*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)  

A small wire-loop antenna is often used in direction finding because of its unique radiation pattern. Let's break down the correct answer and why the other choices are incorrect:

### Correct Answer: A: It has a bidirectional null pattern

1. **Why the correct answer is correct:**
   - A small wire-loop antenna has a radiation pattern that features two nulls, or points of minimum signal reception, which are 180 degrees apart. This is known as a bidirectional null pattern. These nulls are useful in direction finding because they help identify the direction of the signal source by pointing towards the nulls.

### Why the other choices are incorrect:

2. **B: It does not have a clearly defined null**
   - This is incorrect because a small wire-loop antenna actually has very well-defined nulls. These nulls are crucial for its use in direction finding, as they help pinpoint the direction of the incoming signal.

3. **C: It is practical for use only on VHF and higher bands**
   - This is incorrect because small wire-loop antennas can be used on a variety of frequency bands, not just VHF and higher. Their size and construction can be adapted for different frequencies, making them versatile for various applications.

4. **D: All these choices are correct**
   - This is incorrect because only choice A is correct. The other choices do not accurately describe the characteristics of a small wire-loop antenna.

### Useful Information:

- **Radiation Pattern:** The radiation pattern of a small wire-loop antenna is typically a figure-eight shape, with nulls at the ends of the loop.
- **Direction Finding:** The nulls are used in direction finding by rotating the antenna until the signal strength is minimized, indicating the direction of the source is perpendicular to the plane of the loop.

Understanding these characteristics helps in effectively using small wire-loop antennas for direction finding tasks.