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Alternating current (ac)
Electrical current that flows first in one direction in a wire and then in the other. The applied voltage is also changing polarity. This direction reversal continues at a rate that depends on the frequency of the ac.

Amateur operator
A person holding a written authorization to be the control operator of an amateur station.

Amateur service
A radiocommunication service for the purpose of self-training, intercommunication and technical investigations carried out by amateurs, that is, duly authorized persons interested in radio technique solely with a personal aim and without pecuniary interest.

Amateur station
A station licensed in the amateur service, including necessary equipment, used for amateur communication.

Ammeter
A test instrument that measures current

Ampere (A)
The basic unit of electrical current. Current is a measure of the electron flow through a circuit. If we could count electrons, we would find that if there are 6.24 X 1018 electrons moving past a point in one second, we have a current of one ampere. We abbreviate amperes as amps. (Numbers written as a multiple of some power are expressed in exponential notation, as shown here.

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 (535-1710 kHz). Few amateurs use double-sideband voice AM, but a variation, known as single sideband, is very popular.

Antenna
A device that picks up or sends out radio frequency energy.

Antenna switch
A switch used to connect one transmitter, receiver or transceiver to several different antennas.

Antenna tuner
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.

Autopatch
A device that allows repeater users to make telephone calls through a repeater.

Balun
Contraction for balanced to unbalanced. A device to couple a balanced load to an unbalanced source, or vice versa.

Band spread
A receiver quality used to describe how far apart stations on different nearby frequencies will seem to be. We usually express band spread as the number of kilohertz that the frequency changes per tuning-knob rotation. Band spread and frequency resolution are related. The amount of band spread determines how easily signals can be tuned.

Band-pass filter
A circuit that allows signals to go through it only if they are within a certain range of frequencies. It attenuates signals above and below this range.

Bandwidth
The width of a frequency band outside of which the mean power is attenuated at least 26 dB below the mean power of the total emission, including allowances for transmitter drift or Doppler shift. Bandwidth describes the range of frequencies that a radio transmission occupies.

Battery
A device that converts chemical energy into electrical energy.

Beacon station
An amateur station transmitting communications for the purposes of observation of propagation and reception or other related experimental activities.

Beam antenna
A directional antenna. A beam antenna must be rotated to provide coverage in different directions.

Beat-frequency oscillator (BFO)
A receiver circuit that provides a signal to the detector. The BFO signal mixes with the incoming signal to produce an audio tone for CW reception. A BFO is needed to copy CW and SSB signals.

Block diagram
A drawing using boxes to represent sections of a complicated device or process. The block diagram shows the connections between sections.

Broadcasting
Transmissions intended to be received by the general public, either direct or relayed.