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https://github.com/NawfalMotii79/PLFM_RADAR.git
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e01c2ae424
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.
