Coaxial Cable Impedance Calculator

Coaxial Cable Impedance Formula:

\[ Z_0 = \frac{138}{\sqrt{\epsilon_r}} \times \log_{10}\left(\frac{D}{d}\right) \]

Relative Permittivity (ε_r):

dimensionless

Outer Conductor Diameter (D):

meters

Inner Conductor Diameter (d):

meters

Characteristic Impedance (Z₀):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Coaxial Cable Impedance?

The characteristic impedance (Z₀) of a coaxial cable is a crucial parameter that determines how electrical signals propagate through the cable. It depends on the cable's physical dimensions and the dielectric material between the conductors.

2. How Does the Calculator Work?

The calculator uses the coaxial impedance formula:

\[ Z_0 = \frac{138}{\sqrt{\epsilon_r}} \times \log_{10}\left(\frac{D}{d}\right) \]

Where:

\( Z_0 \) — Characteristic impedance (Ω)
\( \epsilon_r \) — Relative permittivity of the dielectric material
\( D \) — Inner diameter of the outer conductor (m)
\( d \) — Outer diameter of the inner conductor (m)

Explanation: The equation shows that impedance increases with the ratio of outer to inner conductor diameters and decreases with higher dielectric constants.

3. Importance of Impedance Calculation

Details: Proper impedance matching is essential for minimizing signal reflections and maximizing power transfer in RF systems. Common values are 50Ω (general RF) and 75Ω (video/cable TV).

4. Using the Calculator

Tips: Enter the relative permittivity of the dielectric material and the diameters of both conductors in meters. Ensure the outer diameter is larger than the inner diameter.

5. Frequently Asked Questions (FAQ)

Q1: What are typical values for relative permittivity?
A: Common values: Air (1.0), PTFE (2.1), Polyethylene (2.25), PVC (3.0-4.0).

Q2: Why is 50Ω impedance common in RF systems?
A: 50Ω represents a compromise between power handling (lower impedance) and signal loss (higher impedance) for air-dielectric coaxial lines.

Q3: What happens if there's an impedance mismatch?
A: Mismatches cause signal reflections, leading to standing waves, reduced power transfer, and potential equipment damage.

Q4: Can this calculator be used for other transmission lines?
A: No, this formula is specific to coaxial cables. Other transmission lines (twisted pair, microstrip) have different impedance formulas.

Q5: How does frequency affect impedance?
A: At high frequencies, skin effect and dielectric losses become significant, but the characteristic impedance remains relatively constant.