Migration Guide

This guide helps users coming from Meep or OpenEMS translate their workflows into rfx. rfx follows the same Yee-cell FDTD physics, so the core concepts are familiar — the API surface is different.

Concept Mapping

Concept	Meep	OpenEMS	rfx
Grid setup	`Simulation(resolution=N)`	`InitCSX()` + `InitFDTD()`	`Simulation(freq_max=...)` or `Simulation.auto(...)`
Cell size	`resolution` (cells/unit)	`SetDeltaUnit(1e-3)`	`dx=` in metres (auto-calculated from `freq_max`)
Source	`EigenModeSource`, `Source`	`AddExcitation`	`add_port()`, `add_source()`
S-parameters	`add_flux()` + post-processing	`CalcPort`	`compute_s_params=True` in `run()`
Resonance finding	`harminv(...)`	Manual FFT	`result.find_resonances()`
Auto-stop	`stop_when_fields_decayed`	`EndCriteria`	`run(until_decay=1e-3)`
Materials	`Medium(epsilon=...)`	`AddMaterial`	`sim.add(shape, material="fr4")` or `sim.add_material(...)`
PEC	`PerfectElectricalConductor`	`AddMetal`	`material="pec"`
PML / ABC	`PML(thickness)`	`AddPML`	`boundary="cpml"`
Dispersive media	`LorentzianSusceptibility`	`AddLorentzMaterial`	`DebyePole`, `LorentzPole`, `drude_pole()`
Differentiable	Not available	Not available	`jax.grad(loss_fn)(params)`
Inverse design	Not native (adjoint plugin)	Not native	`rfx.optimize(sim, design_region, objective)`
Subgridding	Not available	Not available	experimental / specialized
Non-uniform mesh	Not native	`SmoothMeshLines`	`dz_profile` or `auto_configure()`

Quick Translation

Meep: Rectangular Cavity Resonance

# Meep
import meep as mp

sim = mp.Simulation(
    cell_size=mp.Vector3(0.1, 0.1, 0.05),
    resolution=50,
    boundary_layers=[],
)
sim.sources = [mp.Source(
    mp.GaussianSource(frequency=2.0, fwidth=0.5),
    component=mp.Ez,
    center=mp.Vector3(0.03, 0.03, 0.02),
)]
sim.run(mp.at_beginning(mp.output_epsilon),
        until_after_sources=mp.stop_when_fields_decayed(50, mp.Ez, mp.Vector3(), 1e-6))

# rfx equivalent
from rfx import Simulation

sim = Simulation(freq_max=5e9, domain=(0.1, 0.1, 0.05), boundary="pec")
sim.add_source(position=(0.03, 0.03, 0.02), component="ez")
sim.add_probe(position=(0.06, 0.06, 0.02), component="ez")
result = sim.run(until_decay=1e-3)
freqs = result.find_resonances()

OpenEMS: Waveguide S-Parameters

% OpenEMS (MATLAB)
CSX = InitCSX();
FDTD = InitFDTD('EndCriteria', 1e-5);
CSX = AddMetal(CSX, 'PEC');
CSX = AddBox(CSX, 'PEC', 10, [0 0 0], [40 20 10]);
[CSX, port{1}] = AddRectWaveGuidePort(CSX, 0, 1, ...);
RunOpenEMS(Sim_Path, Sim_CSX, '--engine=multithreaded');
port = calcPort(port, Sim_Path, freq);
s11 = port{1}.uf.ref ./ port{1}.uf.inc;

# rfx equivalent
from rfx import Simulation, Box

sim = Simulation(freq_max=15e9, domain=(0.04, 0.02, 0.01), boundary="cpml")
sim.add(Box((0, 0, 0), (0.04, 0.02, 0.01)), material="pec")
sim.add_port(
    position=(0.005, 0.01, 0.005),
    component="ez",
    waveform=GaussianPulse(f0=10e9),
)
result = sim.run(until_decay=1e-5, compute_s_params=True)
s11 = result.s_params[0, 0, :]

Meep Adjoint -> rfx Inverse Design

# Meep (requires meep.adjoint plugin)
opt = mpa.OptimizationProblem(...)
opt.update_design([design_params])
f, g = opt()  # forward + adjoint

# rfx (native JAX autodiff)
import jax

def loss(eps_r):
    sim = Simulation(freq_max=8e9, domain=(0.04, 0.01, 0.01), boundary="cpml")
    sim.add_port(position=(0.008, 0.005, 0.005), component="ez")
    result = sim.run(n_steps=150, compute_s_params=True, eps_r_override=eps_r)
    return -jnp.abs(result.s_params[0, 0, 50])

grad_fn = jax.grad(loss)
gradient = grad_fn(initial_eps_r)

What rfx Does Differently

JAX-Native, GPU-Accelerated

rfx runs on GPU out of the box via JAX. No separate engine binary, no file-based I/O between the Python front-end and a C++ solver. The full time-stepping loop is JIT-compiled and executes on GPU (~3,000 Mcells/s on RTX 4090).

Differentiable by Default

Every rfx simulation is differentiable with jax.grad. Gradients propagate through the entire FDTD time-stepping, sources, probes, and post-processing. This enables gradient-based inverse design without adjoint plugins or finite-difference approximations.

Declarative Builder API

rfx uses a single Simulation object that accumulates geometry, materials, sources, and probes. Call run() once — S-parameters, resonances, and field snapshots are computed automatically based on what was requested.

Built-in Material Library

Eleven common RF materials (fr4, rogers4003c, alumina, silicon, copper, pec, etc.) are available by name. No need to look up permittivity tables for standard substrates and conductors.

Auto-Configuration

Simulation(freq_max=N) automatically sets cell size, time step, CPML thickness, and source waveform. For most antenna and waveguide problems you only need to specify freq_max and domain.

Non-Uniform Z for Thin Substrates

For PCB and patch-style problems, the most practical recent rfx workflow is graded z meshing via dz_profile / auto_configure(), rather than assuming one globally fine uniform mesh.

Common Gotchas

Issue	Solution
Coordinates are in metres, not mm or cell units	All positions use SI metres
`freq_max` is in Hz, not normalized frequency	Use `freq_max=5e9` for 5 GHz
PML is outside the domain you specify	`domain=` is the physical region; CPML pads are added automatically
`run()` returns a `Result` object	Access fields via `result.s_params`, `result.time_series`, `result.find_resonances()`
No mesh file export	rfx solves in-process; use `sim.grid` to inspect the mesh