The importance of dielectric losses in planar microstrip structures is eval
uated with the finite-difference time-domain (FDTD) method, This analysis w
as previously not possible in many FDTD simulators due to a lack of absorbi
ng boundary conditions (ABCs), which appropriately terminate air/dielectric
interfaces for which the dielectric is lossy. The newly proposed lossy two
-time derivative Lorentzian material (L2TDLM) model ABC allows for these te
rminations and is presented and implemented here for three-dimensional FDTD
simulations. The effect of dielectric losses on several well-known planar
microstrip structures is evaluated. It is shown that the inclusion of these
losses in FDTD simulations, which is facilitated by the L2TDLM ABC, is, in
fact, important to predict the performance of resonant structure on lossy
dielectric substrates.