How to model a lumped inductor in HFSS for printed PCB antennas

Use HFSS’s Lumped RLC element between two conductor terminals (via a tiny bridging sheet) to represent the inductor, align it with the intended current path, and include its series resistance and, if needed, frequency dependence.

What does “lumped inductor boundary” mean?

A discrete inductor on a PCB is modeled in HFSS as a two-terminal element that imposes V = jωL·I (plus ESR and any parasitic C). In HFSS 3D this is Assign Excitation > Lumped RLC on a small sheet connecting two metal pads; in HFSS 3D Layout it’s a Discrete RLC component between nets. The element is electrical only; the physical body is not modeled unless you explicitly draw it.

Geometry and terminal setup in HFSS 3D

Draw a rectangular sheet that bridges the two copper pads where the inductor sits. Make its length equal to the pad-to-pad gap (for example 0.2–0.5 mm for 0402 pads) and width no smaller than the pad width to keep current distribution realistic. Assign Lumped RLC to that sheet, pick the two opposite edges as Positive and Negative terminals such that the E-field arrow points along the intended current direction. Enter L in henries and include ESR as Rseries; leave C blank unless you want to approximate parasitics. If the inductor is to ground (shunt), one terminal must land on the ground pad or a ground via pad; do not rely on proximity—ensure electrical continuity by metal contact.

HFSS 3D Layout workflow

Import or build the PCB stackup, define nets, and place a Discrete Inductor between two pads (series or shunt). Attach vendor data if available: S2P of the inductor or a frequency-dependent RLC model. Co-simulate with HFSS via electromagnetic components for the radiator region if coupling is strong.

Step-by-step setup checklist

• Define stackup: dielectric thickness and loss tangent (e.g., FR-4 εr ≈ 4.2–4.6, tanδ ≈ 0.015–0.02 at 2.4 GHz; RF laminate εr ≈ 2.9–3.7, tanδ ≈ 0.001–0.004)

• Draw the antenna geometry: example microstrip IFA/monopole with feed and ground

• Add the lumped inductor: small sheet + Lumped RLC (3D) or Discrete RLC (3D Layout)

• Ports: microstrip feed as wave port at board edge, port size ≥ 3× trace width and ≥ 3× dielectric height; or a lumped port across feed trace and ground for local excitation

• Boundary: Radiation box or PML, at least 0.25–0.5 free-space wavelengths from all radiating edges; for PML, ≥ 0.16 wavelengths is typical

• Mesh: locally refine around the inductor sheet and feed; target element size ≤ 1/5 of the smallest pad dimension; ensure at least 2–3 layers through dielectric thickness if modeling finite thickness conductors

• Frequency sweep: discrete points around the band (e.g., 2.2–2.8 GHz), with a fast sweep for interpolation after an initial discrete solve

How should I pick L and ESR?

At 2.45 GHz, X_L = 2πfL gives X_L ≈ 105 Ω for 6.8 nH and ≈ 77 Ω for 5 nH. Practical 0402/0603 RF inductors show ESR of about 0.3–1.5 Ω and self-resonant frequencies of 4–10+ GHz depending on value and vendor. Start with the datasheet’s typical ESR at your band, include ±5–10% L tolerance, and if available, import the S2P so HFSS captures the self-resonance and Q(f) without guesswork.

How far should the radiation boundary be?

Place the radiation surface 0.25–0.5 wavelengths away in all directions from the antenna edges in the highest-permittivity path to avoid truncation. For a 2.45 GHz PCB antenna, that’s typically 30–60 mm in free space. If size is tight or the pattern is directive, use PML with ≥ 0.16 wavelengths spacing and a minimum of one PML cell thickness equal to 0.1–0.2 wavelengths.

Ports and grounding details that commonly break results

A wave port must see a complete cross-section of the guiding structure. If the microstrip’s reference plane is a partial ground, extend ground to the port plane and stitch with vias near the port. For lumped ports, reference them to a real ground metal, not to a radiation box. Avoid touching the radiation boundary with any metal; keep at least a few millimeters margin.

Substrate loss, copper roughness, and thickness matter

Loss tangent and surface roughness can shift bandwidth and efficiency by several percent. If operating above about 6 GHz or pushing miniature antennas, enable a roughness model (e.g., Huray or Hammerstad), use measured εr/tanδ at frequency, and include real copper thickness (e.g., 17 or 35 μm). For 50 Ω microstrip on 1.6 mm FR-4, the trace width is roughly 2.9–3.2 mm depending on εr; on 0.8 mm it’s roughly 1.5–1.7 mm.

Validating the model

Check S11 and input impedance versus the target band. Inspect current density on the inductor sheet to ensure it carries the intended current (no unintended shorts). Sweep L ±10% to see tuning sensitivity; a 5 nH step at 2.45 GHz shifts X_L by ~77 Ω, which can move resonance by tens of MHz in small antennas. Compare a model with ideal L+R to one using the vendor S2P; expect a narrower bandwidth with the real part due to finite Q.

From simulation to fabrication

When you are ready to build, we can manufacture boards to IPC Class 2 or Class 3 acceptance criteria and support RoHS and REACH compliance. Our quality management system is ISO 9001:2015 certified (certificate CN-2023-Q1156, SGS, valid to June 2027). Quotes are typically returned within 24 hours and DFM feedback within 1 business day. For RF substrates, our partner factory manufactures its own high-frequency laminates and supplies RF PCB fabricators in China, including publicly listed PCB companies.