How does parallel conductor transmission line compare to coaxial cable with a plastic dielectric?
The correct answer is A: Lower loss. Parallel conductor transmission line (like ladder line) compared to coaxial cable with a plastic dielectric has lower loss per unit length. Air-insulated parallel line has less dielectric loss than solid-dielectric coax.
Parallel conductor line (open-wire, ladder line) typically uses air as the dielectric (or minimal dielectric), which has very low loss. Coaxial cable with solid plastic dielectric has dielectric losses in the insulating material. At HF frequencies, the difference can be significant - parallel line might have 0.1 dB per 100 feet while coax might have 0.5-1 dB per 100 feet. This lower loss is why many operators use open-wire or ladder line for long feed lines, especially at higher power levels where losses become significant. The air dielectric minimizes losses.
Exam Tip
Parallel line vs coax = Lower loss. Remember: Parallel conductor transmission line (air-insulated) has lower loss than coaxial cable with plastic dielectric because air has less dielectric loss than solid plastic.
Memory Aid
"**P**arallel **L**ine vs **C**oax = **L**ower **L**oss (think 'PL vs C = LL')"
Real-World Application
You compare a 200-foot run of ladder line (parallel conductor, air-insulated) to 200 feet of RG-8X coax (solid dielectric). The ladder line has about 0.2 dB loss, while the coax has about 1.0 dB loss. The parallel line's air dielectric has much less loss than the coax's solid plastic dielectric. This lower loss is especially important at higher power levels.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Parallel line doesn't have higher SWR. SWR depends on impedance matching, not the line type itself.
Option C: Incorrect. Parallel line doesn't have a smaller reflection coefficient. Reflection coefficient depends on impedance match, not line type.
Option D: Incorrect. Parallel line typically has higher velocity factor (closer to 1.0) than plastic dielectric coax, not lower.
题目解析
The correct answer is A: Lower loss. Parallel conductor transmission line (like ladder line) compared to coaxial cable with a plastic dielectric has lower loss per unit length. Air-insulated parallel line has less dielectric loss than solid-dielectric coax. Parallel conductor line (open-wire, ladder line) typically uses air as the dielectric (or minimal dielectric), which has very low loss. Coaxial cable with solid plastic dielectric has dielectric losses in the insulating material. At HF frequencies, the difference can be significant - parallel line might have 0.1 dB per 100 feet while coax might have 0.5-1 dB per 100 feet. This lower loss is why many operators use open-wire or ladder line for long feed lines, especially at higher power levels where losses become significant. The air dielectric minimizes losses.
考试技巧
Parallel line vs coax = Lower loss. Remember: Parallel conductor transmission line (air-insulated) has lower loss than coaxial cable with plastic dielectric because air has less dielectric loss than solid plastic.
记忆口诀
**P**arallel **L**ine vs **C**oax = **L**ower **L**oss (think 'PL vs C = LL')
实际应用示例
You compare a 200-foot run of ladder line (parallel conductor, air-insulated) to 200 feet of RG-8X coax (solid dielectric). The ladder line has about 0.2 dB loss, while the coax has about 1.0 dB loss. The parallel line's air dielectric has much less loss than the coax's solid plastic dielectric. This lower loss is especially important at higher power levels.
错误选项分析
Option B: Incorrect. Parallel line doesn't have higher SWR. SWR depends on impedance matching, not the line type itself. Option C: Incorrect. Parallel line doesn't have a smaller reflection coefficient. Reflection coefficient depends on impedance match, not line type. Option D: Incorrect. Parallel line typically has higher velocity factor (closer to 1.0) than plastic dielectric coax, not lower.
知识点
Parallel conductor line, Coaxial cable, Loss, Dielectric loss
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.