What is the velocity factor of a transmission line?
The correct answer is D: The velocity of a wave in the transmission line divided by the velocity of light in a vacuum. The velocity factor of a transmission line is the velocity of a wave in the transmission line divided by the velocity of light in a vacuum. It's a ratio, typically between 0.5 and 1.0.
The velocity factor (VF) tells you how fast signals travel in the transmission line compared to the speed of light. VF = v_line / c, where v_line is the wave velocity in the line and c is the speed of light. For example, if VF = 0.66, signals travel at 66% of the speed of light. The velocity factor depends on the dielectric material - air-insulated lines have VF ≈ 0.95-1.0, while solid dielectric coax might have VF ≈ 0.66. The velocity factor is important for calculating electrical length from physical length.
Exam Tip
Velocity factor = Line velocity / Light speed. Remember: Velocity factor = velocity in transmission line / velocity of light in vacuum. It's typically 0.5-1.0 (signals travel slower than light).
Memory Aid
"**V**elocity **F**actor = **L**ine **V**elocity / **L**ight **S**peed (think 'VF = LV/LS')"
Real-World Application
You have coaxial cable with velocity factor 0.66. This means signals travel at 66% of the speed of light in this cable. If you need a 1/4-wavelength section at 14 MHz, the physical length is shorter than the free-space length because of the velocity factor. The velocity factor accounts for the slower wave propagation in the cable's dielectric.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Velocity factor isn't about impedance ratios. It's about wave velocity, not impedance relationships.
Option B: Incorrect. This is the inverse of the correct relationship. Velocity factor is line velocity divided by light speed, not the other way around.
Option C: Incorrect. Velocity factor isn't multiplied by light speed - it's divided by light speed. Signals in transmission lines travel slower than light, so VF < 1.
题目解析
The correct answer is D: The velocity of a wave in the transmission line divided by the velocity of light in a vacuum. The velocity factor of a transmission line is the velocity of a wave in the transmission line divided by the velocity of light in a vacuum. It's a ratio, typically between 0.5 and 1.0. The velocity factor (VF) tells you how fast signals travel in the transmission line compared to the speed of light. VF = v_line / c, where v_line is the wave velocity in the line and c is the speed of light. For example, if VF = 0.66, signals travel at 66% of the speed of light. The velocity factor depends on the dielectric material - air-insulated lines have VF ≈ 0.95-1.0, while solid dielectric coax might have VF ≈ 0.66. The velocity factor is important for calculating electrical length from physical length.
考试技巧
Velocity factor = Line velocity / Light speed. Remember: Velocity factor = velocity in transmission line / velocity of light in vacuum. It's typically 0.5-1.0 (signals travel slower than light).
记忆口诀
**V**elocity **F**actor = **L**ine **V**elocity / **L**ight **S**peed (think 'VF = LV/LS')
实际应用示例
You have coaxial cable with velocity factor 0.66. This means signals travel at 66% of the speed of light in this cable. If you need a 1/4-wavelength section at 14 MHz, the physical length is shorter than the free-space length because of the velocity factor. The velocity factor accounts for the slower wave propagation in the cable's dielectric.
错误选项分析
Option A: Incorrect. Velocity factor isn't about impedance ratios. It's about wave velocity, not impedance relationships. Option B: Incorrect. This is the inverse of the correct relationship. Velocity factor is line velocity divided by light speed, not the other way around. Option C: Incorrect. Velocity factor isn't multiplied by light speed - it's divided by light speed. Signals in transmission lines travel slower than light, so VF < 1.
知识点
Velocity factor, Transmission line, Wave velocity, Speed of light
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.