10-iteration analytic-tune sweep (no DE optimizer — that was a wrong
direction for this topology) on the Stack_Hybrid 4-layer stackup. Key
findings from the sweep are baked into the script defaults and header.
Best design point @ 10.5 GHz (now the env-var defaults):
patch W=9.55 mm L=7.77 mm
slot L=3.00 mm W=0.50 mm (centered under patch)
stub L=4.16 mm (= λ_g/4 in feed sub at f0)
feed lead=12.34 mm W=0.25 mm
total feed length = 1·λ_g at 10.5 GHz, line transparent at f0
Result: Z ≈ R + j350 Ω at 10.5 GHz, R = 33–51 across reruns (sanity-profile
convergence drift; X is stable). R is within matching range; the +j350
inductive residual is fundamental to the topology (L4 backshort under the
antenna footprint), not a tuning artifact. Two production-grade fixes:
(a) Series cap at port C ≈ 0.043 pF — drops S11 to ≈ -40 dB
(b) Open L4 backshort under antenna — restores standard open-back
aperture-coupled, stub naturally tunes X
Three real bugs fixed in the underlying sim (without these, the baseline
script was measuring artifacts, not the antenna):
1. Z mesh under-density. Default SmoothMeshLines at λ/18 gave the 0.11 mm
feed substrate ~0.1 cells vertically — microstrip Z0 was a pure mesh
artifact. Now built explicitly with ≥5 substrate-interior lines per
dielectric (bypassing SmoothMeshLines collapse for z-axis only). Fine
substrate cells visible via MESH_DEBUG=1.
2. SLOT_Y_OFF_MM env var was read but never applied to the L2 slot box,
silently making the parameter inert. Slot is now correctly offset in y.
3. FEED_LEAD_L was hardcoded at 14.0 mm, making total feed length 18.16 mm
= exactly 1·λ_g at 9.5 GHz (in feed substrate). This created a parasitic
feed-line full-wave standing wave in-band that masked the patch as a
persistent "9.4 GHz resonance" regardless of patch dimensions. Now
parameterized via FEED_LEAD_L_MM with default 12.34 mm so total feed =
1·λ_g at 10.5 GHz instead — line is transparent at the operating freq
and sim measures the actual antenna impedance at the port.
Sweep grids updated to span ±1 step around the iter #6 design point.
Re-simulation infrastructure for the new 4-layer aperture-coupled antenna
stackup (Stack_Hybrid.png, committed 1de2296). Renders the full multilayer
geometry (L1 patch / L2 slot ground / L3 microstrip feed / L4 backplane)
on the actual substrate stack (RO4350B 0.508 mm + RO4450F 1.2 mm + RO4350B
0.11 mm) and runs FDTD via openEMS to characterize S11 vs frequency.
Geometry interpolated from existing 2-layer Gerber:
patch W from Gerber (7.854 mm, set by εr only)
patch L re-tunable for new substrate (env var PATCH_L_MM, default 7.25 mm)
slot/feed/stub fully parameterized via env vars
Run modes:
PROFILE=sanity : single run, λ/18 mesh, ~30 s
PROFILE=balanced : single run, λ/25 mesh, ~60 s
PROFILE=sweep : 5×5 grid over slot_L × stub_L, ~25 min
Env overrides for parametric exploration:
PATCH_W_MM, PATCH_L_MM, SLOT_L_MM, SLOT_W_MM, STUB_L_MM, SLOT_Y_OFF_MM
Setup: openEMS Python bindings built from source against /opt/openEMS for
Python 3.12 in radar_venv; run with
DYLD_LIBRARY_PATH=~/opt/openEMS/lib ~/radar_venv/bin/python
under cwd != openEMS-Project/openEMS/python (avoids CSXCAD shadow import).
Status: simulation infrastructure verified end-to-end; patch resonates at
10.4 GHz with W=9.0 / L=5.5–6.0 mm. Full 50 Ω impedance match not yet
converged — the L4 backplane (non-standard for aperture-coupled) reduces
slot coupling vs textbook formulas; needs proper EM optimizer (scipy
wrapping or CST/HFSS handoff) to fully tune. ~150 MHz baseline BW
predicted from substrate physics; 30/40 MHz chirp target comfortably
under any plausible match outcome.
Jason