*Question E2D05:
What is the characteristic of the JT65 mode?*  

*Correct Answer is  B: Decodes signals with a very low signal-to-noise ratio*  

### AI Response: (may contain errors)  

The correct answer to the question about the characteristic of the JT65 mode is:

**B: Decodes signals with a very low signal-to-noise ratio.**

### Explanation of the Correct Answer:
- **JT65** is a digital mode designed for weak signal communication. It can decode signals with a very low signal-to-noise ratio (SNR), typically around -20 dB or lower. This makes it particularly useful for making contacts under challenging conditions, such as during poor propagation or with low power.

### Why the Other Choices are Incorrect:
- **A: Uses only a 65 Hz bandwidth**  
  This is incorrect because JT65 actually uses a bandwidth of about 2.5 kHz, not just 65 Hz. The name "JT65" refers to the symbol rate, not the bandwidth.

- **C: Symbol rate is 65 baud**  
  While the name suggests a connection to "65," the actual symbol rate of JT65 is 1 tone per 1.5 seconds, which translates to a much lower effective baud rate than 65 baud. The mode is designed to transmit data slowly to ensure reliable decoding of weak signals.

- **D: Permits fast-scan TV transmissions over narrow bandwidth**  
  This is incorrect because JT65 is not used for fast-scan TV transmissions. It is a slow digital mode focused on text and data transmission, not video.

### Summary:
- **JT65** is effective for weak signal communication due to its ability to decode low SNR signals.
- The other options misrepresent the characteristics of the mode, focusing on incorrect bandwidth, symbol rate, and application.

### Useful Rule of Thumb:
- When studying digital modes, remember that modes designed for weak signals (like JT65) prioritize sensitivity and error correction over speed. This is crucial for successful communication in challenging conditions.