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Mayday: A most urgent distress call (français: «m'aidez» - help me) solely for use in life-threatening emergencies. The CW equivalent is SOS. Lesser incidents are signalled using "pan-pan" (français: «panne» - mechanical breakdown, typically a ship drifting helplessly or an aircraft's emergency landing) or "security" (français: «sécurité» - safety, for warnings such as reports of icebergs in shipping lanes).
Megahertz: One million Hertz (HZ) - same as one million cycles per second.
Microphone : A device that converts voice into electrical signals.
Microwave : signals above 1GHZ in frquency.
Microwave Bands : Division of the range of frequencies above 1GHz into a series of bands.
Mobile: A mode of operation in which a station may operate from a moving vehicle (mobile) or a ship at sea (maritime mobile). In CW, mobile and maritime mobile operators may identify using /M and /MM respectively. As a class of transceiver, mobile units fall between hand-held (5W or less) and base stations in both physical size and transmitted power. These units typically operate from the vehicle's battery and often transmit 50 watts or more using a vertical antenna mounted to use the metal vehicle body as a ground plane.
Mode : The way electromagnetic waves are changed so that transmission of information is possible.
Modulation : The process of adding information to a carrier signal.
Modulator : A device that adds information to a carrier signal.
MP73N : A narrow band SSTV Mode
MUF : Maximum usable frequency. The highest frequency at which a skywave is reflected back to Earth at the ionosphere under current propagation conditions. Can vary rapidly depending on time of day and sunspot activity.
MW : Medium Wave, 300-3000 kHz. This range includes the 160 metres amateur band, the AM broadcast band and the 2182 kHz marine radiotelephone band.