Calculate transmission line impedance, propagation delay, wavelength, VSWR, reflection coefficient, and signal loss for PCB traces, coaxial cables, and RF applications.
Transmission line analysis is critical for high-frequency circuit design, signal integrity, and RF engineering. When signal wavelengths approach the physical length of interconnects, simple wire models fail — voltages and currents vary along the line, creating reflections, standing waves, and signal distortion. The Transmission Line Calculator handles impedance computation, propagation characteristics, and mismatch analysis for microstrip, stripline, and coaxial geometries.
For PCB designers, maintaining proper impedance (typically 50Ω single-ended or 100Ω differential) prevents signal reflections that corrupt high-speed data. Mismatched impedance creates standing waves characterized by VSWR (Voltage Standing Wave Ratio) — a VSWR of 1.0 is perfect; above 2.0 requires attention. This calculator computes VSWR, reflection coefficient (Γ), and return loss from any source/load impedance combination.
Whether you're designing RF circuits, PCB high-speed interconnects, antenna feedlines, or studying telecommunications, this calculator provides the transmission line parameters you need: characteristic impedance from geometry, propagation delay, electrical length, and complete mismatch analysis.
Use this calculator when you need to size or verify a line before the signal path becomes a problem. It is useful for PCB traces, coax, RF feedlines, and mismatch checks where reflections or delay matter. It helps you connect physical geometry to the electrical behavior you actually need to control.
Microstrip Z₀ ≈ 87/√(εr+1.41) × ln(5.98h/(0.8w+t)). Coaxial Z₀ = 138/√εr × log(D/d). VSWR = (1+|Γ|)/(1-|Γ|). Γ = (ZL-Z₀)/(ZL+Z₀). Return Loss = -20log|Γ| dB. Propagation delay = √εeff / c.
Result: Z₀ = 50.2Ω, VSWR = 1.49, Return Loss = 14.0 dB
A 0.3mm wide microstrip on 0.2mm FR-4 (εr=4.4) has ~50Ω impedance. With 75Ω load, VSWR is 1.49 and return loss is 14 dB — acceptable for many digital applications but poor for precision RF.
Microstrip traces run on the outer layer of a PCB with a ground plane below. They're easy to route and probe but have higher radiation, lower isolation, and slightly lower impedance accuracy because the electric field is partially in air. Effective dielectric constant is roughly (εr + 1)/2 for narrow traces.
Stripline traces are sandwiched between two ground planes. They offer better impedance control, lower crosstalk, and reduced radiation but are harder to probe and require vias for component connections. Stripline is preferred for high-speed buses and RF distribution networks.
A transmission line exactly λ/4 long has special properties: it transforms impedance as Z_in = Z₀²/Z_L. This makes quarter-wave sections powerful matching tools. A 61.2Ω quarter-wave section matches 50Ω to 75Ω (√(50×75) = 61.2). Quarter-wave stubs (open or short-circuited) act as resonant circuits and are widely used in filters and power dividers.
Modern digital signals with very fast rise times require meticulous impedance control. At high data rates, a PCB trace becomes electrically significant in just a few millimeters. Reflections from impedance discontinuities at vias, connectors, and trace geometry changes create inter-symbol interference and degrade bit error rates. Simulation tools model these effects, but this calculator provides quick sanity checks for impedance targets.
Characteristic impedance (Z₀) is the ratio of voltage to current for a wave traveling along the transmission line. It depends on the line's geometry and dielectric material, not its length. Common values: 50Ω (RF), 75Ω (video/CATV), 100Ω (differential PCB pairs).
VSWR (Voltage Standing Wave Ratio) measures impedance mismatch. VSWR = 1.0 means perfect match (no reflections). VSWR = 2.0 means 11% of power is reflected. Above 3.0, significant signal degradation occurs. For transmitting antennas, high VSWR can damage the transmitter.
Return loss is the ratio (in dB) of incident to reflected power. Higher is better: 20 dB means only 1% power reflected (VSWR ≈ 1.22). 10 dB means 10% reflected (VSWR ≈ 1.93). Most specifications require >15 dB return loss.
Any trace where the propagation delay exceeds about 1/6 of the signal rise time needs impedance control. For modern digital signals with 100-500ps rise times, this means traces longer than about 15-50mm may need controlled impedance.
FR-4: εr ≈ 4.2-4.5. Rogers 4350B: εr ≈ 3.48. PTFE/Teflon: εr ≈ 2.1. Polyethylene (coax): εr ≈ 2.25. The effective dielectric for microstrip is between 1 and εr because fields partially travel through air above the trace.
Common techniques: quarter-wave transformer (λ/4 section of Z = √(Z₁Z₂)), L-network (series + shunt reactive elements), stub matching (short/open transmission line stubs). For broadband matching, multi-section transformers or tapered lines are used.