What is characteristic impedance?

Characteristic impedance (Z0) is the ratio of voltage to current in a transmission line carrying a wave in one direction only. For a lossless line it depends purely on geometry and the dielectric constant. Mismatches cause reflections.

What is a typical coaxial cable impedance?

The two most common standards are 50 Ω (used in RF/microwave and test equipment) and 75 Ω (used in cable TV and video). The calculator lets you verify dimensions that hit those targets.

How does the dielectric constant affect impedance?

A higher dielectric constant (εr) lowers characteristic impedance and slows the propagation velocity. Air (εr ≈ 1) gives the highest speed; PTFE (εr ≈ 2.1) and FR4 (εr ≈ 4.5) are common solid dielectrics.

What is the velocity factor?

Velocity factor (VF) is the ratio of the wave speed on the line to the speed of light in vacuum. VF = 1 / √εr for TEM lines. A VF of 0.66 means the signal travels at 66% of c.

What microstrip formula does this use?

For narrow traces (W/h < 1) the tool uses the standard closed-form approximation by Wheeler; for wide traces (W/h ≥ 1) it uses the wide-strip form. Effective permittivity accounts for partial field in air above the substrate.

Why is my microstrip impedance so different from the coaxial result?

Microstrip fields are split between substrate and air above, giving an effective εr lower than the bulk substrate value. Coaxial fields are entirely within the dielectric, so the full εr applies.

Transmission Line Parameters

Name: Transmission Line Parameters
Availability: InStock
Author: Nham Vu

Calculate characteristic impedance, propagation velocity, and wavelength for coaxial, microstrip, and two-wire transmission lines.

Line Type

Inner conductor diameter, d (mm)

Inner diameter of outer conductor, D (mm)

Relative permittivity, εr

Air = 1.0 · PTFE ≈ 2.1 · PE ≈ 2.3 · FR4 ≈ 4.5

Frequency, f (MHz) — for wavelength

Select a line type, enter dimensions, and click Calculate.

Summary

Calculate characteristic impedance, propagation velocity, and wavelength for coaxial, microstrip, and two-wire transmission lines.

How it works

Select the transmission line type: coaxial, microstrip, or two-wire.
Enter the physical dimensions in millimeters.
Enter the relative permittivity (dielectric constant) of the insulating material.
Click Calculate to see characteristic impedance Z0, propagation velocity, and wavelength.
Adjust any input and recalculate to compare different configurations.

Use cases

Design coaxial cables for RF and microwave circuits.
Calculate microstrip trace impedance for PCB layout.
Verify transmission line matching in antenna feed networks.
Determine signal propagation delay in high-speed digital interconnects.
Compute guided wavelength for quarter-wave transformers.
Cross-check cable specs against manufacturer datasheets.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu