How does the radiation pattern of a horizontally-polarized antenna mounted above a long slope compare with the same antenna mounted above flat ground?
The correct answer is B: The main lobe takeoff angle decreases in the downhill direction. A horizontally-polarized antenna mounted above a long slope compared to flat ground has decreased main lobe takeoff angle in the downhill direction. The slope acts like the antenna is higher in that direction, creating a lower take-off angle.
When mounted on a slope, the effective height is different in different directions. In the downhill direction, the ground is effectively further away (the antenna appears higher relative to the ground in that direction), which creates a lower take-off angle. This is similar to raising the antenna - lower take-off angles are better for long-distance communications. The slope effectively tilts the elevation pattern, with better low-angle radiation in the downhill direction.
Exam Tip
Horizontal on slope = Lower take-off angle downhill. Remember: A horizontally-polarized antenna on a slope has decreased main lobe takeoff angle in the downhill direction, where the antenna is effectively higher.
Memory Aid
"**H**orizontal **S**lope = **L**ower **A**ngle **D**ownhill (think 'HS = LAD')"
Real-World Application
You install a horizontal dipole on a hillside, with the antenna running along the slope. In the downhill direction, the ground is effectively further away (the antenna appears higher), creating a lower take-off angle. This lower angle improves long-distance performance in the downhill direction. The slope effectively tilts your elevation pattern.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The takeoff angle doesn't increase downhill - it decreases. The slope makes the antenna effectively higher in the downhill direction.
Option C: Incorrect. The takeoff angle doesn't increase uphill. The question specifically asks about the downhill direction.
Option D: Incorrect. The takeoff angle doesn't decrease uphill. The improvement is in the downhill direction where the antenna is effectively higher.
题目解析
The correct answer is B: The main lobe takeoff angle decreases in the downhill direction. A horizontally-polarized antenna mounted above a long slope compared to flat ground has decreased main lobe takeoff angle in the downhill direction. The slope acts like the antenna is higher in that direction, creating a lower take-off angle. When mounted on a slope, the effective height is different in different directions. In the downhill direction, the ground is effectively further away (the antenna appears higher relative to the ground in that direction), which creates a lower take-off angle. This is similar to raising the antenna - lower take-off angles are better for long-distance communications. The slope effectively tilts the elevation pattern, with better low-angle radiation in the downhill direction.
考试技巧
Horizontal on slope = Lower take-off angle downhill. Remember: A horizontally-polarized antenna on a slope has decreased main lobe takeoff angle in the downhill direction, where the antenna is effectively higher.
记忆口诀
**H**orizontal **S**lope = **L**ower **A**ngle **D**ownhill (think 'HS = LAD')
实际应用示例
You install a horizontal dipole on a hillside, with the antenna running along the slope. In the downhill direction, the ground is effectively further away (the antenna appears higher), creating a lower take-off angle. This lower angle improves long-distance performance in the downhill direction. The slope effectively tilts your elevation pattern.
错误选项分析
Option A: Incorrect. The takeoff angle doesn't increase downhill - it decreases. The slope makes the antenna effectively higher in the downhill direction. Option C: Incorrect. The takeoff angle doesn't increase uphill. The question specifically asks about the downhill direction. Option D: Incorrect. The takeoff angle doesn't decrease uphill. The improvement is in the downhill direction where the antenna is effectively higher.
知识点
Horizontal antenna, Slope, Take-off angle, Elevation pattern
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.