This work encompasses some relevant issues that arise when using a broadband method to
characterize the even and odd mode propagation constants of symmetric microstrip coupled lines using
four-port pure-mode differential and common mode S-Parameters. Correct excitations of differential and
common modes play a significant role in experimental results. From these 4-port measurements, parameters
corresponding to the even and odd modes can be extracted from the multi-mode generalized S-parameters
obtained by the vector network analyzer. Differential and common mode sub-matrices are considered as
two independent equivalent 2-port transmission lines. The method will be applied to extract the propagation
constants of even and odd modes of coupled microstrip transmission lines. A discussion is included on
ensuring the correct excitation of the modes in the structure and avoiding possible unwanted effects such
as radiation, that negatively affects the final estimation, both in phase and attenuation. Two figures of merit
are defined to evaluate how transitions affect the performance of the method. These figures of merit allow
a reasonable estimation of how the final experiment is affected by selected excitation of modes. To assess
the theory presented, two sets of seven lines of different lengths are manufactured. The experimental results
show an excellent agreement, both with electromagnetic simulations and with the analytical model used, for
the phase constant in the improved case of 30° bends, and an improvement in the useful bandwidth of the attenuation constant. This fact verifies the correct functioning of the method. In addition, the measurements are
presented in the range of 0.01–67 GHz, which represents a significant advance in bandwidth for this type of
structure.