噪声系数计算器
计算接收机或放大器的噪声系数。
mm
mm
Characteristic Impedance
53.7 Ω
Velocity Factor0.659
Diameter Ratio3.89
Characteristic Impedance vs Outer Conductor ID
公式
## Coaxial Cable Impedance ### Formula **Z0 = (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 cables: RG-58 (50 ohm), RG-59 (75 ohm), RG-6 (75 ohm).
计算示例
RG-58-like: D = 3.5 mm, d = 0.9 mm, PTFE dielectric (Er = 2.3).
- 01Ratio = 3.5 / 0.9 = 3.889
- 02Z0 = 138 / sqrt(2.3) x log10(3.889)
- 03Z0 = 91.0 x 0.5898 = 53.7 ohms
常见问题
Why 50 ohms?
50 ohms is a compromise between minimum attenuation (77 ohms) and maximum power handling (30 ohms) for air-dielectric coax.
Why 75 ohms for video?
75 ohms minimizes signal attenuation in coax. Video prioritizes signal fidelity over power handling.
What is velocity factor?
Signal speed as a fraction of c. Solid PE: 0.66. Foam PE: 0.78. PTFE: 0.70. Air: 1.0.
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