Page 47 - The Illustrated Dictionary of Electronics

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32 anomalous propagation • antenna current

atmospheric layers. 2. Unusual, bizarre, or unex- antenna amplifier 1. A radio-frequency amplifier,
plainable electromagnetic-wave propagation (e.g., often installed at the antenna, used to boost sig-
apparent F-layer ionospheric effects in the FM nals before they reach a receiver (also called an
broadcast band). 3. Rapid fluctuation of a sonar RF preamplifier). 2. Occasionally, the first RF am-
echo because of variations in propagation. plifier stage of a receiver, also known as the front
anoxemia toximeter An electronic instrument for end.
measuring or alerting against the onset of anox- antenna array See ARRAY.
emia (deficiency of oxygen in the blood)—espe- antenna bandwidth The frequency range through-
cially in airplane pilots. out which an antenna will operate at a specified
AN radio range A navigational facility entailing four efficiency without needing alteration or adjust-
zones of equal signal strength. When the aircraft ment.
deviates from course, an aural Morse-code signal, antenna beamwidth A measure of the extent to
A (DIT DAH) or N (DAH DIT) is heard; but when the which a directional antenna focuses a transmit-
aircraft is on course, a continuous tone is heard. ted electromagnetic field, or focuses its response
ANSI Acronym for American National Standards In- to incoming electromagnetic fields. Expressed as
stitute. the angle in degrees between opposite half-power
AN signal The signal provided by an AN radio range points in the main lobe of the directional pattern.
to apprise aircraft pilots of course deviation. Usually determined in the horizontal plane, but
answerback The automatic response of a terminal occasionally in the vertical plane.
station to a remote-control signal. antenna coil The primary coil of the input RF
answer cord In a telephone system, the cord used transformer of a receiver, or the secondary coil of
for answering subscribers’ calls and incoming the output RF transformer of a transmitter.
trunk calls.
answering machine A device that automatically
answers a telephone and records an audio mes-
sage from the caller.
answer lamp A telephone switchboard lamp that
lights when an answer cord is plugged into a line
jack; it switches off when the telephone answers
and lights when the call is completed.
ant Abbreviation of ANTENNA.
antenna In a communications system, a special-
ized transducer that converts incoming electro-
magnetic fields into alternating electric currents
having the same frequencies (receiving antenna),
or converts an alternating current at a specific
frequency into an outgoing electromagnetic field
at the same frequency (transmitting antenna). An antenna coincidence The condition in which two
antenna can be a simple wire or rod, or a compli- directional antennas are pointed directly toward
cated structure. Thousands of geometries and each other.
specifications are possible. The optimum antenna antenna-conducted interference Extraneous sig-
type for a given situation depends on the commu- nals generated in a transmitter or receiver and
nications frequency, the distance to be covered, presented to the antenna, from which they are ra-
and various other factors. diated.
antenna ammeter An RF ammeter, usually of the antenna core A ferrite rod or slab around which a
thermocouple type, employed to measure current coil of wire is wound to act as a self-contained an-
flowing to a transmitting antenna. tenna, usually in a miniature receiver.
antenna coupler A device consisting of an induc-
tor, RF transformer, or a combination of induc-
tor(s) and capacitor(s), used to match the
impedance of an antenna to that of a transmitter
or receiver. Also known as a transmatch or an-
tenna tuner.
antenna coupling Inductive and/or capacitive
coupling used to optimize the transfer of energy
from an antenna to a receiver, or from a trans-
mitter to an antenna.
antenna current 1. Radio-frequency current flow-
ing from a transmitter into an antenna. 2. Radio-
frequency current flowing from a receiving
antenna into a receiver.