmission Lines with Periodic Structures

Abstract

This paper describes the calculation for characteristic impedance of transmission line with periodic structures such as defected ground structure (DGS) and photonic bandgap (PBG). The previous method which uses the λ/4 transmission line model is reviewed and its disadvantage that the calculated characteristic impedance is strongly dependent on the frequency is discussed. The characteristic impedance of transmission lines with periodic structures are calculated using the λ/4 transmission line model and analytic method. The calculated characteristic impedance by the latter method is an almost constant value while that from the first method depends on the frequency strongly. In addition, the characteristic impedance of the transmission line with PBG is calculated and proposed, while it has been rarely studied ever. S-parameters are obtained from the measurement using the fabricated sample as well as simulation, and used for calculating the characteristic impedances and comparison. The characteristic impedances calculated from the measured S-parameters agree well with the simulated results. It is well described that the analytic method to calculate the characteristic impedance of transmission lines on uniform dielectric structures can be applied successfully to the transmission lines with periodic structures such as DGS and PBG.