# Tutorial: Patch Antenna Design

Channel: development; package: 2.0.0rc1; source: 2c32b7cf79b796fae825bfa08fc73d9914e61723

Page: https://remilab.ai/rfx/examples/patch-antenna/
Source: https://github.com/bk-squared/rfx/blob/2c32b7cf79b796fae825bfa08fc73d9914e61723/docs/public/examples/patch-antenna.mdx

[Your First Patch Antenna](https://remilab.ai/rfx/examples/first-patch/) builds a patch model that
passes preflight and runs. This page is the roadmap from there to numbers you
can report (resonance, return loss, radiation pattern) and then to design
iteration. Each step links to the page that covers it in detail.

## The workflow

| Step | Goal | Where |
|---|---|---|
| 1. Model | size the patch, grade the z mesh, declare foils as sheets, clear preflight | [Your First Patch Antenna](https://remilab.ai/rfx/examples/first-patch/) |
| 2. Resonance | run until the ring-down settles, extract modes, identify the radiating one | [Probes and S-Parameters](https://remilab.ai/rfx/measure/s-parameters/) |
| 3. Return loss | replace the soft source with a port and compute S11 | [Sources and Ports](https://remilab.ai/rfx/model/ports/) |
| 4. Radiation | add an NTFF box and compute pattern and directivity | [Far-Field and RCS](https://remilab.ai/rfx/measure/far-field-and-rcs/) |
| 5. Convergence | refine the mesh and report how much each number moves | [Convergence Study](https://remilab.ai/rfx/evidence/mesh-convergence/) |
| 6. Design iteration | sweep or optimize the dimensions and substrate | [Parametric Sweeps](https://remilab.ai/rfx/design/sweeps/), [Inverse Design](https://remilab.ai/rfx/design/optimization/) |

`examples/tutorials/patch_antenna_demo.py` runs steps 1, 2 and 4 end to end on
the openEMS "Simple Patch Antenna" geometry, including far-field mode
identification. [Benchmarks](https://remilab.ai/rfx/guide/benchmarks/) shows how rfx's patch
cases compare with openEMS.

## Modelling rules

- **Finite ground, open domain.** Draw the ground plane as a conductor inside an
  all-CPML domain. A PEC boundary wall is infinite and closes the structure into
  a cavity, which moves the resonance.
- **Foil is a sheet.** Declare copper foil (ground and patch) with
  `add_thin_conductor()` on a zero-thickness `Box`. Use a thick `Box` of metal
  only when the metal really is that thick. See
  [Materials and Geometry](https://remilab.ai/rfx/model/materials/).
- **Resolve the substrate.** A thin substrate needs several cells through its
  thickness. Use a graded `dz_profile` rather than refining the whole domain,
  and put the substrate faces on cell boundaries. If you build the profile with
  `smooth_grading`, derive every z coordinate from the final profile: smoothing
  inserts transition cells that move the fine band. See
  [Non-Uniform Mesh](https://remilab.ai/rfx/model/domain-mesh/).
- **Keep the drawn size.** Preflight warns when patch edges fall between grid
  planes. Refine the mesh rather than rounding L and W to the grid, because
  rounding changes the antenna.

## Resonance

A patch rings down slowly, so run for many periods and check
`result.settling_db` against a -40 dB bar before you quote a frequency. With a
soft current source feeding the patch, `run(until_decay=...)` may never stop:
the source leaves a static charge between patch and ground that neither
radiates nor decays. Use a fixed `num_periods` and read the settling level
instead.

`result.find_resonances(freq_range=...)` returns several modes. A rectangular
patch has at least two low-order modes (one along each side). Identify the
radiating design mode by its far field: a broadside beam and the peak of the
radiated power. Don't pick it by amplitude, or by which mode is closest to the
formula estimate. Both have picked the wrong mode on real patches.

## Return loss

A soft source has no impedance, so it cannot give S11. Feed the patch with a
port instead:

- a **wire port** (`add_port(..., extent=...)`) spanning the substrate, for a
  probe feed;
- a **microstrip port** (`add_msl_port`), for an edge or inset feed.

For a lumped or wire port, use about 60 cells per wavelength at the highest
frequency of interest for one-port S11. At that resolution the magnitude on
resistive loads is within 0.54 dB of closed form. For a microstrip port,
the substrate thickness sets the cell (at least 4 cells through it; preflight
warns below that). See
[Sources and Ports](https://remilab.ai/rfx/model/ports/) and the
[S-parameter support matrix](https://github.com/bk-squared/rfx/blob/2c32b7cf79b796fae825bfa08fc73d9914e61723/docs/guides/sparameter_support_matrix.md).

## Radiation

Add a near-to-far-field box with `sim.add_ntff_box(...)` around the antenna,
with each face about half a wavelength from the radiator where the domain
allows. The face below the ground plane often cannot be that far; preflight
tells you, and you should report it with the result. Evaluate the pattern at
the radiating mode's frequency. See [Far-Field and RCS](https://remilab.ai/rfx/measure/far-field-and-rcs/).

## Design iteration

- **A few dimensions** (length, width, inset depth): sweep them with
  `parametric_sweep` or a `ParameterSweep` grid. Each case is a full run, so
  start coarse and refine the best region.
- **Material or shape regions**: gradient optimization through `forward()` and
  `optimize()`. Use an S11 objective from a port (`port_s11_freqs=`), check the
  gradient first, and verify the final design with steps 2–5 on a converged
  mesh. See [Inverse Design](https://remilab.ai/rfx/design/optimization/).

## Limits

- **A short run, or a clean preflight, is not a result.** Resonance, S11, Q and
  pattern all need a settled record and a mesh study.
- **Coarse meshes shift the resonance, in either direction.** An under-resolved
  substrate tends to read high and a staircased patch edge low, so the error on
  a coarse mesh is not a fixed bias. Refine both and report the spread.
- **Soft-source runs give no S11 and may never meet `until_decay`.** Use a port
  for return loss and a fixed record length for ring-down.
- **The wrong mode is easy to pick.** Confirm the radiating mode from the far
  field.