INCIDENT WAVE—(1) The wave that strikes the
surface of a medium; (2) The wave that travels
from the sending end to the receiving end of a
transmission line.
INDUCTION FIELD—The electromagnetic field
produced about an antenna when current and
voltage are present on the same antenna.
INDUCTION LOSSES—The losses that occur when
the electromagnetic field around a conductor cuts
through a nearby metallic object and induces a
current into that object.
INPUT END—The end of a two-wire transmission
line that is connected to a source. Also known
as a GENERATOR END or a TRANSMITTER
END.
INPUT IMPEDANCE—The impedance presented
to the transmitter by the transmission line and
its load.
INTERFERENCE—Any disturbance that produces
an undesirable response or degrades a wave.
IONOSPHERE—The most important region of the
atmosphere extending from 31 miles to 250 miles
above the earth. Contains four cloud-like layers
that affect radio waves.
IONOSPHERIC STORMS—Disturbances in the
earth’s magnetic field that make communications
practical only at lower frequencies.
IONIZATION—The process of upsetting electrical
neutrality.
IRIS—A metal plate with an opening through which
electromagnetic waves may pass. Used as an
impedance matching device in waveguides.
ISOTROPIC RADIATION—The radiation of energy
equally in all directions.
LEAKAGE CURRENT—The small amount of
current that flows between the conductors of a
transmission line through the dielectric.
LOAD END—See OUTPUT END.
LOAD ISOLATOR—A passive attenuator in which
the loss in one direction is much greater than that
in the opposite direction. An example is a ferrite
isolator for waveguides that allow energy to travel
in only one direction.
LOADING—See LUMPED-IMPEDANCE TUNING.
LOBE—An area of a radiation pattern plotted on a
polar-coordinate graph that represents maximum
radiation.
LONG-WIRE ANTENNA—An antenna that is a
wavelength or more long at its operating fre-
quency.
LONGITUDINAL WAVES—Waves in which the
disturbance (back and forth motion) takes place
in the direction of propagation. Sometimes called
compression waves.
LOOP—(1) The curves of a standing wave or antenna
that represent amplitude of current or voltage;
(2) A curved conductor that connects the ends
of a coaxial cable or other transmission line and
projects into a waveguide or resonant cavity for
the purpose of injecting or extracting energy.
LOWEST USABLE FREQUENCY—The minimum
operating frequency that can be used for commu-
nications between two points.
LUMPED CONSTANTS—The properties of
inductance, capacitance, and resistance in a
transmission line.
LUMPED-IMPEDANCE TUNING—The inser-
tion of an inductor or capacitor in series with an
antenna to lengthen or shorten the antenna
electrically. Also known as LOADING.
LOOSE COUPLING—Inefficient coupling of energy
from one circuit to another that is desirable in
some applications. Also called weak coupling.
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