What emission must be used when operating on the MF distress and calling voice frequency?

When operating on the MF (Medium Frequency) distress and calling voice frequency, the appropriate emission mode is and must be amplitude modulation (A3E). This mode is traditionally used for voice communications in the MF band, particularly for maritime distress and calling frequencies, where reliable voice communication is crucial.

Using amplitude modulation allows for robust communication over distances and provides clear voice signals, which are essential during emergencies. Historical context supports this choice; A3E has been the standard for voice communications in maritime applications due to its effectiveness in ensuring that messages can be heard over potential interference and noise present on these frequencies.

Single sideband telephony (J3E), while efficient, is not mandatory for distress communications and is generally used for more efficient bandwidth utilization. On-off keying (A1A) is primarily used for Morse code and digital transmissions, making it unsuitable for voice communication. Frequency modulation (F3E) is typically employed in VHF communications rather than MF, where amplitude modulation is focused.

In summary, A3E is the designated mode for reliable MF voice communication, especially in distress situations, ensuring that all mariners can communicate effectively when safety is at stake.