Standalone FDTD STEP Model#

This tutorial tells you how to simulate a STEP CAD file with the FDTD (openEMS) backend. You do not have to build a CSXCAD geometry first.

The FEM backend has a related function. See Standalone FEM STEP Model.

Note

This function is a prototype. It can change in a later release.

Make a file with the name inset_fed_patch_step_fdtd.py.

Import the modules#

from pathlib import Path
import numpy as np
from simpleEMS import SimTools
from simpleEMS import simulate_step_FDTD

simulate_step_FDTD reads the STEP file and runs the FDTD solver. SimTools shows and exports the results. All the simpleEMS workflows use SimTools.

See also

Model structure#

A STEP file contains only named solids. It contains no material data and no port data. You must tell the function what each solid is.

Three arguments give the role of each solid:

  • dielectrics={name: (eps_r, tan_d)} — the dielectric solids, for example the substrate

  • pec=[name, ...] — the solids that are perfect conductors

  • ports={name: {"z0": ..., "direction": "x|y|z", "number": ...}} — the ports

Important

Give the name of each solid that you want in the simulation. The FDTD backend does not set a role automatically from the name of a solid. It ignores each solid that these three arguments do not name.

The function stops and shows an error if a name is not in the STEP file. It also stops if you give no ports.

The example uses a STEP file with five named solids:

Solid

Role from

Function

substrate

dielectrics

dielectric substrate

patch_inset

pec

radiant patch

feed

pec

inset feed line

ground

pec

ground plane

port_resist_1

ports

lumped port

Set the parameters#

STEP_FILE = Path(__file__).parent / "structure.step"

resonant_freq = 24.125e9  # Hz
span_freq = 2e9  # Hz
freqs = np.linspace(resonant_freq - span_freq, resonant_freq + span_freq, 501)

STEP_FILE gives the path to the CAD file. freqs gives the frequency points of the results. The lowest and the highest of these frequencies also set the band of the excitation signal.

Simulate the model#

sim_data, nf2ff = simulate_step_FDTD(
    STEP_FILE,
    freqs,
    dielectrics={"substrate": (3.48, 0.0037)},  # (eps_r, tan_d)
    pec=["patch_inset", "feed", "ground"],
    ports={"port_resist_1": {"z0": 50.0, "direction": "z", "number": 1}},
    charac_imp=50.0,
    FDTD_end_criteria=1e-5,
)

One call does all of these steps:

  1. It reads the named solids from the STEP file.

  2. It builds the CSXCAD geometry.

  3. It makes the mesh.

  4. It runs the openEMS solver.

  5. It reads the results back.

These arguments control the solver:

  • FDTD_end_criteria sets when the solver stops. A smaller value gives more accuracy, but the solver runs for a longer time.

  • FDTD_timestep sets the maximum number of time steps.

  • FDTD_mesh_resolution_factor and FDTD_metal_mesh_resolution_factor set how fine the mesh is. The second one applies near the metal.

These arguments control the sequence:

  • show_structure shows the geometry in AppCSXCAD before the simulation. The default value is True. Close the AppCSXCAD window to continue.

  • run starts the solver. The default value is True. Set it to False to examine the geometry only.

The function returns two items:

  • sim_data — the S-parameters, the impedance, the VSWR, and the port power

  • nf2ff — the object that records the far-field data

Note

The function makes the nf2ff object before the solver runs. openEMS records the far-field data only while the solver runs. You cannot get a radiation pattern after the simulation if this object does not exist.

Show the results#

SimTools.plot_s_param(sim_data.freqs, sim_data.s11)
SimTools.plot_smith_chart(
    sim_data.freqs, sim_data.s11, charac_imp=sim_data.ref_impedance
)
SimTools.plot_vswr(sim_data.freqs, sim_data.vswr)
SimTools.plot_impedance(sim_data.freqs, sim_data.z11)
SimTools.plot_2d_directivity(nf2ff, resonant_freq)
SimTools.plot_2d_rad_pattern(nf2ff, resonant_freq)
nf2ff_3d = SimTools.compute_nf2ff_3d(nf2ff, resonant_freq)
SimTools.plot_3d_directivity(nf2ff_3d, resonant_freq)
SimTools.plot_3d_gain(nf2ff_3d, resonant_freq, sim_data.input_power)
SimTools.plot_3d_power(nf2ff_3d, resonant_freq)

SimTools.save_plots()
SimTools.show_plots()

These commands show the S11 curve, the Smith chart, the VSWR, and the input impedance. They also show the 2D and the 3D radiation patterns. save_plots writes each plot to a file. show_plots shows the plots on the screen.

See also

Export the results#

SimTools.export_touchstone(freqs=sim_data.freqs, s11=sim_data.s11, charac_imp=50.0)

This command writes the S11 results to a Touchstone file. Other RF tools can read this file.

Complete script#

The complete script is below. It simulates an inset-fed patch antenna at 24.125 GHz directly from structure.step. Then it shows the S11 curve, the Smith chart, the VSWR, the input impedance, and the radiation patterns. At the end it writes a Touchstone file.

#!/usr/bin/env python3
"""PROTOTYPE: inset-fed patch antenna at 24.125 GHz, FDTD simulation directly
from a standalone STEP file (CSXCAD polyhedron import -> openEMS FDTD)."""

# IMPORTS
from pathlib import Path
import numpy as np
from simpleEMS import SimTools
from simpleEMS import simulate_step_FDTD
# IMPORTS

# PARAMS
STEP_FILE = Path(__file__).parent / "structure.step"

resonant_freq = 24.125e9  # Hz
span_freq = 2e9  # Hz
freqs = np.linspace(resonant_freq - span_freq, resonant_freq + span_freq, 501)
# PARAMS

# SIMULATE
sim_data, nf2ff = simulate_step_FDTD(
    STEP_FILE,
    freqs,
    dielectrics={"substrate": (3.48, 0.0037)},  # (eps_r, tan_d)
    pec=["patch_inset", "feed", "ground"],
    ports={"port_resist_1": {"z0": 50.0, "direction": "z", "number": 1}},
    charac_imp=50.0,
    FDTD_end_criteria=1e-5,
)
# SIMULATE

# PPROCESS
SimTools.plot_s_param(sim_data.freqs, sim_data.s11)
SimTools.plot_smith_chart(
    sim_data.freqs, sim_data.s11, charac_imp=sim_data.ref_impedance
)
SimTools.plot_vswr(sim_data.freqs, sim_data.vswr)
SimTools.plot_impedance(sim_data.freqs, sim_data.z11)
SimTools.plot_2d_directivity(nf2ff, resonant_freq)
SimTools.plot_2d_rad_pattern(nf2ff, resonant_freq)
nf2ff_3d = SimTools.compute_nf2ff_3d(nf2ff, resonant_freq)
SimTools.plot_3d_directivity(nf2ff_3d, resonant_freq)
SimTools.plot_3d_gain(nf2ff_3d, resonant_freq, sim_data.input_power)
SimTools.plot_3d_power(nf2ff_3d, resonant_freq)

SimTools.save_plots()
SimTools.show_plots()
# PPROCESS

# EXPORT
SimTools.export_touchstone(freqs=sim_data.freqs, s11=sim_data.s11, charac_imp=50.0)
# EXPORT