FDTD Transmission Line Simulator
Full-wave 3D FDTD simulation of microstrip stubs, coupled-line filters, via transitions, and step discontinuities using openEMS. Get S-parameters, Smith chart, and impedance analysis.
How It Works
FDTD (Finite-Difference Time-Domain) solves Maxwell's curl equations directly on a 3D Yee mesh by time-stepping electric and magnetic field components. A Gaussian pulse excitation covers the entire frequency band in a single simulation run, and S-parameters are extracted from port voltages and currents via Fourier transforms.
This tool uses the openEMS open-source FDTD solver with CSXCAD geometry. When the solver is unavailable, an analytical fallback using transmission-line ABCD matrix models provides immediate S-parameter results — useful for first-pass design even without a full 3D simulation.
Four structure types are supported:
- Microstrip Open Stub — A parallel open-circuit stub produces a frequency-selective reflection notch at the quarter-wave resonance of the stub.
- Coupled-Line Filter Section — Edge-coupled microstrip lines with even/odd mode impedances form a bandpass section.
- Through-Via Transition — A cylindrical via modelled as series inductance plus shunt pad capacitance; critical for signal integrity above a few GHz.
- Step Discontinuity — A change in trace width creates fringing capacitance and a partial reflection; quantified by the mismatch S11.
Related Calculators
FAQ
How long does a simulation take?+
Coarse mesh ~1 min, normal mesh ~2.5 min, fine mesh ~5 min. Fine mesh is available to Pro users.
What is the analytical fallback?+
If the openEMS FDTD engine is unavailable, the tool uses transmission-line ABCD matrix models. Results are less accurate for 3D discontinuity effects but are useful for initial design and are computed in under a second.
Which substrate should I choose?+
FR-4 is the standard low-cost PCB substrate (εr ≈ 4.4, tan δ ≈ 0.02). Rogers 4350B and Rogers 3003 are high-frequency laminates with tighter tolerances and lower loss — preferred for designs above 5 GHz.
Why does the stub notch not appear at the expected frequency?+
The notch frequency depends on the effective dielectric constant, not just the physical length. The Hammerstad-Jensen model automatically accounts for the microstrip fringing field, but you may need to reduce stub length slightly to hit a precise target due to the T-junction effect.