What is the purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint?
The correct answer is A: To create a similar feed point impedance on multiple bands. The purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint is to create a similar feed point impedance on multiple bands. Off-center feeding helps maintain reasonable impedance across different frequencies.
A center-fed dipole has different impedances on different bands - it might be 50 ohms on one band but very high or very low on others. By feeding off-center (typically around 1/3 from one end), the impedance becomes more consistent across multiple bands. This makes the OCFD easier to match with a single feed system across multiple bands. The off-center feed point creates a more balanced impedance profile, which is why OCFDs are popular for multi-band operation.
Exam Tip
OCFD purpose = Similar impedance on multiple bands. Remember: Off-center-fed dipoles are fed off-center to create similar feed point impedance on multiple bands, making multi-band operation easier.
Memory Aid
"**O**CFD **P**urpose = **S**imilar **I**mpedance **M**ulti-**B**and (think 'OP = SIMB')"
Real-World Application
You build an OCFD for 80/40/20 meters. By feeding it about 1/3 from one end instead of at the center, the impedance is more consistent across all three bands (maybe 200-300 ohms on each). This makes it easier to match with a single balun or matching network, compared to a center-fed dipole which might have very different impedances on each band.
FCC Part 97.3Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Off-center feeding doesn't suppress off-center lobes. The purpose is impedance matching, not pattern control.
Option C: Incorrect. Off-center feeding doesn't make the antenna resonate across a wider range. It helps with impedance matching on multiple bands, not bandwidth.
Option D: Incorrect. Off-center feeding doesn't specifically reduce common-mode current. The main purpose is creating similar impedance on multiple bands.
题目解析
The correct answer is A: To create a similar feed point impedance on multiple bands. The purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint is to create a similar feed point impedance on multiple bands. Off-center feeding helps maintain reasonable impedance across different frequencies. A center-fed dipole has different impedances on different bands - it might be 50 ohms on one band but very high or very low on others. By feeding off-center (typically around 1/3 from one end), the impedance becomes more consistent across multiple bands. This makes the OCFD easier to match with a single feed system across multiple bands. The off-center feed point creates a more balanced impedance profile, which is why OCFDs are popular for multi-band operation.
考试技巧
OCFD purpose = Similar impedance on multiple bands. Remember: Off-center-fed dipoles are fed off-center to create similar feed point impedance on multiple bands, making multi-band operation easier.
记忆口诀
**O**CFD **P**urpose = **S**imilar **I**mpedance **M**ulti-**B**and (think 'OP = SIMB')
实际应用示例
You build an OCFD for 80/40/20 meters. By feeding it about 1/3 from one end instead of at the center, the impedance is more consistent across all three bands (maybe 200-300 ohms on each). This makes it easier to match with a single balun or matching network, compared to a center-fed dipole which might have very different impedances on each band.
错误选项分析
Option B: Incorrect. Off-center feeding doesn't suppress off-center lobes. The purpose is impedance matching, not pattern control. Option C: Incorrect. Off-center feeding doesn't make the antenna resonate across a wider range. It helps with impedance matching on multiple bands, not bandwidth. Option D: Incorrect. Off-center feeding doesn't specifically reduce common-mode current. The main purpose is creating similar impedance on multiple bands.
知识点
Off-center-fed dipole, OCFD, Feed point impedance, Multi-band operation
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.