Deep Dive: G9D11
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G9D
Reference: 2023-2027 Question Pool · G9 - Antennas and Feed Lines
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G9D topic.