Hi Flywke -- In shielded EM tools, it is possible to get Im(Zo) = 0, and to get exactly lossless results. Two tools are AWR Emsight, and Sonnet (I work for Sonnet). For the lossless case, these tools (at least Sonnet, I am not sure about Emsight) use only the imaginary part of the MoM matrix. The real part is zero. This is exactly like the Y or Z parameter matrix for a network that contains only capacitors and inductors, no resistors. This is nice because only half the memory is used for the lossless case. There is no need to use up memory for the real (lossy) part of the matrix because it is all exactly zero. However, keep in mind that an exactly lossless circuit can not be built.