Cable Impedance Calculator

Characteristic impedance is the ratio of voltage to current in a transmission line under matched conditions, measured in ohms. It is determined by the cable's physical geometry and the dielectric properties of its insulation, not by its length. Matching the cable impedance to the source and load impedance minimizes signal reflections and maximizes power transfer.

The characteristic impedance of a coaxial cable is calculated using: Z = (60 / √εr) × ln(D/d), where D is the inner diameter of the outer conductor, d is the outer diameter of the inner conductor, and εr is the relative dielectric constant of the insulating material between the conductors.

For a twisted-pair cable, the characteristic impedance is: Z = (120 / √εr) × acosh(s/d) ≈ (120 / √εr) × ln(2s/d) when s >> d, where s is the center-to-center separation between the wires and d is the wire diameter. The dielectric constant εr accounts for the insulation surrounding the wires.

The most common coaxial cable impedances are 50 Ω (used in RF and microwave applications, test equipment, and most antennas) and 75 Ω (used in cable TV, satellite, and video distribution systems). 93 Ω cables exist for low-capacitance digital applications, though they are rare.

Common dielectric constants include: air or vacuum = 1.0, PTFE (Teflon) = 2.1, solid polyethylene = 2.25–2.35, PVC = 3.0–4.5, and foam polyethylene ≈ 1.4–1.6. The insulation material significantly affects both impedance and signal propagation speed, so using the correct value is important for accurate results.

When a cable's characteristic impedance does not match the impedance of the connected devices, signal reflections occur at the interface. These reflections cause standing waves, signal loss, ringing, and in high-frequency systems, significant degradation of data integrity. Proper impedance matching ensures maximum power transfer and clean signal transmission.

Higher dielectric constants reduce the characteristic impedance of the cable for a given set of conductor dimensions. A higher εr also slows the propagation velocity of signals through the cable (propagation delay increases as √εr) and increases the capacitance per unit length. Using air or foam dielectrics keeps impedance higher and signal speed closer to the speed of light.

Propagation delay is the time it takes for a signal to travel a unit length along the cable, expressed in nanoseconds per meter (ns/m). It equals √(L × C) where L and C are inductance and capacitance per unit length. In high-speed digital systems and precision timing applications, propagation delay affects signal synchronization and must be accounted for in system design.