coax calculator

What this coax calculator does

A coax cable is a transmission line. Its geometry and dielectric material determine how signals travel through it. This calculator helps you quickly compute practical values engineers care about: characteristic impedance, velocity factor, propagation delay, wavelength, and distributed capacitance/inductance.

If you work in RF, ham radio, instrumentation, SDR, broadcast, or high-speed digital testing, these numbers can save time when selecting cable, matching impedance, or trimming phase length.

Inputs explained

1) Inner conductor diameter (d)

This is the center conductor diameter. Use millimeters. The value must be positive and smaller than the shield inner diameter.

2) Outer conductor inner diameter (D)

This is the inside diameter of the shield/tube that surrounds the center conductor. Also in millimeters.

3) Relative permittivity (εr)

εr describes the dielectric between conductors (e.g., solid PE, PTFE, air-spaced foam). A higher εr slows signal velocity and changes impedance.

4) Frequency and physical length

Frequency gives wavelength-related outputs. Length is used to compute total delay, electrical length (degrees), and total distributed values.

Formulas used

• Characteristic impedance: Z₀ = (60 / √εr) ln(D / d)
• Capacitance per meter: C' = (2π ε₀ εr) / ln(D / d)
• Inductance per meter: L' = (μ₀ / 2π) ln(D / d)
• Velocity factor: VF = 1 / √εr
• Wave velocity: v = c · VF
• Wavelength: λ = v / f

These are idealized transmission-line equations. They are highly useful for design estimates and sanity checks.

How to use the results

Impedance matching

If your calculated impedance is near 50 Ω, the cable geometry is suitable for common RF systems. If it is near 75 Ω, it lines up with video, CATV, and some receive-only systems.

Delay and phase alignment

Propagation delay and electrical length are essential for phased arrays, time-domain measurements, and matching feed lines.

Reactive loading

Capacitance and inductance per meter help estimate how line length affects circuits, especially at lower frequencies where long cables can still influence tuning and stability.

Practical tips

• Use manufacturer datasheets when available; this calculator is best for first-pass design.
• Foam dielectrics vary by manufacturer and compression; εr can shift slightly.
• At high frequency, attenuation and connector quality matter as much as impedance.
• For precision phase work, include temperature effects and cable bend radius behavior.

Limitations to keep in mind

This tool assumes non-magnetic materials and a uniform coax geometry. It does not model skin-effect losses, conductor roughness, dielectric loss tangent, connector discontinuities, moisture ingress, or standing-wave behavior under mismatch conditions. For final production design, pair this estimate with measured VNA or TDR data.

Quick FAQ

Can I use inches instead of millimeters?

Yes, if both d and D use the same units, because the equations depend on the ratio D/d. The calculator labels mm for convenience.

Why does changing εr affect velocity?

Electromagnetic waves propagate slower in materials with higher permittivity. That directly lowers velocity factor.

Is this only for RF?

No. It also helps in pulse systems, instrumentation timing, and high-speed digital interconnect planning when coax is used.