Cable Impedance Calculator

Calculate the characteristic impedance of cables based on their dimensions, materials, and electrical properties.

Calculate Your Cable Impedance Calculator

What is Cable Impedance?

Cable impedance, also known as characteristic impedance (Z₀), is the ratio of voltage to current in a transmission line when an electromagnetic wave travels through it. It's a fundamental property of transmission lines that affects signal integrity, power transfer efficiency, and overall system performance in high-frequency applications.

Why Cable Impedance Matters

Proper impedance matching is crucial in various applications:

Signal Integrity: Prevents signal reflections, ensuring clean transmission of data
Power Transfer: Maximizes the power transfer from source to load
Reduced Noise: Minimizes electromagnetic interference and crosstalk
System Performance: Ensures optimal operation of RF, audio, video, and data systems

Cable Impedance Formulas

The impedance formula varies depending on the type of transmission line:

Coaxial Cable:

Z₀ = (138 / √εᵣ) × log₁₀(D/d)

Where:
Z₀ = Characteristic impedance (ohms)
εᵣ = Relative permittivity of the dielectric
D = Inner diameter of the outer conductor
d = Outer diameter of the inner conductor

Twin-Lead / Parallel Wire:

Z₀ = (276 / √εᵣ) × log₁₀(2s/d)

Where:
Z₀ = Characteristic impedance (ohms)
εᵣ = Relative permittivity of the surrounding medium
s = Center-to-center distance between conductors
d = Conductor diameter

Microstrip Line:

Z₀ = (87 / √(εᵣ + 1.41)) × ln(5.98h / (0.8w + t))

Where:
Z₀ = Characteristic impedance (ohms)
εᵣ = Relative permittivity of the substrate
h = Substrate height
w = Trace width
t = Trace thickness

Standard Impedance Values

50Ω: RF transmission lines, test equipment
75Ω: Video, cable TV, antenna systems
100Ω: Twisted pair, ethernet cables
110-120Ω: Differential signaling, CAN bus
300Ω: TV antenna twin-lead

Factors Affecting Impedance

Conductor Dimensions: Diameter, spacing, geometry
Dielectric Material: Permittivity (εᵣ)
Frequency: Some materials change properties at high frequencies
Temperature: Affects both conductors and dielectrics

Impedance Matching

When two systems with different impedances are connected, some of the signal energy is reflected rather than transmitted. This reflection can cause:

Standing waves and resonances
Signal distortion
Reduced power transfer
Increased noise

Impedance matching networks can be used to minimize these reflections, ensuring optimal signal transfer between systems with different impedances.

Applications and Best Practices

RF Systems

For radio frequency applications, maintain consistent 50Ω or 75Ω impedance throughout the signal path. Even small discontinuities can cause reflections at high frequencies.

Digital Signal Integrity

In high-speed digital systems, treat traces as transmission lines when their length exceeds approximately 1/10 of the signal's wavelength. Use termination resistors equal to the characteristic impedance to prevent reflections.

PCB Design

Control impedance in PCB traces by adjusting trace width, substrate height, and dielectric constant. Modern PCB design tools include impedance calculators to help maintain consistent impedance.

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Frequently Asked Questions

Cable impedance is the opposition a cable presents to alternating current flow. It's important because impedance mismatches cause signal reflections, resulting in data loss, slower transmission speeds, and potential system failure in high-frequency applications. Proper impedance matching ensures efficient power transfer and signal integrity.

Several factors affect cable impedance: conductor diameter, dielectric material (insulation) and its thickness, shield design, conductor spacing, and operating frequency. Our calculator considers these variables to determine the characteristic impedance of different cable types.

Characteristic impedance (Z₀) is the ratio of voltage to current in a transmission line, assuming the line is infinitely long. It's determined by the cable's physical properties rather than its length. Common values include 50Ω for RF applications, 75Ω for video, and 100Ω, 110Ω or 120Ω for digital data.

Choose impedance based on your application: 50Ω for RF transmitters, test equipment, and data acquisition; 75Ω for video, CATV, and antenna systems; 100-120Ω for digital signals like Ethernet. Always match cable impedance to both source and load impedances for optimal performance.

Impedance mismatches cause signal reflections, resulting in standing waves that reduce power transfer efficiency. In digital systems, this causes errors, jitter, and degraded signal integrity. In analog systems, mismatches distort the signal and reduce the signal-to-noise ratio. Always match impedances in high-frequency or high-speed applications.

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