What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types?
The correct answer is C: It requires special techniques to prevent moisture in the cable. A disadvantage of air core coaxial cable (compared to foam or solid dielectric) is that it requires special techniques to prevent moisture in the cable. Air core cable uses air as the dielectric (with spacers to keep the center conductor centered), which means water can easily enter and flow through the cable if not properly sealed. Foam and solid dielectric cables are less susceptible to moisture because the dielectric material helps block water entry. For amateur radio operators, air core cable offers lower loss but requires careful installation and sealing to prevent moisture problems.
Exam Tip
Air core = requires moisture prevention. Think 'A'ir core = 'A'llows 'A'ir (and water) entry. The air dielectric means water can enter easily, requiring special sealing techniques. Lower loss is an advantage, not a disadvantage.
Memory Aid
"Air core = requires moisture prevention. Think 'A'ir core = 'A'llows water entry easily. Special sealing techniques needed. Air dielectric = water can flow through if not sealed."
Real-World Application
You install air core coaxial cable for a low-loss feed line. Because the cable uses air as the dielectric, you must carefully seal all connectors and ensure the cable ends are properly terminated to prevent moisture entry. Any opening allows water to enter and flow through the cable, causing failure. Foam or solid dielectric cables are more forgiving because the dielectric material helps block water.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Air core cable typically has less loss per foot than foam or solid dielectric types because air has lower dielectric loss. Lower loss is an advantage, not a disadvantage.
Option B: Incorrect. Air core cable can be used for VHF and UHF antennas - the frequency range isn't limited by the dielectric type.
Option D: Incorrect. Air core cable can be used at below-freezing temperatures - the air dielectric doesn't freeze like water would.
题目解析
The correct answer is C: It requires special techniques to prevent moisture in the cable. A disadvantage of air core coaxial cable (compared to foam or solid dielectric) is that it requires special techniques to prevent moisture in the cable. Air core cable uses air as the dielectric (with spacers to keep the center conductor centered), which means water can easily enter and flow through the cable if not properly sealed. Foam and solid dielectric cables are less susceptible to moisture because the dielectric material helps block water entry. For amateur radio operators, air core cable offers lower loss but requires careful installation and sealing to prevent moisture problems.
考试技巧
Air core = requires moisture prevention. Think 'A'ir core = 'A'llows 'A'ir (and water) entry. The air dielectric means water can enter easily, requiring special sealing techniques. Lower loss is an advantage, not a disadvantage.
记忆口诀
Air core = requires moisture prevention. Think 'A'ir core = 'A'llows water entry easily. Special sealing techniques needed. Air dielectric = water can flow through if not sealed.
实际应用示例
You install air core coaxial cable for a low-loss feed line. Because the cable uses air as the dielectric, you must carefully seal all connectors and ensure the cable ends are properly terminated to prevent moisture entry. Any opening allows water to enter and flow through the cable, causing failure. Foam or solid dielectric cables are more forgiving because the dielectric material helps block water.
错误选项分析
Option A: Incorrect. Air core cable typically has less loss per foot than foam or solid dielectric types because air has lower dielectric loss. Lower loss is an advantage, not a disadvantage. Option B: Incorrect. Air core cable can be used for VHF and UHF antennas - the frequency range isn't limited by the dielectric type. Option D: Incorrect. Air core cable can be used at below-freezing temperatures - the air dielectric doesn't freeze like water would.
知识点
Air core coaxial cable, Moisture prevention, Cable types, Dielectric materials
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.