Cable Impedance Calculator

A cable carrying a rapidly changing signal behaves as a transmission line rather than as a simple wire. Its conductors store magnetic energy through distributed inductance and electric energy through distributed capacitance. The ratio of these two properties determines characteristic impedance, the voltage-to-current relationship of a wave traveling along an infinitely long or perfectly terminated line. Characteristic impedance is measured in ohms, but it is not the same as the direct-current resistance measured across a cable. For an ideal lossless line, characteristic impedance equals the square root of inductance per unit length divided by capacitance per unit length. This calculator accepts inductance in microhenries per meter and capacitance in picofarads per meter, performs the required unit conversion, and returns impedance in ohms. It also calculates propagation delay as the square root of inductance multiplied by capacitance. With these convenient input units, that square root directly gives nanoseconds per meter. Signal velocity is one billion divided by delay in nanoseconds per meter. A common 50-ohm line might have approximately 0.25 microhenries per meter and 100 picofarads per meter. Those values produce a five-nanosecond-per-meter delay and a velocity near two hundred million meters per second, about two thirds of the speed of light in vacuum. Cable dielectric material and geometry set the distributed values. Coaxial diameter ratios, twisted-pair spacing, insulation permittivity, shields, nearby conductors, and manufacturing tolerances all matter. Impedance matching becomes important when cable length is significant relative to a signal’s rise time or wavelength. If source, line, and load impedances differ, part of the traveling wave reflects. Reflections can cause ringing, overshoot, reduced power transfer, data errors, or standing waves. Radio systems commonly use 50-ohm cable, broadcast video often uses 75 ohms, and many differential digital interfaces use controlled differential impedances. These standards are not interchangeable simply because connectors fit. The lossless equation is a useful design estimate, but real cables also have series resistance and shunt conductance that vary with frequency. Skin effect, dielectric loss, connector discontinuities, bends, temperature, and installation conditions alter behavior. For high-frequency or safety-critical work, use manufacturer data or measured network-analyzer results. This cable impedance calculator is best used for checking distributed parameters, comparing cable constructions, teaching transmission-line fundamentals, and obtaining an initial signal-integrity estimate before more complete frequency-dependent analysis.