*Question E2E01:
Which of the following types of modulation is used for data emissions below 30 MHz?*  

*Correct Answer is  B: FSK*  

### AI Response: (may contain errors)  

For data emissions below 30 MHz, Frequency Shift Keying (FSK) is commonly used. Here's why:

1. **Correct Answer: FSK (Frequency Shift Keying)**
   - **Explanation**: FSK is a type of frequency modulation where digital information is transmitted through discrete frequency changes of a carrier wave. It is well-suited for HF (High Frequency) bands below 30 MHz because it efficiently uses the available bandwidth and is resilient to noise, which is common in these frequencies.

2. **Why the Other Choices are Incorrect:**
   - **A: DTMF tones modulating an FM signal**
     - **Explanation**: DTMF (Dual-Tone Multi-Frequency) is used for telephone signaling and not typically for data transmission in amateur radio, especially below 30 MHz. FM (Frequency Modulation) is also not commonly used for data below 30 MHz due to its wide bandwidth requirements.
   
   - **C: Pulse modulation**
     - **Explanation**: Pulse modulation involves varying the pulse characteristics (such as width, position, or amplitude) to encode data. It is not typically used for HF data emissions due to its complexity and bandwidth requirements.
   
   - **D: Spread spectrum**
     - **Explanation**: Spread spectrum techniques spread the signal over a wide frequency band, which is not efficient for the narrow bandwidths available below 30 MHz. It is more commonly used in higher frequency bands where bandwidth is less of a constraint.

3. **Useful Formulas or Rules of Thumb:**
   - **FSK Bandwidth**: The bandwidth of an FSK signal can be estimated using Carson's Rule:
     $$ BW = 2 \times (\Delta f + f_m) $$
     where:
     - $\Delta f$ = frequency deviation
     - $f_m$ = maximum modulating frequency

   - **Efficiency**: FSK is efficient for narrowband HF communications, making it a preferred choice for data below 30 MHz.

Understanding these concepts will help you recognize why FSK is the appropriate choice for data emissions in this frequency range.