AbstractThe definition of frequency dependent strip characteristic impedance for multiconductor transmission lines in inhomogeneous dielectric media is discussed. The considerations are based on orthogonality properties for the eigencurrent and eigenvoltage vectors of propagation modes that have been presented by Marx and Nagai for the TEM case and are now extended to general hybrid‐mode structures. The definition adopted takes care of the reciprocity of transmission line structures and is essential to get consistent simulation results in the CAD of MICs and MMIC