*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)  

JT65 is a digital mode used in amateur radio communications, particularly known for its ability to decode weak signals. Let's break down the characteristics and why the correct answer is correct:

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

1. **Why the correct answer is correct:**
   - JT65 is specifically designed to work with very weak signals, often below the noise floor. It can decode signals with a signal-to-noise ratio (SNR) as low as -28 dB. This makes it highly effective for long-distance communications where signals are weak.

2. **Why the other choices are incorrect:**
   - **A: Uses only a 65 Hz bandwidth**
     - JT65 actually uses a bandwidth of approximately 177 Hz, not 65 Hz. The "65" in JT65 refers to the number of tones used in its modulation scheme, not the bandwidth.
   - **C: Symbol rate is 65 baud**
     - The symbol rate for JT65 is not 65 baud. It is actually much lower, around 1.46 baud. The mode is designed for slow, reliable communication rather than fast data transfer.
   - **D: Permits fast-scan TV transmissions over narrow bandwidth**
     - JT65 is not used for fast-scan TV. It is a digital mode for text-based communication, not video. Fast-scan TV requires much higher bandwidth than what JT65 provides.

### Additional Information:
- **JT65 Modulation:**
  - JT65 uses a multi-frequency shift keying (MFSK) modulation with 65 tones, which contributes to its ability to decode weak signals.
  
- **Signal-to-Noise Ratio (SNR):**
  - The ability to decode signals at low SNR is a key feature of JT65, making it popular for weak-signal work, such as moonbounce (EME) and long-distance HF communications.

Understanding these characteristics will help you recognize the strengths of JT65 and why it is used in specific amateur radio applications.