diff --git a/nbs/_palace_cpw.ipynb b/nbs/_palace_cpw.ipynb
index 215f16a3..69d64790 100644
--- a/nbs/_palace_cpw.ipynb
+++ b/nbs/_palace_cpw.ipynb
@@ -181,9 +181,14 @@
     "sim_lumped.set_geometry(c)\n",
     "\n",
     "# Configure layer stack from active PDK\n",
-    "stack = get_stack(air_above=100.0, air_below=100.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK\n",
     "sim_lumped.set_stack(stack)\n",
     "\n",
+    "# Surround the design with air. Without this the absorbing boundary sits\n",
+    "# directly on the passivation, microns above the metal, and adds large\n",
+    "# spurious loss and reflection.\n",
+    "sim_lumped.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
+    "\n",
     "# Configure left CPW port (single port at signal center)\n",
     "sim_lumped.add_cpw_port(\n",
     "    \"o1\",\n",
@@ -239,6 +244,7 @@
     "\n",
     "# Reuse the same stack from active PDK\n",
     "sim_waveport.set_stack(stack)\n",
+    "sim_waveport.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
     "\n",
     "# Configure left CPW port (single port at signal center)\n",
     "sim_waveport.add_wave_port(\"o1\", layer=\"topmetal2\", max_size=True, mode=1, excited=True)\n",
diff --git a/nbs/_palace_cpw.py b/nbs/_palace_cpw.py
index 09ec0d9c..d258b3b9 100644
--- a/nbs/_palace_cpw.py
+++ b/nbs/_palace_cpw.py
@@ -108,9 +108,14 @@ def gsg_electrode(
 sim_lumped.set_geometry(c)
 
 # Configure layer stack from active PDK
-stack = get_stack(air_above=100.0, air_below=100.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK
 sim_lumped.set_stack(stack)
 
+# Surround the design with air. Without this the absorbing boundary sits
+# directly on the passivation, microns above the metal, and adds large
+# spurious loss and reflection.
+sim_lumped.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
+
 # Configure left CPW port (single port at signal center)
 sim_lumped.add_cpw_port(
     "o1",
@@ -154,6 +159,7 @@ def gsg_electrode(
 
 # Reuse the same stack from active PDK
 sim_waveport.set_stack(stack)
+sim_waveport.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
 
 # Configure left CPW port (single port at signal center)
 sim_waveport.add_wave_port("o1", layer="topmetal2", max_size=True, mode=1, excited=True)
diff --git a/nbs/palace_branch_line_coupler.ipynb b/nbs/palace_branch_line_coupler.ipynb
index 075e233c..4ca2a730 100644
--- a/nbs/palace_branch_line_coupler.ipynb
+++ b/nbs/palace_branch_line_coupler.ipynb
@@ -317,9 +317,14 @@
     "sim.set_geometry(c)\n",
     "\n",
     "# Configure layer stack from active PDK\n",
-    "stack = get_stack(air_above=300.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK\n",
     "sim.set_stack(stack)\n",
     "\n",
+    "# Surround the design with air. Without this the absorbing boundary sits\n",
+    "# directly on the passivation, microns above the metal, and adds large\n",
+    "# spurious loss and reflection.\n",
+    "sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
+    "\n",
     "# Configure via ports (Metal3 ground plane to TopMetal2 signal)\n",
     "for port in c.ports:\n",
     "    sim.add_port(port.name, from_layer=\"metal3\", to_layer=\"topmetal2\", geometry=\"via\")\n",
diff --git a/nbs/palace_branch_line_coupler.py b/nbs/palace_branch_line_coupler.py
index a1f76966..6069c028 100644
--- a/nbs/palace_branch_line_coupler.py
+++ b/nbs/palace_branch_line_coupler.py
@@ -233,9 +233,14 @@ def branch_line_coupler(
 sim.set_geometry(c)
 
 # Configure layer stack from active PDK
-stack = get_stack(air_above=300.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK
 sim.set_stack(stack)
 
+# Surround the design with air. Without this the absorbing boundary sits
+# directly on the passivation, microns above the metal, and adds large
+# spurious loss and reflection.
+sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
+
 # Configure via ports (Metal3 ground plane to TopMetal2 signal)
 for port in c.ports:
     sim.add_port(port.name, from_layer="metal3", to_layer="topmetal2", geometry="via")
diff --git a/nbs/palace_cpw_lumped.ipynb b/nbs/palace_cpw_lumped.ipynb
index 8d0e8f3d..c0ad37d8 100644
--- a/nbs/palace_cpw_lumped.ipynb
+++ b/nbs/palace_cpw_lumped.ipynb
@@ -189,7 +189,7 @@
     "sim.set_output_dir(\"./palace-sim-cpw-lumped\")\n",
     "sim.set_geometry(c)\n",
     "\n",
-    "stack = get_stack(air_above=100.0, air_below=100.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK; air comes from set_airbox() below\n",
     "sim.set_stack(stack)\n",
     "\n",
     "# CPW lumped ports — offset defaults to length/2 (flush with conductor edge)\n",
diff --git a/nbs/palace_cpw_lumped.py b/nbs/palace_cpw_lumped.py
index 72c4ce5d..2bae773d 100644
--- a/nbs/palace_cpw_lumped.py
+++ b/nbs/palace_cpw_lumped.py
@@ -98,7 +98,7 @@ def gsg_electrode(
 sim.set_output_dir("./palace-sim-cpw-lumped")
 sim.set_geometry(c)
 
-stack = get_stack(air_above=100.0, air_below=100.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK; air comes from set_airbox() below
 sim.set_stack(stack)
 
 # CPW lumped ports — offset defaults to length/2 (flush with conductor edge)
diff --git a/nbs/palace_cpw_via.ipynb b/nbs/palace_cpw_via.ipynb
index 3b9a2463..6b9c77e9 100644
--- a/nbs/palace_cpw_via.ipynb
+++ b/nbs/palace_cpw_via.ipynb
@@ -323,9 +323,14 @@
     "sim.set_geometry(c)\n",
     "\n",
     "# Configure layer stack from active PDK\n",
-    "stack = get_stack(air_above=300.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK\n",
     "sim.set_stack(stack)\n",
     "\n",
+    "# Surround the design with air. Without this the absorbing boundary sits\n",
+    "# directly on the passivation, microns above the metal, and adds large\n",
+    "# spurious loss and reflection.\n",
+    "sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
+    "\n",
     "# Configure left CPW port (single port at signal center)\n",
     "sim.add_cpw_port(\"o1\", layer=\"topmetal2\", s_width=20, gap_width=15)\n",
     "\n",
diff --git a/nbs/palace_cpw_via.py b/nbs/palace_cpw_via.py
index b429d1d4..f78425fa 100644
--- a/nbs/palace_cpw_via.py
+++ b/nbs/palace_cpw_via.py
@@ -239,9 +239,14 @@ def gsg_electrode_tm2_tm1_m5(
 sim.set_geometry(c)
 
 # Configure layer stack from active PDK
-stack = get_stack(air_above=300.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK
 sim.set_stack(stack)
 
+# Surround the design with air. Without this the absorbing boundary sits
+# directly on the passivation, microns above the metal, and adds large
+# spurious loss and reflection.
+sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
+
 # Configure left CPW port (single port at signal center)
 sim.add_cpw_port("o1", layer="topmetal2", s_width=20, gap_width=15)
 
diff --git a/nbs/palace_microstrip.ipynb b/nbs/palace_microstrip.ipynb
index 21a1d53f..85c057f6 100644
--- a/nbs/palace_microstrip.ipynb
+++ b/nbs/palace_microstrip.ipynb
@@ -146,9 +146,14 @@
     "sim.set_geometry(c)\n",
     "\n",
     "# Configure layer stack from active PDK\n",
-    "stack = get_stack(air_above=300.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK\n",
     "sim.set_stack(stack)\n",
     "\n",
+    "# Surround the design with air. Without this the absorbing boundary sits\n",
+    "# directly on the passivation, microns above the trace, and adds large\n",
+    "# spurious loss and reflection.\n",
+    "sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
+    "\n",
     "# Configure via ports (Metal1 ground plane to TopMetal2 signal)\n",
     "for port in c.ports:\n",
     "    sim.add_port(port.name, from_layer=\"metal1\", to_layer=\"topmetal2\", geometry=\"via\")\n",
diff --git a/nbs/palace_microstrip.py b/nbs/palace_microstrip.py
index b55ac63f..f4624eb7 100644
--- a/nbs/palace_microstrip.py
+++ b/nbs/palace_microstrip.py
@@ -62,9 +62,14 @@
 sim.set_geometry(c)
 
 # Configure layer stack from active PDK
-stack = get_stack(air_above=300.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK
 sim.set_stack(stack)
 
+# Surround the design with air. Without this the absorbing boundary sits
+# directly on the passivation, microns above the trace, and adds large
+# spurious loss and reflection.
+sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
+
 # Configure via ports (Metal1 ground plane to TopMetal2 signal)
 for port in c.ports:
     sim.add_port(port.name, from_layer="metal1", to_layer="topmetal2", geometry="via")
diff --git a/nbs/palace_width_sweep.ipynb b/nbs/palace_width_sweep.ipynb
index 47b13f5f..10153070 100644
--- a/nbs/palace_width_sweep.ipynb
+++ b/nbs/palace_width_sweep.ipynb
@@ -134,7 +134,7 @@
     "\n",
     "PDK.activate()\n",
     "\n",
-    "stack = get_stack(air_above=300.0)  # auto-detects active PDK\n",
+    "stack = get_stack()  # auto-detects active PDK\n",
     "\n",
     "sims = []\n",
     "\n",
@@ -153,6 +153,10 @@
     "    sim.set_output_dir(f\"./palace-sim-w{w:.1f}\")\n",
     "    sim.set_geometry(c)\n",
     "    sim.set_stack(stack)\n",
+    "    # Surround the design with air. Without this the absorbing boundary sits\n",
+    "    # directly on the passivation, microns above the trace, and adds large\n",
+    "    # spurious loss and reflection.\n",
+    "    sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)\n",
     "\n",
     "    for port in c.ports:\n",
     "        sim.add_port(\n",
diff --git a/nbs/palace_width_sweep.py b/nbs/palace_width_sweep.py
index 9658b87f..27a5bcb8 100644
--- a/nbs/palace_width_sweep.py
+++ b/nbs/palace_width_sweep.py
@@ -44,7 +44,7 @@
 
 PDK.activate()
 
-stack = get_stack(air_above=300.0)  # auto-detects active PDK
+stack = get_stack()  # auto-detects active PDK
 
 sims = []
 
@@ -63,6 +63,10 @@
     sim.set_output_dir(f"./palace-sim-w{w:.1f}")
     sim.set_geometry(c)
     sim.set_stack(stack)
+    # Surround the design with air. Without this the absorbing boundary sits
+    # directly on the passivation, microns above the trace, and adds large
+    # spurious loss and reflection.
+    sim.set_airbox(margin_x=50, margin_y=50, z_above=100, z_below=100)
 
     for port in c.ports:
         sim.add_port(
diff --git a/src/gsim/palace/base.py b/src/gsim/palace/base.py
index 760a2499..33bb6b2e 100644
--- a/src/gsim/palace/base.py
+++ b/src/gsim/palace/base.py
@@ -1328,6 +1328,30 @@ def mesh(
             gmsh_verbosity=verbosity,
         )
 
+        # Without an airbox the absorbing boundary is applied straight to the
+        # outer dielectric surface, often microns from the metal, where it acts
+        # as a lossy sheet and fakes large insertion loss and reflection. Warn
+        # here so it surfaces before any run is submitted.
+        from gsim.palace.mesh.geometry import is_air_like_material
+
+        mesh_result = getattr(self, "_mesh_result", None) or getattr(
+            self, "_last_mesh_result", None
+        )
+        groups = getattr(mesh_result, "groups", None) or {}
+        volumes = groups.get("volumes", {})
+        if groups.get("boundary_surfaces", {}).get("absorbing") and not any(
+            is_air_like_material(stack, name) for name in volumes
+        ):
+            logger.warning(
+                "Absorbing boundary is applied to solid dielectric surfaces "
+                "(%s) because the model has no air region, which produces "
+                "large spurious insertion loss and reflection. Add an airbox "
+                "via set_airbox(margin_x=..., margin_y=..., z_above=..., "
+                "z_below=...). Note that get_stack(air_above=...) is "
+                "deprecated and ignored.",
+                ", ".join(sorted(volumes)),
+            )
+
         # Post-mesh summary: nodes, tets, refined / max sizes (in um).
         stats = result.mesh_stats or {}
         node_count = stats.get("nodes")
diff --git a/src/gsim/palace/mesh/geometry.py b/src/gsim/palace/mesh/geometry.py
index 43006e81..2d1dc668 100644
--- a/src/gsim/palace/mesh/geometry.py
+++ b/src/gsim/palace/mesh/geometry.py
@@ -28,6 +28,50 @@
 logger = logging.getLogger(__name__)
 
 
+def _contains_air_token(name: str | None) -> bool:
+    """Return True when *name* reads as an air/vacuum region name."""
+    if not name:
+        return False
+
+    normalized = name.strip().lower().replace("-", "_")
+    if normalized in {"air", "vacuum"}:
+        return True
+
+    tokens = [tok for tok in normalized.split("_") if tok]
+    return "air" in tokens or "vacuum" in tokens
+
+
+def is_air_like_material(
+    stack: LayerStack,
+    material_name: str,
+    dielectric_name: str | None = None,
+) -> bool:
+    """Return True when a stack material behaves as air/vacuum.
+
+    A material qualifies if it is a dielectric with permittivity 1, or if
+    either the material or region name contains an ``air``/``vacuum`` token.
+    """
+    mat = stack.materials.get(material_name)
+    if isinstance(mat, dict):
+        mat_type = str(mat.get("type", "")).strip().lower()
+        eps = mat.get("permittivity")
+    else:
+        mat_type = str(getattr(mat, "type", "")).strip().lower()
+        eps = getattr(mat, "permittivity", None)
+
+    if mat_type == "dielectric":
+        try:
+            if eps is not None and abs(float(eps) - 1.0) <= 1e-9:
+                return True
+        except (TypeError, ValueError):
+            pass
+
+    if _contains_air_token(material_name):
+        return True
+
+    return _contains_air_token(dielectric_name)
+
+
 @dataclass
 class GeometryData:
     """Container for geometry data extracted from component."""
@@ -93,41 +137,6 @@ def resolve_dielectric_regions(
     xmax_air = xmax0 + margin_x
     ymax_air = ymax0 + margin_y
 
-    def _contains_air_token(name: str | None) -> bool:
-        if not name:
-            return False
-
-        normalized = name.strip().lower().replace("-", "_")
-        if normalized in {"air", "vacuum"}:
-            return True
-
-        tokens = [tok for tok in normalized.split("_") if tok]
-        return "air" in tokens or "vacuum" in tokens
-
-    def _is_air_or_vacuum(
-        material_name: str,
-        dielectric_name: str | None = None,
-    ) -> bool:
-        mat = stack.materials.get(material_name)
-        if isinstance(mat, dict):
-            mat_type = str(mat.get("type", "")).strip().lower()
-            eps = mat.get("permittivity")
-        else:
-            mat_type = str(getattr(mat, "type", "")).strip().lower()
-            eps = getattr(mat, "permittivity", None)
-
-        if mat_type == "dielectric":
-            try:
-                if eps is not None and abs(float(eps) - 1.0) <= 1e-9:
-                    return True
-            except (TypeError, ValueError):
-                pass
-
-        if _contains_air_token(material_name):
-            return True
-
-        return _contains_air_token(dielectric_name)
-
     z_min_all = math.inf
     z_max_all = -math.inf
 
@@ -146,7 +155,9 @@ def _is_air_or_vacuum(
         dielectric_name = str(dielectric.get("name", "dielectric"))
         material = str(dielectric["material"])
 
-        is_air_like = _is_air_or_vacuum(material, dielectric_name=dielectric_name)
+        is_air_like = is_air_like_material(
+            stack, material, dielectric_name=dielectric_name
+        )
         if is_air_like and use_airbox:
             continue