What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?
The correct answer is B: A single null. The feature of a cardioid pattern antenna that makes it useful for direction-finding antennas is a single null. The single, well-defined null clearly indicates the direction of the signal.
Cardioid patterns have one main lobe (forward direction) and one null (reverse direction). This single null is perfect for direction-finding because when you rotate the antenna and find the null, you know exactly which direction the signal is coming from - there's no ambiguity. This is much better than a bidirectional pattern (like a loop without termination) which has two nulls, creating ambiguity. The single null of a cardioid pattern makes it ideal for DF work - you can precisely determine signal direction by finding the null.
Exam Tip
Cardioid for DF = Single null. Remember: A cardioid pattern antenna is useful for direction-finding because it has a single null - when you find the null, you know exactly which direction the signal is coming from, with no ambiguity.
Memory Aid
"**C**ardioid **D**F = **S**ingle **N**ull (think 'CDF = SN')"
Real-World Application
You're using a cardioid-pattern antenna for direction-finding. You rotate it and find a deep null. Because the cardioid has only one null (not two like a bidirectional pattern), you know exactly which direction the signal is coming from - no ambiguity. The single null makes direction-finding precise and unambiguous.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. A cardioid pattern doesn't have a very sharp peak - it has a broad forward lobe. The useful feature for DF is the single null, not a sharp peak.
Option C: Incorrect. Cardioid patterns aren't necessarily broadband. The useful feature for DF is the single null, not broadband response.
Option D: Incorrect. Cardioid patterns don't have high radiation angle - they typically have low take-off angles. The useful feature is the single null.
题目解析
The correct answer is B: A single null. The feature of a cardioid pattern antenna that makes it useful for direction-finding antennas is a single null. The single, well-defined null clearly indicates the direction of the signal. Cardioid patterns have one main lobe (forward direction) and one null (reverse direction). This single null is perfect for direction-finding because when you rotate the antenna and find the null, you know exactly which direction the signal is coming from - there's no ambiguity. This is much better than a bidirectional pattern (like a loop without termination) which has two nulls, creating ambiguity. The single null of a cardioid pattern makes it ideal for DF work - you can precisely determine signal direction by finding the null.
考试技巧
Cardioid for DF = Single null. Remember: A cardioid pattern antenna is useful for direction-finding because it has a single null - when you find the null, you know exactly which direction the signal is coming from, with no ambiguity.
记忆口诀
**C**ardioid **D**F = **S**ingle **N**ull (think 'CDF = SN')
实际应用示例
You're using a cardioid-pattern antenna for direction-finding. You rotate it and find a deep null. Because the cardioid has only one null (not two like a bidirectional pattern), you know exactly which direction the signal is coming from - no ambiguity. The single null makes direction-finding precise and unambiguous.
错误选项分析
Option A: Incorrect. A cardioid pattern doesn't have a very sharp peak - it has a broad forward lobe. The useful feature for DF is the single null, not a sharp peak. Option C: Incorrect. Cardioid patterns aren't necessarily broadband. The useful feature for DF is the single null, not broadband response. Option D: Incorrect. Cardioid patterns don't have high radiation angle - they typically have low take-off angles. The useful feature is the single null.
知识点
Cardioid pattern, Direction-finding, Single null, DF antenna
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.