*Question G9D11:
Which of the following is a disadvantage of multiband antennas?*  

*Correct Answer is  D: They have poor harmonic rejection*  

### AI Response: (may contain errors)  

The correct answer to the question about the disadvantages of multiband antennas is:

**D: They have poor harmonic rejection.**

### Explanation of the Correct Answer:
- **Poor Harmonic Rejection**: Multiband antennas are designed to operate on multiple frequencies, which can lead to the unintended radiation of harmonics. This means that while they can transmit on various bands, they may not effectively suppress signals at harmonic frequencies, potentially causing interference and reducing overall performance.

### Explanation of the Incorrect Choices:
- **A: They present low impedance on all design frequencies**:  
  This statement is incorrect because multiband antennas are typically designed to present a reasonable impedance across their operating frequencies. They may not present low impedance on all frequencies, but rather a suitable impedance that can be matched with a feedline.

- **B: They must be used with an antenna tuner**:  
  While it is true that some multiband antennas may benefit from an antenna tuner to match impedance, it is not a strict requirement. Many multiband antennas are designed to work well without a tuner on their specified bands.

- **C: They must be fed with open wire line**:  
  This is incorrect as multiband antennas can be fed with various types of feedlines, including coaxial cable. Open wire line is not a requirement for all multiband antennas.

### Summary:
- Multiband antennas are versatile but can have issues with harmonic rejection.
- They do not inherently present low impedance on all frequencies, do not always require an antenna tuner, and can be fed with different types of feedlines.

This understanding will help you in recognizing the characteristics and limitations of multiband antennas for the Extra Class exam.