Cable Impedance Calculator
Calculate the characteristic impedance of a transmission line or cable from its inductance and capacitance per unit length.
Inductance per unit length
Capacitance per unit length
How to Use This Calculator
- Enter the inductance per unit length (H/m or nH/m) from cable specifications or measurements.
- Enter the capacitance per unit length (F/m or pF/m) from cable specifications or measurements.
- The calculator displays the characteristic impedance in ohms (Ω).
- Use this to verify cable specifications, design transmission lines, or understand impedance matching.
Cable Impedance Formula
Characteristic impedance is calculated from inductance and capacitance:
Where Z₀ = characteristic impedance (Ω), L = inductance per unit length (H/m), C = capacitance per unit length (F/m). Example: L = 250 nH/m (0.00000025 H/m), C = 100 pF/m (0.0000000001 F/m): Z₀ = √(0.00000025 / 0.0000000001) = √2500 = 50Ω.
Full Description
Characteristic impedance is a fundamental property of transmission lines (cables, PCB traces, etc.) that determines how signals propagate. It's the impedance that an infinitely long transmission line would present, and it's determined by the line's geometry and materials, not its length. Understanding cable impedance is essential for proper signal transmission, impedance matching, and preventing reflections.
Common cable impedances include 50Ω for coaxial cables (RF, test equipment), 75Ω for video and cable TV, 100Ω for twisted pair Ethernet, and 120Ω for RS-485. The impedance depends on the ratio of inductance to capacitance, which is determined by conductor size, spacing, and dielectric material. Impedance matching (matching source, cable, and load impedances) is crucial for maximum power transfer and minimal reflections.
This calculator helps you determine cable impedance from inductance and capacitance measurements or specifications. Enter the values per unit length, and it calculates the characteristic impedance. Use it when designing transmission lines, verifying cable specifications, or understanding impedance matching. The impedance is independent of cable length but depends on the cable's physical construction.
Frequently Asked Questions
What is characteristic impedance?
Characteristic impedance (Z0) is the impedance of a transmission line when it's infinitely long. It's determined by the line's geometry and materials, not its length. Common values: 50Ω (coaxial), 75Ω (video/RF), 100Ω (twisted pair), 120Ω (RS-485).
How is cable impedance calculated?
Z0 = √(L/C), where L is inductance per unit length and C is capacitance per unit length. This formula applies to transmission lines like coaxial cables, twisted pairs, and PCB traces. The impedance depends on conductor geometry and dielectric material.
Why is impedance matching important?
Impedance matching prevents signal reflections that cause distortion, loss, and interference. When source, cable, and load impedances match, maximum power is transferred and reflections are minimized. Mismatched impedances cause standing waves and signal degradation.
What affects cable impedance?
Impedance depends on: conductor diameter, spacing between conductors, dielectric material (and its permittivity), and geometry (coaxial, twisted pair, microstrip, etc.). Changing any of these changes the impedance. Impedance is independent of cable length.