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name: matlab-analyze-em description: "S-parameters, insertion loss, fields, currents, mesh control, and solver selection for RF PCB performance validation. TRIGGER: user asks to compute S-parameters, analyze insertion/return loss, extract fields or currents, compare MoM vs FEM, or control mesh for any RF PCB component. Invoke BEFORE writing sparameters() or solver code — API is non-obvious. SKIP: designing or creating components (use the specific matlab-design-pcb-* skill), material/stackup setup only (use matlab-manage-pcb-material), optimization sweeps (use matlab-optimize-pcb-design), PDN/IR-drop analysis (use matlab-analyze-pcb-pdn)." license: https://www.mathworks.com/content/dam/mathworks/license/pmrl/license.md metadata: author: MathWorks version: "1.0"

Analyzing EM Performance of RF PCB Components

When to Use

• Extracting S-parameters from any RF PCB component (catalog or custom pcbComponent)
• Comparing MoM vs FEM solvers or selecting the right solver for a structure
• Using interpolating sweeps or frequencySweep objects for faster multi-frequency analysis
• Controlling mesh density for accuracy vs speed tradeoffs
• Visualizing E/H fields, surface currents, or charge distributions
• Using behavioral (analytic) S-parameter models for fast estimates or optimization

When NOT to Use

• Building or assembling custom PCB structures — use matlab-assemble-pcb-layout
• Designing standard transmission lines or catalog objects — use matlab-design-pcb-transmission-line
• Defining dielectric or metal materials — use matlab-manage-pcb-material
• Cascading or connecting multiple components into circuits — use matlab-integrate-pcb-circuit
• Importing PCB layouts from Gerber, ODB++, or Allegro — use matlab-read-pcb-layout

Typical Workflow

1. Before: matlab-manage-pcb-material — substrate/conductor setup; then a design skill or matlab-assemble-pcb-layout — create the component
2. This skill: Extract S-parameters, visualize fields and currents, verify performance
3. After: matlab-optimize-pcb-design — tune dimensions if specs not met → matlab-integrate-pcb-circuit — cascade into larger network → matlab-write-pcb-layout — export Gerber

Quick Reference

| Task | Code | |------|------| | S-parameters (MoM) | sp = sparameters(obj, freq, 'SweepOption', 'interp') | | S-params with port Z0 | sp = sparameters(obj, freq, 50, 'SweepOption', 'interp') | | Interpolating sweep | sp = sparameters(obj, freq, 50, 'SweepOption', 'interp') | | Interp with gradient | sp = sparameters(obj, freq, 50, 'SweepOption', 'interpWithGrad') | | Plot S-params | rfplot(sp) or rfplot(sp, [2 1], 1) | | Current distribution | current(obj, fc) | | Charge distribution | charge(obj, fc) | | Feed current | feedCurrent(obj, freq) | | E/H fields | [e, h] = EHfields(obj, fc, points) | | Set mesh | mesh(obj, 'MaxEdgeLength', val) | | Memory estimate | memoryEstimate(obj, fc) | | Switch to FEM solver | pcb.SolverType = 'FEM' | | FEM boundary condition | s = solver(pcb); s.BoundaryCondition = 'absorbing' | | Frequency sweep object | fsweep = frequencySweep; sp = sparameters(obj, freq, 'SweepOption', fsweep) | | Rational model from sweep | rmodel = getRationalModel(fsweep) | | Discover methods | methods(obj) |

S-Parameter Extraction

The sparameters function is the primary analysis method for all RF PCB components.

Basic Usage

obj = design(couplerBranchline, 5e9);
freq = linspace(1e9, 10e9, 101);
sp = sparameters(obj, freq, 'SweepOption', 'interp');
figure;
rfplot(sp);

Specifying Port Impedance

sp = sparameters(obj, freq, 50, 'SweepOption', 'interp');   % 50-ohm reference
sp = sparameters(obj, freq, 75, 'SweepOption', 'interp');   % 75-ohm reference

Plotting Specific S-Parameters

rfplot(sp, [2 1], 1);              % Plot S21 only
rfplot(sp, [1 1], 1);              % Plot S11 only
rfplot(sp, 2:4, 1);               % Plot S21, S31, S41 vs port 1

Extracting Numeric Data

sp = sparameters(obj, freq, 'SweepOption', 'interp');
S21_dB = 20*log10(abs(squeeze(sp.Parameters(2,1,:))));
S11_dB = 20*log10(abs(squeeze(sp.Parameters(1,1,:))));

Solver Selection

RF PCB Toolbox supports two electromagnetic solvers:

| Solver | Property Value | Best For | |--------|---------------|----------| | Method of Moments (MoM) | 'MoM' (default) | Planar structures, open radiators | | Finite Element Method (FEM) | 'FEM' | Shielded catalog elements; also available on pcbComponent via SolverType |

Switching to FEM

FEM is available via pcbComponent:

pcb = pcbComponent(couplerBranchline);
pcb.SolverType = 'FEM';
sp_fem = sparameters(pcb, freq);

FEM Boundary Condition Configuration

After setting SolverType to 'FEM', retrieve the solver object via solver() to configure boundary conditions:

pcb = pcbComponent(catalogObj);
pcb.SolverType = 'FEM';

s = solver(pcb);                          % Returns em.solvers.fem.FEM object
s.BoundaryCondition = 'absorbing';        % or 'perfectly-matched-layer' (default)

| Boundary Condition | Value | Use Case | |--------------------|-------|----------| | Perfectly Matched Layer (PML) | 'perfectly-matched-layer' (default) | Open radiating structures, antennas | | Absorbing | 'absorbing' | Shielded enclosures, waveguide ports |

Gotcha: solver(comp, 'SolverType', 'FEM') errors with "Too many input arguments." SolverType is a property of pcbComponent, not an argument to solver(). BoundaryCondition is a property of the returned FEM solver object, not of the component.

FEM Prerequisites

The FEM solver (introduced R2025a) requires two dependencies:

1. Integro-Differential Modeling Framework for MATLAB (IDMF) — Install via Home > Add-Ons > search "Integro-Differential Modeling Framework for MATLAB". Verify with matlab.addons.installedAddons.
2. Windows Subsystem for Linux (WSL) — Required on Windows. Install via wsl --install from an elevated PowerShell prompt. Verify with wsl --status.

If WSL is available, the FEM solver can be used when designing custom structures via pcbComponent. For shielded catalog elements, FEM is used automatically.

Firewall note: Windows Defender may block the PostgreSQL server used by IDMF (<matlabroot>\sys\postgresql\win64\PostgreSQL\bin\postgres.exe). If FEM solves hang on first use, inform the user of this potential cause and defer to them on what action to take per their IT/security policies. Do not modify firewall settings autonomously.

WSL Memory Tuning

WSL is allocated only 50% of system RAM by default. Large FEM problems may fail with out-of-memory errors. If the user hits OOM during an FEM solve, inform them that WSL memory can be increased by editing C:/Users/%UserProfile%/.wslconfig:

[wsl2]
memory=48GB
swap=8GB

Followed by wsl --shutdown and restart-service LxssManager (elevated PowerShell). Values should be adjusted based on system specs. This may require IT involvement — ask the user to make this change manually and resume when ready. Do not create or modify .wslconfig autonomously.

FEM-Only Properties

When SolverType='FEM', additional properties become available on pcbComponent:

| Property | Purpose | |----------|---------| | Connector | Attach an RFConnector object for coaxial feed modeling (default: 50-ohm, InnerRadius=0.5mm, OuterRadius=1.5mm) | | IsShielded | Add metal shielding box around the structure (dimensions match ground plane) |

FEM Constraints

• No mode impedance extraction: getZEven/getZOdd are not available with the FEM solver. Use S-parameters only for shielded comparisons.
• PEC required: FEM requires Conductivity=Inf (PEC). Finite conductivity metals (e.g., Copper) will error. For shielded vs unshielded comparisons, use PEC for both.
• Connector spacing: The RFConnector outer radius (default 1.5 mm) must fit between adjacent ports. If port spacing is tight, increase Spacing/GroundPlaneWidth or reduce OuterRadius on the connector.

Comparing Solvers

obj = design(couplerBranchline, 5e9);
freq = linspace(1e9, 5e9, 21);

sp_mom = sparameters(obj, freq, 'SweepOption', 'interp');

pcb = pcbComponent(obj);
pcb.SolverType = 'FEM';
sp_fem = sparameters(pcb, freq);  % FEM: interp not applicable

figure;
rfplot(sp_mom); hold on;
rfplot(sp_fem, '--');
legend('MoM', 'FEM');

Interpolating Sweep

For faster multi-frequency analysis, use interpolating sweep instead of discrete point-by-point solves. This is significantly faster, especially for large structures.

Basic Interpolation

freq = linspace(4.5e9, 5.5e9, 101);
sp = sparameters(obj, freq, 50, 'SweepOption', 'interp');

Interpolation with Gradient

More accurate interpolation using gradient information:

freq = [4.5e9, 5.5e9];  % Only need start/end — solver picks internal points
sp = sparameters(obj, freq, 50, 'SweepOption', 'interpWithGrad');

When to Use Interpolation

| Scenario | Recommendation | |----------|---------------| | Narrowband (< 2:1 BW) | 'interpWithGrad' — fastest, accurate | | Wideband (> 2:1 BW) | 'interp' — stable over wide range | | Debugging / single freq | No sweep option (discrete) | | Resonant structures | Discrete or fine 'interp' grid |

frequencySweep Object (R2025a)

For finer control over interpolation-based sweeps, use the frequencySweep object. It exposes error tolerance, iteration limits, and rational fitting — useful when the default 'SweepOption' settings are not sufficient.

fsweep = frequencySweep;
fsweep.SweepType = "interp";        % "interp" (default) | "interpWithGrad"
fsweep.ErrTol = -80;                % dB, default -80
fsweep.NumFreqs = 100;              % points to discretize frequency range, default 100
fsweep.NumIters = 25;               % max fitting iterations, default 25

freq = linspace(1e9, 10e9, 200);
sp = sparameters(comp, freq, 'SweepOption', fsweep);

% Extract rational fitting model after the sweep
rmodel = getRationalModel(fsweep);

| Property | Default | Description | |----------|---------|-------------| | SweepType | "interp" | Interpolation type; "interpWithGrad" uses gradient info | | ErrTol | -80 dB | Max error tolerance between fitting iterations | | NumFreqs | 100 | Number of points to discretize frequency range | | NumIters | 25 | Maximum number of fitting iterations |

Mesh Control

Mesh density directly affects accuracy and computation time.

Setting Maximum Edge Length

Rule of thumb: MaxEdgeLength ≤ λ/8 at the highest frequency.

fc = 10e9;
lambda = 3e8 / fc;
mesh(obj, 'MaxEdgeLength', lambda/8);

Viewing the Mesh

figure;
mesh(obj);                              % Visualize default mesh
figure;
mesh(obj, 'MaxEdgeLength', 1e-3);       % Visualize refined mesh

Mesh Configuration

Switch between automatic and manual meshing:

meshconfig(obj, 'manual');
mesh(obj, 'MaxEdgeLength', 0.5e-3, 'MinEdgeLength', 0.1e-3);

meshconfig(obj, 'auto');   % Revert to automatic meshing

Pre-Solve Checkpoint: Inspect Mesh and Memory

Catalog components generate dense auto-meshes that can dominate runtime even with interpolating sweep. Always inspect before committing to a full solve:

fc = 10e9;
memoryEstimate(obj, fc, 'RetainMesh', true);  % Estimate RAM; retain mesh for inspection
mesh(obj);                                     % Visualize — check if overly dense

% If too dense or memory too high, coarsen
lambda = physconst('LightSpeed') / fc;
mesh(obj, 'MaxEdgeLength', lambda/6);          % Relax from default
memoryEstimate(obj, fc, 'RetainMesh', true);   % Re-check after coarsening

The 'RetainMesh', true option keeps the generated mesh attached to the object so you can visualize it immediately. Without it, the mesh is discarded af