A common approach to perform dispersive analyses of waveguides periodic along one direction is based on the electromagnetic simulation of a single spatial period of the structure. However, the resulting equivalent two-port network representation of the single cell may lead to inaccurate modal results, since mutual coupling between cells has been neglected. When a finite number of adjacent cells are simulated, with the aim of improving the accuracy of the analysis, spurious solutions are introduced; they are shown here to be related to the nonuniqueness of root-extraction operations in the complex plane. A simple automatic method is proposed to recover the correct solution and to test the convergence of the analysis as the number of simulated cells is increased.
Valerio, G., Paulotto, S., Baccarelli, P., Burghignoli, P., & Galli, A. (2011). Accurate Bloch Analysis of 1-D Periodic Lines Through the Simulation of Truncated Structures. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, 59(6 PART 2), 2188-2195 [10.1109/tap.2011.2143667].