What is the purpose of placing an electrostatic shield around a small-loop direction-finding antenna?
The correct answer is B: It eliminates unbalanced capacitive coupling to the antenna's surroundings, improving the depth of its nulls. The purpose of placing an electrostatic shield around a small-loop direction-finding antenna is to eliminate unbalanced capacitive coupling to the antenna's surroundings, improving the depth of its nulls. The shield prevents the antenna from coupling capacitively to nearby objects.
Small loop antennas for direction-finding rely on sharp nulls to determine direction. Without a shield, the loop can couple capacitively to nearby objects (ground, buildings, etc.), which distorts the pattern and makes the nulls less sharp. An electrostatic shield (typically a Faraday shield - conductive but not a complete electrical circuit) blocks electric field coupling while allowing magnetic field coupling. This eliminates the capacitive coupling, making the nulls deeper and sharper, which improves direction-finding accuracy. The shield is essential for good DF performance.
Exam Tip
Electrostatic shield purpose = Eliminate capacitive coupling, improve nulls. Remember: An electrostatic shield around a small-loop DF antenna eliminates unbalanced capacitive coupling to surroundings, making the nulls deeper and sharper for better direction-finding.
Memory Aid
"**E**lectrostatic **S**hield = **E**liminate **C**apacitive **C**oupling, **I**mprove **N**ulls (think 'ES = ECCIN')"
Real-World Application
You're using a small loop for direction-finding. Without a shield, the loop couples capacitively to nearby objects (ground, your body, etc.), making the nulls shallow and hard to find. You add an electrostatic shield (Faraday shield) around the loop. This blocks electric field coupling while allowing magnetic fields, making the nulls much deeper and sharper. Your direction-finding accuracy improves significantly.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. The shield doesn't add capacitive loading or increase bandwidth. Its purpose is to eliminate unwanted capacitive coupling, not add capacitance.
Option C: Incorrect. The shield doesn't eliminate tracking errors from out-of-band signals. It eliminates capacitive coupling to surroundings.
Option D: Incorrect. The shield doesn't increase signal strength or improve feed line match. Its purpose is to improve null depth for direction-finding.
题目解析
The correct answer is B: It eliminates unbalanced capacitive coupling to the antenna's surroundings, improving the depth of its nulls. The purpose of placing an electrostatic shield around a small-loop direction-finding antenna is to eliminate unbalanced capacitive coupling to the antenna's surroundings, improving the depth of its nulls. The shield prevents the antenna from coupling capacitively to nearby objects. Small loop antennas for direction-finding rely on sharp nulls to determine direction. Without a shield, the loop can couple capacitively to nearby objects (ground, buildings, etc.), which distorts the pattern and makes the nulls less sharp. An electrostatic shield (typically a Faraday shield - conductive but not a complete electrical circuit) blocks electric field coupling while allowing magnetic field coupling. This eliminates the capacitive coupling, making the nulls deeper and sharper, which improves direction-finding accuracy. The shield is essential for good DF performance.
考试技巧
Electrostatic shield purpose = Eliminate capacitive coupling, improve nulls. Remember: An electrostatic shield around a small-loop DF antenna eliminates unbalanced capacitive coupling to surroundings, making the nulls deeper and sharper for better direction-finding.
记忆口诀
**E**lectrostatic **S**hield = **E**liminate **C**apacitive **C**oupling, **I**mprove **N**ulls (think 'ES = ECCIN')
实际应用示例
You're using a small loop for direction-finding. Without a shield, the loop couples capacitively to nearby objects (ground, your body, etc.), making the nulls shallow and hard to find. You add an electrostatic shield (Faraday shield) around the loop. This blocks electric field coupling while allowing magnetic fields, making the nulls much deeper and sharper. Your direction-finding accuracy improves significantly.
错误选项分析
Option A: Incorrect. The shield doesn't add capacitive loading or increase bandwidth. Its purpose is to eliminate unwanted capacitive coupling, not add capacitance. Option C: Incorrect. The shield doesn't eliminate tracking errors from out-of-band signals. It eliminates capacitive coupling to surroundings. Option D: Incorrect. The shield doesn't increase signal strength or improve feed line match. Its purpose is to improve null depth for direction-finding.
知识点
Electrostatic shield, Loop antenna, Direction-finding, Null depth
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.