Antenna pattern from FDTD

Wave 69 + 69.B closed the loop between cad-sim's existing 3D FDTD solver and the Huygens-equivalence Near-to-Far-Field transform — the only MIT/Apache FDTD → NTFF antenna pattern code in the open-source ecosystem.

See the NTFF algorithm page for the math.

What you need

The current Wave 69 pipeline is a Rust library — there's no MCP tool yet (Wave 69.C will add run_fdtd_radiation_pattern). The library sequence is:

  1. Configure the FDTD grid (FdtdConfig, FdtdGrid).
  2. Place a source — FdtdSource::GaussianPulse at the radiating structure's feed point.
  3. Define a Huygens surface enclosing the structure: HuygensSurface::new_box(box_idx, &grid, frequency_hz).
  4. Time-step the simulation. On every step, after update_e and update_h and any source application: 
    #![allow(unused)]
fn main() {
grid.update_h();
grid.apply_source(&source);
grid.update_e();
surface.step_sample(&grid, t_now);
}
  5. When stepping completes, convert to NTFF input: let samples = surface.extract_currents();
  6. Compute the radiation pattern: let result = ntff_radiation_pattern(&samples, freq, n_theta, n_phi);

The FarFieldResult exposes per-(θ, φ) gain in dBi, |E_θ|, |E_φ|, peak direction, etc.

Validating against analytic patterns

The Wave 69 tests show this loop reproduces:

  • Hertzian dipole sin θ pattern shape to ratios within 10⁻⁶ at 9 sample angles.
  • Two-element broadside array at z = ±λ/4 in phase — the classic main-lobe at θ = 90° dominating the near-axis values by ≥ 5×.
  • Pattern sweep peak lands within 10° of θ = 90° (broadside) for a z-oriented dipole.

The Wave 69.B Huygens-surface sampling tests verify:

  • A static Ex = 1 V/m sampled at t = 0 accumulates exactly (dt, 0) in the running DFT phasor (pure cosine projection).
  • The same field at t = T/4 accumulates exactly (0, −dt) (quarter-period phase rotation correct).

What's coming (Wave 69.C)

run_fdtd_radiation_pattern(geometry, frequency_hz, n_theta, n_phi, grid_steps) MCP tool will encapsulate the whole loop so agents can compute patterns over the network. The current path requires the substrate running in-process.

When to bother

For boards using antenna-in-package parts (the Infineon BGT60TR13C 60 GHz radar in aether-t, for example), the antenna sits inside the chip — the PCB carries no RF trace to it and no NTFF is needed.

For boards with on-PCB antennas (LoRa monopoles, GNSS patches, Wi-Fi PIFAs, mmWave horns), NTFF validates the radiation pattern before fabrication. The substrate's NTFF is matched against analytic shapes at lib-test time; for novel geometries, run an external reference (HFSS / openEMS / CST) once and use the substrate's result as the go-to for design-iteration speed.