Question
Which of the following types of modulation is used for data emissions below 30 MHz?
Answer Options
- A) DTMF tones modulating an FM signal
- B) FSK
- C) Pulse modulation
- D) Spread spectrum
Correct Answer: B
Explanation
Data emissions on the HF bands (below 30 MHz) are subject to stringent bandwidth limitations to ensure spectrum efficiency. The modulation methods used must be narrow-band and compatible with propagation characteristics common to HF, where signals may experience fading and interference.
The most commonly used and authorized modulation type for data emissions below 30 MHz is FSK (Frequency-Shift Keying). In FSK, the digital data (binary 1s and 0s) is represented by shifting the carrier or audio tone between two discrete frequencies. This simple and robust method is used by modes like RTTY, PACTOR, and some AFSK modes that are overlaid onto an SSB signal.
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