*Question E9H05:
What challenge is presented by a small wire-loop antenna for direction finding?*  

*Correct Answer is  A: It has a bidirectional null pattern*  

### AI Response: (may contain errors)  

The correct answer to the question about the challenges presented by a small wire-loop antenna for direction finding is:

**A: It has a bidirectional null pattern.**

### Explanation of the Correct Answer:
- A small wire-loop antenna typically exhibits a bidirectional null pattern, meaning it has two main lobes of radiation and two nulls (areas of minimal reception) that are 180 degrees apart. This can complicate direction finding because it may not provide a clear indication of the direction of the incoming signal, as signals can be received from two opposite directions equally well.

### Explanation of the Incorrect Choices:
- **B: It does not have a clearly defined null.**
  - This statement is incorrect because a small wire-loop antenna does have defined nulls, albeit they are bidirectional. The presence of nulls is a characteristic of the antenna's radiation pattern, which can be used for direction finding, even if it is not straightforward.

- **C: It is practical for use only on VHF and higher bands.**
  - This choice is misleading. While small wire-loop antennas are often more effective at VHF and higher frequencies due to their size relative to the wavelength, they can still be used at lower frequencies. However, their performance may not be as efficient or practical at those lower frequencies.

- **D: All these choices are correct.**
  - Since choices B and C are incorrect, this option cannot be correct either. Therefore, not all choices are valid.

### Useful Rules of Thumb:
- **Antenna Gain and Patterns:**
  - The gain of a small loop antenna is typically low, and its radiation pattern can be complex. Understanding the pattern is crucial for effective direction finding.
  
- **Direction Finding Techniques:**
  - When using a loop antenna for direction finding, it is important to rotate the antenna to identify the nulls and lobes accurately. This helps in determining the direction of the incoming signal.

In summary, the small wire-loop antenna's bidirectional null pattern is a significant challenge for direction finding, while the other options either misrepresent the antenna's characteristics or are incorrect.