*Question E9H11:
What feature of a cardioid pattern antenna makes it useful for direction-finding antennas?*  

*Correct Answer is  B: A single null*  

### AI Response: (may contain errors)  

The correct answer to the question about the cardioid pattern antenna is **B: A single null**. Here's why this is the correct choice and why the others are not:

### Explanation of the Correct Answer (B: A single null)
- **Single Null**: A cardioid antenna pattern has a unique characteristic of having a single null point, which is the direction where the antenna does not radiate any energy. This feature is particularly useful in direction-finding applications because it allows the operator to easily identify the direction of the incoming signal. By rotating the antenna, the operator can determine the direction of the signal source by finding the null point.

### Explanation of Incorrect Choices
- **A: A very sharp peak**: While some antennas may have sharp peaks in their radiation patterns, the cardioid pattern is more defined by its null rather than a peak. A sharp peak does not provide the same directional sensitivity as a null does for locating signals.

- **C: Broadband response**: Cardioid antennas are not specifically known for their broadband response. Their primary feature is the directional sensitivity provided by the null. Other antenna types, like log-periodic or dipole antennas, are more recognized for their broadband capabilities.

- **D: High radiation angle**: Cardioid antennas typically have a lower radiation angle in the direction of the peak and a null in the opposite direction. High radiation angles are not a defining feature of cardioid antennas and do not contribute to their effectiveness in direction finding.

### Summary
In summary, the cardioid pattern's single null is what makes it particularly effective for direction-finding applications, allowing users to pinpoint the direction of incoming signals. The other options do not accurately describe the key features of a cardioid antenna.