Which of the following types of feed line has the lowest loss at VHF and UHF?
The correct answer is C: Air-insulated hardline. Air-insulated hardline has the lowest loss at VHF and UHF among the listed feed line types. Hardline uses air as the dielectric (with spacers), which has very low dielectric loss. This makes it the lowest-loss option, though it's more expensive and less flexible than flexible coax. For amateur radio operators, hardline is used when minimum loss is critical, such as long feed line runs or high-power applications. Understanding this helps when selecting feed lines for VHF/UHF operation.
Exam Tip
Lowest loss at VHF/UHF = air-insulated hardline. Think 'A'ir-insulated = 'A'lmost no loss. Air dielectric has minimal loss. Flexible coax (50 or 75 ohm) has more loss due to solid/foam dielectric.
Memory Aid
"Lowest loss at VHF/UHF = air-insulated hardline. Think 'A'ir-insulated = 'A'lmost no loss. Air dielectric provides minimal loss. Best for long runs and high-power applications."
Real-World Application
You need a 200-foot feed line for a 70-cm repeater. Air-insulated hardline might have only 1 dB loss, while flexible coax would have 4-5 dB loss. The hardline delivers significantly more power to the antenna. However, hardline is expensive, rigid, and requires careful installation. It's the choice when minimum loss is critical.
Key Concepts
Why Other Options Are Wrong
Option A (50-ohm flexible coax): Incorrect. Flexible coax has more loss than hardline due to its solid or foam dielectric. Hardline's air dielectric has lower loss.
Option B (Multi-conductor unbalanced cable): Incorrect. This isn't a standard feed line type and would likely have high loss. Not suitable for VHF/UHF.
Option D (75-ohm flexible coax): Incorrect. 75-ohm coax has similar loss characteristics to 50-ohm coax, but both have more loss than air-insulated hardline.
题目解析
The correct answer is C: Air-insulated hardline. Air-insulated hardline has the lowest loss at VHF and UHF among the listed feed line types. Hardline uses air as the dielectric (with spacers), which has very low dielectric loss. This makes it the lowest-loss option, though it's more expensive and less flexible than flexible coax. For amateur radio operators, hardline is used when minimum loss is critical, such as long feed line runs or high-power applications. Understanding this helps when selecting feed lines for VHF/UHF operation.
考试技巧
Lowest loss at VHF/UHF = air-insulated hardline. Think 'A'ir-insulated = 'A'lmost no loss. Air dielectric has minimal loss. Flexible coax (50 or 75 ohm) has more loss due to solid/foam dielectric.
记忆口诀
Lowest loss at VHF/UHF = air-insulated hardline. Think 'A'ir-insulated = 'A'lmost no loss. Air dielectric provides minimal loss. Best for long runs and high-power applications.
实际应用示例
You need a 200-foot feed line for a 70-cm repeater. Air-insulated hardline might have only 1 dB loss, while flexible coax would have 4-5 dB loss. The hardline delivers significantly more power to the antenna. However, hardline is expensive, rigid, and requires careful installation. It's the choice when minimum loss is critical.
错误选项分析
Option A (50-ohm flexible coax): Incorrect. Flexible coax has more loss than hardline due to its solid or foam dielectric. Hardline's air dielectric has lower loss. Option B (Multi-conductor unbalanced cable): Incorrect. This isn't a standard feed line type and would likely have high loss. Not suitable for VHF/UHF. Option D (75-ohm flexible coax): Incorrect. 75-ohm coax has similar loss characteristics to 50-ohm coax, but both have more loss than air-insulated hardline.
知识点
Air-insulated hardline, Low-loss feed line, VHF/UHF feed lines, Feed line losses
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.