TNT is an amazing open-source 2D/2.5D field solver for transmission line design. It works excellently for planar structures, but also supports circular conductors. It can even generate W-Element Models for system-level analysis. The best use-case for TNT is in validating controlled impedance geometries. The biggest downside is that it does not model dielectric losses, so the conductance parameter (G in the RLGC model) is not present thus not suitable for modeling long channels where loss is critical.
Go to: http://mmtl.sourceforge.net/
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