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 frequency.
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 communications between two points.
LUMPED CONSTANTS—The properties of inductance, capacitance, and resistance in a transmission line.
LUMPED-IMPEDANCE TUNING—The insertion 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.
AI-5
