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Malicious (harmful) interference
Intentional, deliberate obstruction of radio transmissions.

Maximum useable frequency (MUF)
The highest-frequency radio signal that will reach a particular destination using sky-wave propagation, or skip. The MUF may vary for radio signals sent to different destinations.

MAYDAY
From the French m'aidez (help me), MAYDAY is used when calling for emergency assistance in voice modes.

Mega
The metric prefix for 106, or times 1,000,000.

Metric prefixes
A series of terms used in the metric system of measurement. We use metric prefixes to describe a quantity as compared to a basic unit. The metric prefixes indicate multiples of 10.

Metric system
A system of measurement developed by scientists and used in most countries of the world. This system uses a set of prefixes that are multiples of 10 to indicate quantities larger or smaller than the basic unit.

Micro
The metric prefix for 10- 6, or divide by 1,000,000.

Microphone
A device that converts sound waves into electrical energy.

Milli
The metric prefix for 10 - 3, or divide by 1000.

Mobile device
A radio transmitting device designed to be mounted in a vehicle. A push-to-talk (PTT) switch activates the transmitter.

Modem
Short for modulator/demodulator. A modem modulates a radio signal to transmit data and demodulates a received signal to recover transmitted data.

Modulate
To vary the amplitude, frequency, or phase of a radio-frequency signal.

Modulation
The process of varying an RF carrier in some way (the amplitude or the frequency, for example) to add an information signal to be transmitted.

Monitor mode
One type of packet radio receiving mode. In monitor mode, everything transmitted on a packet frequency is displayed by the monitoring TNC. This occurs whether or not the transmissions are addressed to the monitoring station.

Morse code
see CW.

Multimeter
An electronic test instrument used to measure current, voltage and resistance in a circuit. Describes all meters capable of making these measurements, such as the volt-ohm-milliammeter (VOM), vacuum-tube voltmeter (VTVM) and field-effect transistor VOM (FET VOM).

Multimode transceiver
Transceiver capable of SSB, CW and FM operation.

National Electrical Code
A set of guidelines governing electrical safety, including antennas.

Network
A term used to describe several packet stations linked together to transmit data over long distances.

Nonionizing radiation
Electromagnetic radiation that does not have sufficient energy to knock electrons free from their atoms. Radio frequency (RF) radiation is nonionizing.

NPN transistor
A transistor that has a layer of P-type semiconductor material sandwiched between layers of N-type semiconductor material.