Which of the following is a disadvantage of multiband antennas?
The correct answer is D: They have poor harmonic rejection. A disadvantage of multiband antennas is that they have poor harmonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well - harmonics may also be in-band. For amateur radio operators, this can cause interference. Understanding this helps when using multiband antennas.
Exam Tip
Multiband antenna disadvantage = poor harmonic rejection. Think 'M'ultiband = 'M'any frequencies = 'P'oor 'H'armonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well. Not low impedance, not tuner requirement, not open wire - just poor harmonic rejection.
Memory Aid
"Multiband antenna disadvantage = poor harmonic rejection. Think 'M'ultiband = 'P'oor 'H'armonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well. Can cause interference."
Real-World Application
A multiband antenna: It works on 20m, 40m, and 80m. If you transmit on 20m, the second harmonic (40m) may also be radiated because the antenna is resonant on 40m. Multiband antennas have poor harmonic rejection - harmonics may be in-band. This can cause interference. This is a disadvantage of multiband antennas.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Multiband antennas don't present low impedance on all frequencies - they may have varying impedance. Low impedance on all isn't the disadvantage.
Option B: Incorrect. Multiband antennas don't necessarily require antenna tuners - some are designed to work without tuners. Tuner requirement isn't the disadvantage.
Option C: Incorrect. Multiband antennas don't have to be fed with open wire line - they can use coax. Open wire line isn't the disadvantage.
题目解析
The correct answer is D: They have poor harmonic rejection. A disadvantage of multiband antennas is that they have poor harmonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well - harmonics may also be in-band. For amateur radio operators, this can cause interference. Understanding this helps when using multiband antennas.
考试技巧
Multiband antenna disadvantage = poor harmonic rejection. Think 'M'ultiband = 'M'any frequencies = 'P'oor 'H'armonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well. Not low impedance, not tuner requirement, not open wire - just poor harmonic rejection.
记忆口诀
Multiband antenna disadvantage = poor harmonic rejection. Think 'M'ultiband = 'P'oor 'H'armonic rejection. Multiband antennas work on multiple frequencies, so they don't reject harmonics well. Can cause interference.
实际应用示例
A multiband antenna: It works on 20m, 40m, and 80m. If you transmit on 20m, the second harmonic (40m) may also be radiated because the antenna is resonant on 40m. Multiband antennas have poor harmonic rejection - harmonics may be in-band. This can cause interference. This is a disadvantage of multiband antennas.
错误选项分析
Option A: Incorrect. Multiband antennas don't present low impedance on all frequencies - they may have varying impedance. Low impedance on all isn't the disadvantage. Option B: Incorrect. Multiband antennas don't necessarily require antenna tuners - some are designed to work without tuners. Tuner requirement isn't the disadvantage. Option C: Incorrect. Multiband antennas don't have to be fed with open wire line - they can use coax. Open wire line isn't the disadvantage.
知识点
Multiband antennas, Poor harmonic rejection, Harmonic interference, Antenna disadvantages
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.