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I have been searching for a good free 3D field simulator for a while. It's getting to the point where I'm seriously considering writing my own FDTD engine.A quick summary of the options that *are* free:NEC-2 is a public domain Method-of-Moments (MoM) simulator suitable mainly for wire antennas and some planar antennas which do not rely on dielectric structures (so no patch antenna simulations). It was originally written in the 1970s at Lawrence Livermore National Lab, so the interface leaves much to be desired. It's a command line program that takes a text file input, where each line of text represents an 80 column punched card in a card stack. There are a few graphical interfaces available (4NEC2 is the most popular free one) but they are all pretty old. There's definitely a demand for a modern 3D CAD type frontend, if anyone's looking for a weekend project.OpenEMS is a free FDTD (Finite Difference Time Domain) simulator suitable for a wide range of computational EM problems. As an FDTD package, it will be a lot more memory heavy than a FEM or MoM simulator, but RAM is cheap these days. The main drawback is the interface - there's no UI, rather, the software is used programmatically through a MATLAB/Octave frontend. Again - making a UI would be a great weekend project if anyone's looking. If you can handle the lack of UI, this package is the best truly free option right now.MEEP is another FDTD package. Looks more like it's geared towards photonics applications, although the physics are the same. Again, no UI. The main drawback I can see is that as it's more of a physics research package it doesn't include postprocessing features helpful for EE, namely s-parameter output and far-field radiation pattern computation - you'd need to write your own postprocessor.EmGine is another FDTD package. Unlike the others, it has a UI. Unfortunately, development has stalled and apparently it's difficult to run on Windows on the latest Python. Also, the license prohibits commercial use.Sonnet Lite is the free version of Sonnet, which is a planar EM simulator based on the Method of Moments. Very similar product to Agilent Momentum that's included in ADS. It's good for simple printed transmission line structures like microstrip, hybrid couplers, and filters. You can also do patch antennas with it, but you can't view the far-field patterns unless you buy the full version, so it'd be good for tuning return loss only.FEKO Lite is the free version of FEKO (registration required). FEKO is a professional EM field sim comparable to HFSS or CST which has a big list of solvers: MoM, FEM, FDTD, and UTD, as well as several hybrid MoM-FEM solvers. It's got most of the features of the full version but the mesh size is heavily restricted. Still, you can solve small problems with it, like basic microstrip antennas.Basically, there's a demonstrable need for a user-friendly 3D high frequency electromagnetics simulator in the open-source space, but nobody's picked it up yet. Of course, if this is for personal use, you could "acquire" a "time unlimited evaluation" of HFSS.