What is the far field of an antenna?
The correct answer is D: The region where the shape of the radiation pattern no longer varies with distance. The far field of an antenna is the region where the shape of the radiation pattern no longer varies with distance. Beyond this distance, the pattern is stable and predictable.
The far field (also called Fraunhofer region) begins at a distance where the radiation pattern has stabilized. In the far field, the pattern shape is constant regardless of distance - only the signal strength decreases with distance. The near field is closer, where the pattern shape changes with distance. The far field typically starts at distances greater than 2D²/λ, where D is the antenna's largest dimension and λ is wavelength. Far field measurements are used for antenna testing because the pattern is stable and representative of the antenna's actual performance.
Exam Tip
Far field = Pattern shape constant. Remember: The far field is the region where the radiation pattern shape no longer varies with distance. The pattern is stable and predictable in the far field.
Memory Aid
"**F**ar **F**ield = **P**attern **S**hape **C**onstant (think 'FF = PSC')"
Real-World Application
You're testing an antenna. Close to the antenna (near field), the pattern shape changes as you move. At greater distances (far field), the pattern shape stabilizes and no longer changes with distance - only the signal strength decreases. Far field measurements (typically beyond 2D²/λ) give you the true antenna pattern.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The far field isn't about the ionosphere. It's about the distance from the antenna where the pattern stabilizes, not about ionospheric propagation.
Option B: Incorrect. The far field isn't about power dissipation over time. It's about the spatial region where the pattern shape is constant.
Option C: Incorrect. Field strengths aren't constant in the far field - they decrease with distance. What's constant is the pattern shape, not the field strength.
题目解析
The correct answer is D: The region where the shape of the radiation pattern no longer varies with distance. The far field of an antenna is the region where the shape of the radiation pattern no longer varies with distance. Beyond this distance, the pattern is stable and predictable. The far field (also called Fraunhofer region) begins at a distance where the radiation pattern has stabilized. In the far field, the pattern shape is constant regardless of distance - only the signal strength decreases with distance. The near field is closer, where the pattern shape changes with distance. The far field typically starts at distances greater than 2D²/λ, where D is the antenna's largest dimension and λ is wavelength. Far field measurements are used for antenna testing because the pattern is stable and representative of the antenna's actual performance.
考试技巧
Far field = Pattern shape constant. Remember: The far field is the region where the radiation pattern shape no longer varies with distance. The pattern is stable and predictable in the far field.
记忆口诀
**F**ar **F**ield = **P**attern **S**hape **C**onstant (think 'FF = PSC')
实际应用示例
You're testing an antenna. Close to the antenna (near field), the pattern shape changes as you move. At greater distances (far field), the pattern shape stabilizes and no longer changes with distance - only the signal strength decreases. Far field measurements (typically beyond 2D²/λ) give you the true antenna pattern.
错误选项分析
Option A: Incorrect. The far field isn't about the ionosphere. It's about the distance from the antenna where the pattern stabilizes, not about ionospheric propagation. Option B: Incorrect. The far field isn't about power dissipation over time. It's about the spatial region where the pattern shape is constant. Option C: Incorrect. Field strengths aren't constant in the far field - they decrease with distance. What's constant is the pattern shape, not the field strength.
知识点
Far field, Radiation pattern, Antenna field regions, Fraunhofer region
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.