RF-simulations

This repo contains my learning progress in openEMS and related software for RF simulation and visualization. It makes extensive use of FreeCAD and Python Scripting.

The rf_simulations-folder contains various antenna designs, filter designs and other rf-experiments.

Simulation Process

Currrently, the process of simulating a structure works as follows.

Step 1: Design the objects as bodies in FreeCad

• Sketch the shape of the antenna / air / dielectric
• Extrude the shape as required
• Export the bodies as stl-files

Step 2: Load the shapes into python.

Use the AddPolyhedronReader function in CSXCAD. e.g.:

stl_path = os.path.join(currDir, "freecad")
materialList = {}
materialList['monopole'] = CSX.AddMetal('monopole')
mifa_filepath = os.path.join(stl_path, "monopole.stl")
polyhedrons = {}
polyhedrons['monopole']  = materialList['monopole'].AddPolyhedronReader(mifa_filepath,  priority=5)
assert(polyhedrons['monopole'] .ReadFile(), f"Found file")

Step 3: Create the OpenEMS mesh

This is the critical part, depending on the type of material and the complexity of the shape use different methods.

Meshing through Python Libraries

For an air-boundary / simulation boundary:

mesh_lists_air = add_poly_mesh_boundary(polyhedrons['air'], wavelength_u)
mesh.x = np.concatenate((mesh.x, mesh_lists_air[0]))
mesh.y = np.concatenate((mesh.y, mesh_lists_air[1]))
mesh.z = np.concatenate((mesh.z, mesh_lists_air[2]))
print(f"AIR MESH LIST: \n- {[sorted(lst) for lst in mesh_lists_air]}")
find_poly_min_max(polyhedrons['air'])

For a substrate boundary:

mesh_lists_fr4 = add_poly_mesh_substrate(polyhedrons['FR4'], 1/3)
mesh.x = np.concatenate((mesh.x, mesh_lists_fr4[0]))
mesh.y = np.concatenate((mesh.y, mesh_lists_fr4[1]))
mesh.z = np.concatenate((mesh.z, mesh_lists_fr4[2]))
print(f"FR4 MESH LIST: \n- {[sorted(lst) for lst in mesh_lists_fr4]}")
find_poly_min_max(polyhedrons['FR4'])

For a conductor (PEC, Metal) boundary:

mesh_lists_monopole = add_poly_mesh_pec(polyhedrons['monopole'], wavelength_u, 1/3, unit=1e-3)
mesh.x = np.concatenate((mesh.x, mesh_lists_monopole[0]))
mesh.y = np.concatenate((mesh.y, mesh_lists_monopole[1]))
mesh.z = np.concatenate((mesh.z, mesh_lists_monopole[2]))
print(f"monopole MESH LIST: \n- {[sorted(lst) for lst in mesh_lists_monopole]}")
find_poly_min_max(polyhedrons['monopole'])

Meshing through Freecad Macro

I wrote this macro, which allows for creation of a sketch with names (sketch_xmesh, sketch_ymesh, sketch_zmesh) in freecad, and exports the points in the sketch as a .npy file.

Use this mostly for complicated shapes like meandered antennas. These files can then be loaded into the simulation script:

mesh_0 = np.load(os.path.join(stl_path, "mesh_0.npy"))
mesh_1 = np.load(os.path.join(stl_path, "mesh_1.npy"))
mesh_2 = np.load(os.path.join(stl_path, "mesh_2.npy"))

Step 4: Add excitation and probes

Excitation

Mostly electric field applied from a copper waveguide to a ground plane (i.e. a voltage).

Probes

• Far-field probe: make sure the antenna is situated centrally with respect to the far-field box.

Tips

FreeCad

• In the next antenna project, try to make the sheet belong to the same part, and try to have multiple bodies belong to that part.

Indicating lengths in FreeCad

Do indicate the right length in freecad use

1. Select 2-lengths.
2. Length: length in the normal direction compared to main face
3. Length2: length in the direction parallel but opposite to the normal.

So if you want to have an object of length 2 mm, floating 1 mm from the normal then

• length = 3 mm
• length2 = -1 mm

note that for some annoying reason the length2 can't be 0, so we need to use a very small number here instead

Creating VIAS in FreeCad

1. Sketch a single via (hollow cylinder)
2. Open the Draft-tool
3. The "Position" needs to be set to the position where the x, y and z repetition needs to be started from. E.g.: the left/right upper/down most-position.
4. The x/y/z-interval x/y/z needs to be set to the distance between each repetition
5. Use the copy and rotate tool to make multiple copies of the same via-structure and rotate if necessary
6. Make sure to add all of these together inside a single "part"-container
7. Export as STL.

Meandering

• When meandering the antenna, the impedance increases because there is inductive coupling with meanders (so inductance goes up)
• An increased ground plane decreases impedance, since it reduces certain parasitics.

Resources

Learning FTDT method

Quick overview of EM theory, finite-method code architecture, types of materials and their models, ..:

• https://github.com/john-b-schneider/uFDTD

Learning about the design of openEMS itself

OpenEMS tutorials:

• https://docs.openems.de/python/openEMS/Tutorials/index.html FOSDEM19 intro:
• https://archive.fosdem.org/2019/schedule/event/openems/attachments/slides/3367/export/events/attachments/openems/slides/3367/openEMS_FOSDEM19.pdf
• https://av.tib.eu/media/44458 OpenEMS field simulations in Paraview
• https://www.youtube.com/watch?v=XfZS62qvCN4

Nice add-ons

• https://github.com/SiliconLabs/hardware_design_examples/tree/AntennaSimulationWorkbench/examples Converts HyperLynx to matlab for electromagnetic simulation or PDF for printing.
• https://github.com/koendv/hyp2mat KiCad PCB files to openEMS models
• https://github.com/jcyrax/pcbmodelgen Repository containing a high-level Python interface for mesh generation, kicad footprint generation and much more
• https://github.com/matthuszagh/pyems