混叠频率计算器 — 公式

Coaxial Cable Characteristic Impedance

The characteristic impedance of a coaxial transmission line depends on the ratio of conductor diameters and the dielectric material.

Formula

Z_0 = (138 / sqrt(Er)) x log10(D / d)

Where D is the outer conductor inner diameter, d is the inner conductor outer diameter, and Er is the relative dielectric constant.

Common Values

  • 50 ohm: Standard for RF and instrumentation
  • 75 ohm: Video and cable TV
  • 93 ohm: Early computer networks

    • The velocity factor is 1/sqrt(Er), which determines signal propagation speed.

    计算示例

    Coaxial cable with D=7.0 mm, d=1.63 mm, polyethylene dielectric (Er=2.3).

    1. Diameter ratio: 7.0 / 1.63 = 4.294
    2. log10(4.294) = 0.633
    3. Z0 = 138 / sqrt(2.3) x 0.633 = 138 / 1.517 x 0.633 = 57.6 ohms
    4. Velocity factor: 1 / sqrt(2.3) = 0.659