What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?
The correct answer is B: Additional lobes form with major lobes increasingly aligned with the axis of the antenna. As the wire length of an unterminated long wire antenna is increased, additional lobes form with major lobes increasingly aligned with the axis of the antenna. Longer wires create more lobes, and the main lobes point more along the wire direction.
Long wire antennas create multiple lobes as length increases. With a 1-wavelength wire, you might have 2 main lobes. As you go to 2 wavelengths, 3 wavelengths, etc., more lobes appear. The major lobes tend to align more with the wire axis as length increases. This is why very long wires (several wavelengths) have their strongest radiation along the wire direction. The pattern becomes more complex with more lobes, but the main radiation is increasingly end-fire (along the wire).
Exam Tip
Longer long wire = More lobes, aligned with axis. Remember: As unterminated long wire length increases, additional lobes form, and major lobes increasingly align with the antenna axis (end-fire direction).
Memory Aid
"**L**onger **L**ong **W**ire = **M**ore **L**obes **A**xis (think 'LLW = MLA')"
Real-World Application
You have a 1-wavelength long wire with 2 main lobes. You extend it to 3 wavelengths. Now you have more lobes, and the major lobes are more aligned with the wire direction. The longer wire creates a more complex pattern, but the strongest radiation is increasingly along the wire axis.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Longer wires don't create fewer lobes - they create more lobes. And the lobes don't move closer to broadside; they align more with the axis.
Option C: Incorrect. The pattern doesn't become omnidirectional. Long wires create directional patterns with multiple lobes.
Option D: Incorrect. The pattern doesn't become unidirectional. Long wires create complex patterns with multiple lobes.
题目解析
The correct answer is B: Additional lobes form with major lobes increasingly aligned with the axis of the antenna. As the wire length of an unterminated long wire antenna is increased, additional lobes form with major lobes increasingly aligned with the axis of the antenna. Longer wires create more lobes, and the main lobes point more along the wire direction. Long wire antennas create multiple lobes as length increases. With a 1-wavelength wire, you might have 2 main lobes. As you go to 2 wavelengths, 3 wavelengths, etc., more lobes appear. The major lobes tend to align more with the wire axis as length increases. This is why very long wires (several wavelengths) have their strongest radiation along the wire direction. The pattern becomes more complex with more lobes, but the main radiation is increasingly end-fire (along the wire).
考试技巧
Longer long wire = More lobes, aligned with axis. Remember: As unterminated long wire length increases, additional lobes form, and major lobes increasingly align with the antenna axis (end-fire direction).
记忆口诀
**L**onger **L**ong **W**ire = **M**ore **L**obes **A**xis (think 'LLW = MLA')
实际应用示例
You have a 1-wavelength long wire with 2 main lobes. You extend it to 3 wavelengths. Now you have more lobes, and the major lobes are more aligned with the wire direction. The longer wire creates a more complex pattern, but the strongest radiation is increasingly along the wire axis.
错误选项分析
Option A: Incorrect. Longer wires don't create fewer lobes - they create more lobes. And the lobes don't move closer to broadside; they align more with the axis. Option C: Incorrect. The pattern doesn't become omnidirectional. Long wires create directional patterns with multiple lobes. Option D: Incorrect. The pattern doesn't become unidirectional. Long wires create complex patterns with multiple lobes.
知识点
Long wire antenna, Wire length, Radiation pattern, Lobes
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.