Tutorial: Patch Antenna Design

Your First Patch Antenna 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.

Step	Goal	Where
1. Model	size the patch, grade the z mesh, declare foils as sheets, clear preflight	Your First Patch Antenna
2. Resonance	run until the ring-down settles, extract modes, identify the radiating one	Probes and S-Parameters
3. Return loss	replace the soft source with a port and compute S11	Sources and Ports
4. Radiation	add an NTFF box and compute pattern and directivity	Far-Field and RCS
5. Convergence	refine the mesh and report how much each number moves	Convergence Study
6. Design iteration	sweep or optimize the dimensions and substrate	Parametric Sweeps, Inverse Design

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 shows how rfx’s patch cases compare with openEMS.

• 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.
• 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.
• 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.

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.

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 and the S-parameter support matrix.

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.

• 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.

• 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.