Investigation of the longitudinal multiconductor transmission line functions for symmetric coupled-microstrip systems

In inhomogeneous multiconductor transmission line (MTL) structures such as coupled microstrip, propagation is characterized by multiple quasi-TEM modes with distinct propagation constants. These "mode delays" cause the MTL functions to exhibit longitudinal behavior that superficially appears problematic in the context of passive lossless reciprocal systems. This paper presents a thorough investigation of the longitudinal MTL functions. Using MTL formulation and computer simulation, we explain the mathematics and physics of mode delays so that their effects are not misinterpreted or attributed to error in the numerical analysis of MTLs.