Coax Cable Calculator

Coax Cable Calculator:

Enter the values of Outer Conductor Diameter, D_(mm), Inner Conductor Diameter, d_(mm), and Relative Permittivity, pr, to determine the value of Coaxial Cable Impedance, Z_(Ω).

Enter Outer Conductor Diameter:		mm
Enter Inner Conductor Diameter:		mm
Enter Relative Permittivity:		mm
Result – Coaxial Cable Impedance:		Ω

Coax Cable:

Coaxial cable impedance refers to the opposition that a coaxial transmission line presents to the flow of alternating current (AC), determined by the geometric and material properties of its inner conductor, insulating dielectric, and surrounding outer shield.

It determines how signals travel through the cable and how well they are transmitted with minimal loss. Impedance is important for ensuring maximum power transfer and minimizing reflections or losses in communication systems, especially in high-frequency applications.

A mismatch in impedance between source, cable, and load can lead to signal degradation. The characteristic impedance depends on the physical geometry of the cable and the properties of the dielectric material between the conductors.

Specifically, it depends on the ratio of the outer conductor diameter (D) to the inner conductor diameter (d), and the relative permittivity (pr) of the dielectric. The typical impedance values used are 50 ohms and 75 ohms, depending on the application.

Coax cables used for data, video, and RF signals must be carefully chosen based on their impedance characteristics. A lower dielectric constant results in a higher impedance, and vice versa.

Coaxial Cable Impedance, Z_(Ω) in ohms equals 138 multiplied by the base 10 logarithm of the ratio of Outer Conductor Diameter, D_(mm), to Inner Conductor Diameter, d_(mm), divided by the square root of the Relative Permittivity, pr.

Coaxial Cable Impedance, Z_(Ω)= 138 * log₁₀(D_(mm)/ d_(mm)) / √pr

Z_(Ω)= coaxial cable impedance in ohms, Ω.

D_(mm)= outer conductor diameter in millimetres, mm.

d_(mm)= inner conductor diameter in millimetres, mm.

pr = relative permittivity.

Coax Cable:

1. A coaxial cable has an outer conductor diameter 7.0 mm, an inner conductor diameter 2.0 mm, and a dielectric constant 2.3. Calculate the cable impedance.

Given: D_(mm)= 7.0mm, d_(mm)= 2.0mm, pr = 2.3.

Coaxial Cable Impedance, Z_(Ω)= 138 * log₁₀(D_(mm)/ d_(mm)) / √pr

Z_(Ω)= 138 * log₁₀(7.0 / 2.0) / √2.3

Z_(Ω)= 138 * log₁₀(3.5) / 1.5166

Z_(Ω)= 75.0858 / 1.5166

Z_(Ω)= 49.48Ω.

2. A coaxial cable has an outer diameter 6.0 mm, inner diameter 1.8 mm, and measured impedance 75Ω. Find the dielectric constant pr.

Given: D_(mm)= 6.0mm, d_(mm)= 1.8mm, Z_(Ω)= 75Ω.

Coaxial Cable Impedance, Z_(Ω)= 138 * log₁₀(D_(mm)/ d_(mm)) / √pr

75 = 138 * log₁₀(6.0 / 1.8) / √pr

75 = 138 * 0.5229 / √pr

√pr = 72.1602 / 75

pr = 0.9621²

pr = 0.9257.