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

Choices:  
A: Uses only a 65 Hz bandwidth  
B: Decodes signals with a very low signal-to-noise ratio  
C: Symbol rate is 65 baud  
D: Permits fast-scan TV transmissions over narrow bandwidth    

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

To understand JT65:

1) Why the correct answer is correct
- JT65 was designed for Moonbounce (EME) and weak-signal work. It uses long symbol durations, tight tone spacing, and strong forward error correction (FEC), allowing decoding at very low SNR, typically around −24 dB (in a 2.5 kHz reference bandwidth). This is its hallmark characteristic.

2) Why the other choices are incorrect
- A: Uses only a 65 Hz bandwidth  
  Incorrect. JT65 occupies roughly 175 Hz, not 65 Hz. The “65” refers to the number of tones, not the bandwidth.
- C: Symbol rate is 65 baud  
  Incorrect. The symbol rate is much lower. For JT65A, it’s about 2.69 baud (long symbols).
- D: Permits fast-scan TV transmissions over narrow bandwidth  
  Incorrect. JT65 is a weak-signal digital QSO mode, not a TV mode. Fast-scan TV is wideband and unrelated to JT65.

3) Useful formulas and rules of thumb
- JT65 bandwidth (approximate):
  $$B \approx (M-1)\,\Delta f$$
  where $M=65$ tones and $\Delta f \approx 2.69\ \text{Hz}$ (JT65A), giving $B \approx 64 \times 2.69 \approx 172\ \text{Hz}$.
- Symbol rate (baud):
  $$\text{baud} = \frac{1}{T_s}$$
  JT65A uses long symbol durations, $T_s \approx 0.372\ \text{s}$, so baud ≈ 2.69.
- Weak-signal insight (why negative SNR decodes are possible):
  $$\frac{E_b}{N_0} = \text{SNR} \cdot \frac{B}{R}$$
  With low data rate $R$, narrow occupied bandwidth $B$, long integrations, and strong FEC, JT65 achieves decodes at very low measured SNR.
- Decode thresholds (rule of thumb, 2.5 kHz ref BW):  
  JT65 ≈ −24 dB; FT8 ≈ −20 dB; WSPR ≈ −28 dB.

Summary: JT65’s defining feature is its ability to decode signals far below the noise floor, thanks to narrow signaling, long symbol times, and strong FEC.