Comparison between two optimisation algorithms to compute the complex permittivity of dielectric multilayer structures using a free-space quasi-optical method in W-band

Two optimisation algorithms utilised in calculating the complex permittivity and the thickness of individual dielectric layers of multilayer structures are described. The algorithms are the genetic algorithm and the sequential quadratic programming (SQP) based on the quasi-Newton method (Broyden-Fletcher-Goldfarb-Shanno) (BFGS-SQP); the setup procedure of both algorithms is presented. The reflection and the transmission coefficients are measured by a quasi-optical system in the W-band. The performance of the measurement setup is first checked using different single-layer slabs. Moreover, a detailed comparison between the algorithms is reported.