Which of the following has the biggest effect on the velocity factor of a transmission line?
The correct answer is C: The insulating dielectric material. The insulating dielectric material has the biggest effect on the velocity factor of a transmission line. Different dielectrics slow down the wave by different amounts.
The velocity factor depends primarily on the dielectric constant (εᵣ) of the insulating material: VF ≈ 1/√εᵣ. Air (εᵣ ≈ 1) gives VF ≈ 1.0. Solid polyethylene (εᵣ ≈ 2.3) gives VF ≈ 0.66. Foam dielectric (εᵣ ≈ 1.5) gives VF ≈ 0.81. The dielectric material is the primary factor - other parameters like characteristic impedance, line length, or conductor resistivity have much smaller effects on velocity factor. This is why air-insulated lines have high velocity factors (close to 1.0) while solid dielectric lines have lower velocity factors.
Exam Tip
Velocity factor determined by = Dielectric material. Remember: The insulating dielectric material has the biggest effect on velocity factor. VF ≈ 1/√εᵣ, where εᵣ is the dielectric constant.
Memory Aid
"**V**elocity **F**actor = **D**etermined by **D**ielectric (think 'VF = DD')"
Real-World Application
You compare two transmission lines - one with air dielectric (VF ≈ 0.95) and one with solid polyethylene (VF ≈ 0.66). The dielectric material is the key difference. Air has low dielectric constant, so signals travel almost at light speed. Solid polyethylene has higher dielectric constant, so signals travel slower. The dielectric material determines the velocity factor.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Characteristic impedance affects impedance matching but has minimal effect on velocity factor. The dielectric is the primary factor.
Option B: Incorrect. Transmission line length doesn't affect velocity factor. Length affects electrical length, but the velocity factor (how fast waves travel) is determined by the dielectric.
Option D: Incorrect. Center conductor resistivity affects losses but has minimal effect on velocity factor. The dielectric material is the primary determinant.
题目解析
The correct answer is C: The insulating dielectric material. The insulating dielectric material has the biggest effect on the velocity factor of a transmission line. Different dielectrics slow down the wave by different amounts. The velocity factor depends primarily on the dielectric constant (εᵣ) of the insulating material: VF ≈ 1/√εᵣ. Air (εᵣ ≈ 1) gives VF ≈ 1.0. Solid polyethylene (εᵣ ≈ 2.3) gives VF ≈ 0.66. Foam dielectric (εᵣ ≈ 1.5) gives VF ≈ 0.81. The dielectric material is the primary factor - other parameters like characteristic impedance, line length, or conductor resistivity have much smaller effects on velocity factor. This is why air-insulated lines have high velocity factors (close to 1.0) while solid dielectric lines have lower velocity factors.
考试技巧
Velocity factor determined by = Dielectric material. Remember: The insulating dielectric material has the biggest effect on velocity factor. VF ≈ 1/√εᵣ, where εᵣ is the dielectric constant.
记忆口诀
**V**elocity **F**actor = **D**etermined by **D**ielectric (think 'VF = DD')
实际应用示例
You compare two transmission lines - one with air dielectric (VF ≈ 0.95) and one with solid polyethylene (VF ≈ 0.66). The dielectric material is the key difference. Air has low dielectric constant, so signals travel almost at light speed. Solid polyethylene has higher dielectric constant, so signals travel slower. The dielectric material determines the velocity factor.
错误选项分析
Option A: Incorrect. Characteristic impedance affects impedance matching but has minimal effect on velocity factor. The dielectric is the primary factor. Option B: Incorrect. Transmission line length doesn't affect velocity factor. Length affects electrical length, but the velocity factor (how fast waves travel) is determined by the dielectric. Option D: Incorrect. Center conductor resistivity affects losses but has minimal effect on velocity factor. The dielectric material is the primary determinant.
知识点
Velocity factor, Dielectric material, Dielectric constant, Transmission line
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.