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Advanced mode

Advanced mode hands you the mesh, air margins, PML depth, run length, and convergence criteria that Auto mode derives for you. Boundary face types live on Boundaries. This page covers everything else the Advanced toggle exposes.

Switching to Advanced

The Simulate tab has an Auto / Advanced toggle. In Advanced mode the quality slider stops driving the mesh: you set dx_mm, dy_mm, dz_mm, the air margins, the PML depth, and the run-length fields, and the run uses them exactly as written. Nothing is snapped or re-derived behind you.

Advanced values persist in the project independently of the auto values. Every Advanced field has an adv_* shadow, so a project saved in Auto mode keeps its Advanced entries and reopening in Advanced mode restores them. Transfer to Advanced copies the current auto-derived mesh, margins, PML, and boundary preset into the Advanced fields as a starting point, then you edit from there.

Manual mesh

The Mesh / Cell Size card sets the three base cell sizes.

dx_mm, dy_mm, and dz_mm are the Yee cell dimensions in millimetres, default 0.2 each. The run uses them exactly. FDTD tips covers picking cell sizes.

dz is the trap. In Advanced mode it is used exactly, never snapped, so if it does not evenly divide the conductor stackup height the substrate lands between grid lines. validate warns (dz does not divide the substrate height) and reports the closest dz that would fit, but it does not change your value. The reasoning and how to pick dz are on FDTD tips.

Mesh grading

Grading keeps fine cells in the board region and lets them grow toward the air and PML, cutting total cells while holding resolution where the fields are. Enable it with the grading checkbox. The controls:

  • Refine margin (mesh_grading_margin_mm, default 1.0): how far the fine-cell region extends past the board in every direction.
  • Per-axis multiplier (mesh_grading_{x,y,z}_mult): the board-region refinement factor, 1.0 meaning no refinement. Default 2.0 in Z (half the Z cell near the board), 1.0 in X and Y.
  • Per-axis max ratio (mesh_grading_{x,y,z}_max_ratio, default 1.10): the largest cell-to-cell size step allowed in that axis' transition. Must exceed 1.0, or no transition can satisfy it.

Air margins

The air gap between the geometry and the boundary has three modes, selected by the Air Margins mode combo, each with per-face around / above / below values.

  • Cell count (air_cells, default 6): the same number of air cells on every face. Keyed to the mesh, so the physical gap follows the cell size.
  • Millimetres (air_around_mm, air_above_mm, air_below_mm, default 10.0): each face set in mm directly.
  • Frequency fraction (auto_air_sides_factor, auto_air_above_factor, auto_air_below_factor, default 0.25): each face sized as a fraction of the wavelength at f_min.

PML depth has its own three modes (pml_cells_manual default 8, pml_thickness_mm default 4.0, auto_pml_factor default 0.10), set on the same card and covered under Boundaries.

Run length

The Energy Ringdown card sets when the run stops.

The ring-down criterion is the remaining field energy relative to peak at which the solver stops. It is edited in dB and stored as a ratio (ringdown_end_crit), default -50 dB (ratio 1e-5). More negative is a longer, more converged run. The floor is -120 dB, since the control is logarithmic and zero has no dB value. The check interval and minimum-steps floor default to 20 and 20000 steps and are adjustable through the settings registry (see CLI equivalent below).

Limit max steps adds a hard timestep cap (max_steps). Leave it off and the run has no cap and stops on ring-down alone. If the cap is hit before ring-down, the S-parameter and radiation results are not converged. See FDTD tips.

S-param early stop (sparam_early_stop, off by default) stops the run once the S-parameters stop changing by more than sparam_delta_crit_db (default -40 dB) between checks. sparam_min_steps (default 10000) is a floor so it cannot fire during the source excitation transient.

Mesh convergence

The Mesh Convergence card runs a multi-pass study that refines the cell size until the tracked resonances stabilise. Its Mesh Convergence Study checkbox writes auto_mesh_enabled. It starts from the Cell Size above and refines only the mesh each pass.

  • Source (conv_source, default S11) and resonance count (conv_num_resonances, default 1): which S-parameter and how many of its deepest dips to track, matched between passes by rank.
  • Freq shift (conv_freq_threshold_db, default -20 dB) and depth diff (conv_depth_threshold_db, default -20 dB): the per-resonance thresholds a pass must meet to count as converged. -20 dB is about 1 percent.
  • Mesh step (conv_mesh_refine_factor, default 1.26): the division factor each pass. 1.26 is about cbrt(2), roughly doubling total cells.
  • Max passes (conv_max_passes, default 8): the hard iteration limit.

CLI equivalent

Every field above is a project setting reachable headless: override one with --setting <name>=<value> or carry the whole set in a JobSpec. See Scripting and specs. The full field list with units and bounds is on Settings reference.