Coaxial Line Calculator
Coaxial cable has a central conductor surrounded by a tubular outer conductor. Its characteristic impedance depends only on geometry and the dielectric. Uses closed-form equations for Z₀, phase velocity, capacitance, inductance, and cutoff frequency.
Equations & Parameters ▸
\(Z_0 = \dfrac{60}{\sqrt{\varepsilon_r}}\ln\!\dfrac{D}{d}\)
| d | Inner conductor diameter (mm). |
| D | Outer conductor (shield) inner diameter (mm). |
| εᵣ | Dielectric constant of the insulator. PE ≈ 2.25, PTFE ≈ 2.1, foam ≈ 1.5. |
| Z₀ | Characteristic impedance = (60/√εᵣ)·ln(D/d). |
| vₚ | Phase velocity = c/√εᵣ. Sets wavelengths inside the cable. |
| fₓ | Cutoff frequency of TE₁₁ mode — stay well below this. |
Physical constants used
| c | Speed of light = 2.998×10⁸ m/s |
| µ₀ | Permeability of free space = 4π×10⁻⁷ H/m ≈ 1.2566×10⁻⁶ H/m |
| ε₀ | Permittivity of free space = 8.854×10⁻¹² F/m |
Inputs
mm
Central conductormm
Shield inner diameterPE=2.25, PTFE=2.1, foam≈1.5
Results
Electrical
Characteristic impedance, Z₀—
Phase velocity, vₚ—
Cutoff frequency (TE₁₁)—
Per-unit-length
Capacitance, C—
Inductance, L—
Diagram