How does antenna height affect the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees?
The correct answer is C: If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional. How antenna height affects the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees is that if the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional. Low height reduces the directional pattern, making it more omnidirectional. For amateur radio operators, this explains why low dipoles are less directional. Understanding this helps when installing dipoles.
Exam Tip
Low horizontal dipole (< λ/2 high) = azimuthal pattern almost omnidirectional. Think 'L'ow 'H'eight = 'L'ess 'H'eight = 'O'mnidirectional. Low height reduces directional pattern, making it more omnidirectional. Not unpredictable, not no effect, not eliminates end-fire - just more omnidirectional.
Memory Aid
"Low horizontal dipole (< λ/2 high) = azimuthal pattern almost omnidirectional. Think 'L'ow 'H'eight = 'O'mnidirectional. Low height reduces directional pattern, making it more omnidirectional. Important for dipole installation."
Real-World Application
A horizontal dipole at low height (e.g., 1/4 wavelength): At elevation angles above 45°, the azimuthal pattern is almost omnidirectional - it radiates fairly equally in all horizontal directions. A dipole at higher height (e.g., 1 wavelength) has a more directional pattern. Low height makes the pattern more omnidirectional.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Antenna being too high doesn't make pattern unpredictable - high antennas have predictable patterns. Unpredictable isn't the effect.
Option B: Incorrect. Antenna height does affect the pattern - low height makes pattern more omnidirectional. No effect is wrong.
Option D: Incorrect. Low height doesn't eliminate radiation off ends - it makes pattern more omnidirectional, but doesn't eliminate end-fire radiation. Elimination isn't the effect.
题目解析
The correct answer is C: If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional. How antenna height affects the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees is that if the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional. Low height reduces the directional pattern, making it more omnidirectional. For amateur radio operators, this explains why low dipoles are less directional. Understanding this helps when installing dipoles.
考试技巧
Low horizontal dipole (< λ/2 high) = azimuthal pattern almost omnidirectional. Think 'L'ow 'H'eight = 'L'ess 'H'eight = 'O'mnidirectional. Low height reduces directional pattern, making it more omnidirectional. Not unpredictable, not no effect, not eliminates end-fire - just more omnidirectional.
记忆口诀
Low horizontal dipole (< λ/2 high) = azimuthal pattern almost omnidirectional. Think 'L'ow 'H'eight = 'O'mnidirectional. Low height reduces directional pattern, making it more omnidirectional. Important for dipole installation.
实际应用示例
A horizontal dipole at low height (e.g., 1/4 wavelength): At elevation angles above 45°, the azimuthal pattern is almost omnidirectional - it radiates fairly equally in all horizontal directions. A dipole at higher height (e.g., 1 wavelength) has a more directional pattern. Low height makes the pattern more omnidirectional.
错误选项分析
Option A: Incorrect. Antenna being too high doesn't make pattern unpredictable - high antennas have predictable patterns. Unpredictable isn't the effect. Option B: Incorrect. Antenna height does affect the pattern - low height makes pattern more omnidirectional. No effect is wrong. Option D: Incorrect. Low height doesn't eliminate radiation off ends - it makes pattern more omnidirectional, but doesn't eliminate end-fire radiation. Elimination isn't the effect.
知识点
Horizontal dipole, Antenna height, Azimuthal pattern, Omnidirectional
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.