18WuD2x

32 anomalous propagation • antenna current
atmospheric layers. 2. Unusual, bizarre, or unexplainable
electromagnetic-wave propagation (e.g.,
apparent F-layer ionospheric effects in the FM
broadcast band). 3. Rapid fluctuation of a sonar
echo because of variations in propagation.
anoxemia toximeter An electronic instrument for
measuring or alerting against the onset of anoxemia
(deficiency of oxygen in the blood)—especially
in airplane pilots.
AN radio range A navigational facility entailing four
zones of equal signal strength. When the aircraft
deviates from course, an aural Morse-code signal,
A (DIT DAH) or N (DAH DIT) is heard; but when the
aircraft is on course, a continuous tone is heard.
ANSI Acronym for American National Standards Institute.
AN signal The signal provided by an AN radio range
to apprise aircraft pilots of course deviation.
answerback The automatic response of a terminal
station to a remote-control signal.
answer cord In a telephone system, the cord used
for answering subscribers’ calls and incoming
trunk calls.
answering machine A device that automatically
answers a telephone and records an audio message
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 specialized
transducer that converts incoming electromagnetic
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 can be a simple wire or rod, or a complicated
structure. Thousands of geometries and
specifications are possible. The optimum antenna
type for a given situation depends on the communications
frequency, the distance to be covered,
and various other factors.
antenna ammeter An RF ammeter, usually of the
thermocouple type, employed to measure current
flowing to a transmitting antenna.
antenna amplifier 1. A radio-frequency amplifier,
often installed at the antenna, used to boost signals
before they reach a receiver (also called an
RF preamplifier). 2. Occasionally, the first RF amplifier
stage of a receiver, also known as the front
end.
antenna array See ARRAY.
antenna bandwidth The frequency range throughout
which an antenna will operate at a specified
efficiency without needing alteration or adjustment.
antenna beamwidth A measure of the extent to
which a directional antenna focuses a transmitted
electromagnetic field, or focuses its response
to incoming electromagnetic fields. Expressed as
the angle in degrees between opposite half-power
points in the main lobe of the directional pattern.
Usually determined in the horizontal plane, but
occasionally in the vertical plane.
antenna coil The primary coil of the input RF
transformer of a receiver, or the secondary coil of
the output RF transformer of a transmitter.
antenna coincidence The condition in which two
directional antennas are pointed directly toward
each other.
antenna-conducted interference Extraneous signals
generated in a transmitter or receiver and
presented to the antenna, from which they are radiated.
antenna core A ferrite rod or slab around which a
coil of wire is wound to act as a self-contained antenna,
usually in a miniature receiver.
antenna coupler A device consisting of an inductor,
RF transformer, or a combination of inductor(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 antenna
tuner.
antenna coupling Inductive and/or capacitive
coupling used to optimize the transfer of energy
from an antenna to a receiver, or from a transmitter
to an antenna.
antenna current 1. Radio-frequency current flowing
from a transmitter into an antenna. 2. Radiofrequency
current flowing from a receiving
antenna into a receiver.