An adjoint variable method for time-domain TLM with wide-band Johns matrix boundaries

We present a novel algorithm for efficient estimation of objective function sensitivities for time-domain transmission-line modeling (TLM) with dispersive Johns matrix boundaries. The original electromagnetic structure is simulated using TLM. An adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the objective function with respect to all designable parameters are estimated using only the original and adjoint simulations. We show that the Johns matrix of the adjoint problem at each time step is the transpose of the corresponding matrix for the original structure. Our approach is illustrated through the estimation of the sensitivities of objective functions with respect to the dimensions of waveguide discontinuities. A very good match is obtained between our sensitivity estimates and those obtained using central difference approximation.