  This work presents, for the first time, a generalized bimode Fosters
equivalent circuit for characterization of 2-D Planar Periodic Structures
(PPSs) with arbitrary geometry at oblique incidence. It considers the
interactions between the fundamental TE and TM modes without any restriction
within the bimode bandwidth of the geometry. The proposed circuit is only
composed of frequency-independent LC elements, which can be extracted
systematically from electromagnetic (EM) simulations. The reactive immittances
obtained in the process fulfill the Fosters theorem, enabling the design
process of PPS-based devices using standardized synthesis techniques from
circuit theory. To demonstrate its viability and general nature, equivalent
circuits are extracted for different single- and multilayer PPS composed of
rotated dipoles under oblique incidence theta=20 deg,phi=30 deg, and including
dielectrics. Excellent agreement is found between the response of the circuit
model and the EM simulation in all cases. Finally, to validate experimentally
the proposed equivalent circuit and highlight its applicability, a 90 deg
reflective LinearPolarization (LP) Rotator centered at 25 GHz and under oblique
incidence, theta=30 deg, phi=0 deg (TE), is designed, manufactured, and tested.
The agreement between the circuit response, the EM simulation and the
measurement underlines the potential of the new equivalent circuit for PPS
design under oblique incidence.
