Why is the electrical length of a coaxial cable longer than its physical length?
The correct answer is D: Electromagnetic waves move more slowly in a coaxial cable than in air. The electrical length of a coaxial cable is longer than its physical length because electromagnetic waves move more slowly in a coaxial cable than in air. The slower velocity means the wave takes longer to travel the same distance, making it electrically longer.
Electrical length is measured in wavelengths or degrees. Because waves travel slower in coax (due to the dielectric), a given physical length represents more electrical length than the same physical length in air. For example, if a wave travels at 66% of light speed in the cable, a 10-meter physical length is electrically equivalent to about 15 meters in free space. The electrical length is longer because the wave is slower - it takes more time (more phase) to travel the same physical distance.
Exam Tip
Electrical length > Physical length because waves are slower. Remember: Electrical length is longer than physical length because electromagnetic waves move more slowly in coaxial cable (due to dielectric) than in air.
Memory Aid
"**E**lectrical **L**ength **L**onger = **W**aves **S**lower (think 'ELL = WS')"
Real-World Application
You have a 10-meter piece of coax with velocity factor 0.66. The wave travels at 66% of light speed. Electrically, this 10-meter cable is equivalent to about 15 meters in free space because the slower wave takes more phase (more electrical length) to travel the same physical distance. The electrical length is longer because the wave is slower.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Skin effect doesn't make electrical length longer. Skin effect affects conductor resistance at high frequencies, not wave velocity or electrical length.
Option B: Incorrect. More pronounced skin effect doesn't explain longer electrical length. The issue is wave velocity, not skin effect.
Option C: Incorrect. Waves don't move faster in coax - they move slower due to the dielectric. This slower velocity makes electrical length longer than physical length.
题目解析
The correct answer is D: Electromagnetic waves move more slowly in a coaxial cable than in air. The electrical length of a coaxial cable is longer than its physical length because electromagnetic waves move more slowly in a coaxial cable than in air. The slower velocity means the wave takes longer to travel the same distance, making it electrically longer. Electrical length is measured in wavelengths or degrees. Because waves travel slower in coax (due to the dielectric), a given physical length represents more electrical length than the same physical length in air. For example, if a wave travels at 66% of light speed in the cable, a 10-meter physical length is electrically equivalent to about 15 meters in free space. The electrical length is longer because the wave is slower - it takes more time (more phase) to travel the same physical distance.
考试技巧
Electrical length > Physical length because waves are slower. Remember: Electrical length is longer than physical length because electromagnetic waves move more slowly in coaxial cable (due to dielectric) than in air.
记忆口诀
**E**lectrical **L**ength **L**onger = **W**aves **S**lower (think 'ELL = WS')
实际应用示例
You have a 10-meter piece of coax with velocity factor 0.66. The wave travels at 66% of light speed. Electrically, this 10-meter cable is equivalent to about 15 meters in free space because the slower wave takes more phase (more electrical length) to travel the same physical distance. The electrical length is longer because the wave is slower.
错误选项分析
Option A: Incorrect. Skin effect doesn't make electrical length longer. Skin effect affects conductor resistance at high frequencies, not wave velocity or electrical length. Option B: Incorrect. More pronounced skin effect doesn't explain longer electrical length. The issue is wave velocity, not skin effect. Option C: Incorrect. Waves don't move faster in coax - they move slower due to the dielectric. This slower velocity makes electrical length longer than physical length.
知识点
Electrical length, Coaxial cable, Wave velocity, Physical vs electrical length
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.