What is the approximate physical length of an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz?
The correct answer is C: 10.6 meters. For an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz, the approximate physical length is 10.6 meters. Air-insulated lines have velocity factor ≈ 0.95-1.0, so physical length is close to electrical length.
Calculation: Wavelength at 14.10 MHz = 300 / 14.10 = 21.28 meters. Half-wavelength = 10.64 meters. For air-insulated line (VF ≈ 0.95-1.0), physical length ≈ electrical length, so about 10.6 meters. Air-insulated parallel line (like ladder line) has high velocity factor because air is the dielectric, so the physical length is very close to the free-space half-wavelength. This is why open-wire feed lines are popular - they have high velocity factors and low loss.
Exam Tip
1/2λ at 14.10 MHz = 10.6 meters (air-insulated). Remember: Half-wavelength at 14.10 MHz = 300/(2×14.10) = 10.64 meters. For air-insulated line (VF≈1.0), physical length ≈ 10.6 meters.
Memory Aid
"**1**/**2**λ **1**4.**1**0 **M**Hz = **1**0.**6** **m**eters (think '1/2λ 14.10MHz = 10.6m')"
Real-World Application
You need a 1/2-wavelength section of air-insulated parallel line at 14.10 MHz. The free-space half-wavelength is 10.64 meters. Since air-insulated line has velocity factor close to 1.0, the physical length is about 10.6 meters. You cut your ladder line to this length for the matching section.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. 7.0 meters would be too short. That's about 1/3 wavelength, not 1/2 wavelength.
Option B: Incorrect. 8.5 meters is still too short. The half-wavelength at 14.10 MHz is about 10.6 meters.
Option D: Incorrect. 13.3 meters would be too long - that's closer to 3/4 wavelength. The half-wavelength is about 10.6 meters.
题目解析
The correct answer is C: 10.6 meters. For an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz, the approximate physical length is 10.6 meters. Air-insulated lines have velocity factor ≈ 0.95-1.0, so physical length is close to electrical length. Calculation: Wavelength at 14.10 MHz = 300 / 14.10 = 21.28 meters. Half-wavelength = 10.64 meters. For air-insulated line (VF ≈ 0.95-1.0), physical length ≈ electrical length, so about 10.6 meters. Air-insulated parallel line (like ladder line) has high velocity factor because air is the dielectric, so the physical length is very close to the free-space half-wavelength. This is why open-wire feed lines are popular - they have high velocity factors and low loss.
考试技巧
1/2λ at 14.10 MHz = 10.6 meters (air-insulated). Remember: Half-wavelength at 14.10 MHz = 300/(2×14.10) = 10.64 meters. For air-insulated line (VF≈1.0), physical length ≈ 10.6 meters.
记忆口诀
**1**/**2**λ **1**4.**1**0 **M**Hz = **1**0.**6** **m**eters (think '1/2λ 14.10MHz = 10.6m')
实际应用示例
You need a 1/2-wavelength section of air-insulated parallel line at 14.10 MHz. The free-space half-wavelength is 10.64 meters. Since air-insulated line has velocity factor close to 1.0, the physical length is about 10.6 meters. You cut your ladder line to this length for the matching section.
错误选项分析
Option A: Incorrect. 7.0 meters would be too short. That's about 1/3 wavelength, not 1/2 wavelength. Option B: Incorrect. 8.5 meters is still too short. The half-wavelength at 14.10 MHz is about 10.6 meters. Option D: Incorrect. 13.3 meters would be too long - that's closer to 3/4 wavelength. The half-wavelength is about 10.6 meters.
知识点
Transmission line length, Half-wavelength, Air-insulated, Physical length
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.