Named after André-Marie Ampère (short history).
Amphour(s) (Ah) (mAh).
The storage capacity of a rechargable battery or cell. The measurement denotes how long a new cell or battery can supply one Amp for. i.e. a 20 Ah battery would be able to supply 1 amp for 20 hours, conversely it could also supply 20 amps for 1 hour. mAh (milliAmphour(s)) is used on smaller batteries and cells, denoting how long 1 milliAmp can be supplied for.
Amplitude Modulation (AM) .
A method of combining an information signal and an RF (radio-frequency) carrier. In double-sideband voice AM transmission, we use the voice information to vary (modulate) the amplitude of an RF carrier.
Shortwave broadcast stations use this type of AM, as do stations in the Standard Broadcast Band (535-1710kHz). Few amateurs use double-sideband voice AM, but a variation, known as single sideband (SSB), is very popular.
AMSAT.
Amateur Radio SATellite organizations world-wide, but in particular the Radio Amateur Satellite Corporation (AMSAT-NA) with its headquarters at Silver Spring, Maryland, near Washington DC. AMSAT organizations design, build, arrange launches for, and then operate (command) satellites carrying amateur radio payloads. Other informally affiliated national organizations exist, such as AMSAT Germany (AMSAT-DL) and AMSAT Japan (JAMSAT).
AMTOR.
See: Amateur Teletype Over Radio.
AMU.
Antenna Matching Unit. See: Antenna Tuning unit.
Analog.
See: Analogue.
Analogue (Analog).
In electronic systems it is a continuously variable signal. In contrast to digital signals which usually take only two different levels. The term analogue describes the proportional relationship between a signal and a voltage or current that represented the signal.
ANF.
See: Automatic Notch Filter.
ANL.
See: Automatic Noise Limiter.
ANSI.
See: American National Standards Institute.
Antenna.
A device that picks up or sends out radio frequency energy. Can be 'teamed' with other antennas for the same frequency into an Antenna Array.
Schematic symbol: 
Antenna Array.
A grouping of 2 or more antennas for the same frequency band all coupled to the same receiver, transmitter or transceiver, and used by the equipment at the same time.
Antenna Farm.
A term used to describe the area where antennas are located. Often, in USA, it could quite litterally be the size of a "farm" as we understand them in the UK!
Antenna Ground System.
A term used for a RF reference potential for some types of antennas. Most unbalanced or asymmetrical antennas need a good RF ground
Antenna Impedance.
The impedance of an antenna at its resonant frequency. An antenna's impedance fluctuates with the frequency of operation. Most transceivers require that the connected antenna should be of 50Ω impedance.
Antenna Match.
When the antenna's impedance at resonance is at optimum performance for your transmitter output circuit.
Antenna Matching.
The 'art' of tuning the antenna's impedance at resonance to optimum performance for your transmitter output circuit either by physically changing the length of the antenna or by using an ATU.
Antenna Matching Network.
See: Antenna Tuning Unit.
Antenna Separation.
The physical spacing between transmit and receive antennas, when separate antennas are used.
Antenna Tuner.
See: Antenna Tuning Unit.
AMU.
See: Antenna Tuning Unit.
Antenna Switch.
A switch used to connect one transmitter, receiver or transceiver to several different antennas.
Antenna Tuner / Antenna Tuning Unit (ATU).
Sometimes called an Antenna Matching Unit (AMU). A device that matches the antenna system input impedance to the transmitter, receiver or transceiver output impedance. Also called an Antenna Matching Network, Impedance Matching Network or Transmatch.
APC.
See: Automatic Power Control.
APO.
See: Automatic Power Off.
APRS.
See: Automatic Packet Reporting System.
ARDF.
See: Amateur Radio Direction Finding.
ARQ.
See: Automatic Repeat Request.
ARRL.
American Radio Relay League, the national amateur radio organization in the USA. Founded in May 1914 by Hiram Percy Maxim.
ARS.
See: Automatic Repeater Shift.
ARTS.
See: Automatic Range Transponder System.
ASCII.
See: American Standard Code for Information Interchange.
ASL.
Quite simply: Above Sea Level. Usually refers to antenna height.
ATT.
See: Attenuator.
Attenuator (ATT).
A device designed to reduce the level, or strength, of a signal.
ATU.
See: Antenna tuning unit.
ATV.
See: Amateur Television.
Audio Frequency (AF).
The range of frequencies between 20Hz 20,000Hz, the human hearing range.
Audio Frequency Shift Keying (AFSK).
A modulation technique by which digital data is represented as changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones:
AFSK differs from regular frequency-shift keying in that the modulation is performed at baseband frequencies. In radio applications, the AFSK-modulated signal is normally used to modulate an RF carrier (using a conventional technique, such as AM FM
See also: PSK, FSK, MSK and the Digital Modes page.
Audion Tube.
The first vacuum tube (valve) invented by Lee De Forest in 1906. Unfortunately it was not until 1912 that the Audion Tube was understood after Edwin Howard Armstrong experimented with it and discovered its properties and devised a circuit (regenerative circuit) to make good use of it.
De Forest was determined to retain control of the use of his invention and, after several court cases, was granted ownership of the patent for the regenerative design.
Auto ATU.
See: Automatic Antenna Tuning Unit.
Autodialer.
Outputs a sequence of stored, preprogrammed DTMF tones.
Auto Power Off.
See: Automatic Power Off.
Auto Repeater Shift.
See: Automatic Repeater Shift.
Automatic Antenna Tuning Unit (AATU / Auto ATU).
An Automatic Antenna Tuning Unit, controlled by a CPU.
Automatic Frequency Control (AFC).
A method of automatically maintaining tuning of a radio receiver to desired signal frequency. If a receiver is nearly tuned to the desired signal, a circuit in the receiver develops an error voltage proportional to the degree to which the receiver is mistuned. This error voltage is then fed back to the tuning circuit in such a way that the tuning error is reduced. In most frequency modulation (FM) detectors an error voltage of this type is easily available. In tuning circuits that use frequency synthesis, in which digital circuitry produces precise signals, automatic tuning control is not necessary.
Automatic Gain Control (AGC).
A feedback voltage in the receiver circuit to prevent fading of a received signal.
Automatic Level Control (ALC).
A feedback voltage in the transmitter's output amplifier used to prevent amplifier overdriving. Also used as feedback from a linear amplifier back to the exciter again to prevent overdriving.
Automatic Notch Filter (ANF).
An automatic version of a Notch Filter.
Automatic Noise Limiter (ANL).
A circuit that, when switched on, eliminates impulse and static noise peaks on the receiver.
Automatic Packet Reporting System (APRS).
An amateur radio based system for digital communications, most commonly used to automatically report the position (GPS coordinates) of a person or object, or weather data at a remote location. It was developed by Bob Bruninga, WB4APR, at the United States Naval Academy. The acronym 'APRS' was originally derived from his callsign, and subsequently expanded to 'Automatic Position Reporting System'. As the system was modified and expanded to handle non-position packet data, the meaning of the 'P' in the acronym was again changed to 'Packet' rather than 'Position'. Today, it is common for amateurs to use either expansion of the acronym.
Automatic Position Reporting System.
See: Automatic Packet Reporting System.
Automatic Power Control (APC).
A circuit that limits the current of a power amplifier circuit to prevent damage to finals in high SWR conditions.
Automatic Power Off (APO).
An inbuilt feature in some equipment that switches the equipment off after a predetermined time.
Automatic Range Transponder System (ARTS).
Uses DCS signaling to inform both parties when you and another ARTS™ equipped station are within communications range. This may be particularly useful during Search and Rescue situations, where is important to stay in contact with other members of your group.
Automatic Repeat Request (ARQ) .
An error control method for data transmission which makes use of acknowledgments and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame. Usually, if the transmitter does not receive an acknowledgment before the timeout occurs (within a reasonable amount of time after sending the data frame), it retransmits the frame until it is either correctly received or the error persists beyond a predetermined number of retransmissions.
Automatic Repeater Shift (ARS).
Automatically selects the input shift for repeater use when you select a repeaters output frequency.
AWG.
See: American Wire Gauge.
Azimuthal-Equidistant Projection Map.
A map made with its center at one geographic location and the rest of the continents projected from that point. Also called a great-circle map, this map is the most useful type for determining where to point a directional antenna to communicate with a specific location.
Glossary page selection.
Index & Appendices.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0-9