*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)  

Multiband antennas are designed to operate on multiple frequency bands, which can be very convenient for amateur radio operators. However, they come with certain disadvantages, one of which is poor harmonic rejection.

### Explanation:

1. **Correct Answer: D - They have poor harmonic rejection**
   - Multiband antennas are often designed to resonate at multiple frequencies, which can include harmonics of the fundamental frequencies. This means they may inadvertently transmit or receive signals on harmonic frequencies, leading to interference and reduced performance on the desired band.

2. **Why the other choices are incorrect:**
   - **A: They present low impedance on all design frequencies**
     - This is incorrect because multiband antennas are typically designed to have a reasonable impedance match on their intended bands. They do not inherently present low impedance on all design frequencies.
   - **B: They must be used with an antenna tuner**
     - While an antenna tuner can be beneficial, it is not a requirement for all multiband antennas. Some are designed to work without a tuner on their specified bands.
   - **C: They must be fed with open wire line**
     - Multiband antennas can be fed with various types of transmission lines, including coaxial cable. Open wire line is not a necessity for all multiband antennas.

### Additional Notes:

- **Harmonic Rejection:**
  - Harmonics are integer multiples of a fundamental frequency. For example, if an antenna is designed for 7 MHz, it might also resonate at 14 MHz, 21 MHz, etc., which are harmonics.
  - Poor harmonic rejection can lead to unwanted transmission or reception on these harmonic frequencies, causing interference.

Understanding these characteristics helps in selecting the right antenna for your needs and mitigating potential issues with harmonic interference.