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1-19 DIRECTIONAL ANTENNA—An antenna that radiates most effectively in only one direction [18]. DIRECTIONAL COUPLER—A device that samples the energy traveling in a waveguide in one direction only [11]. DIRECTIVITY—The ability of an antenna to radiate or receive more energy in some directions than in others. The degree of sharpness of the antenna beam [10] [11] [18]. DIRECTLY HEATED CATHODE—A wire, or filament, designed to emit the electrons that flow from cathode to plate. The filament is designed so that a current is passed through it; the current heats the filament to the point where electrons are emitted [6]. DIRECTOR—The parasitic element of an array that reinforces energy coming from the driver element [10]. DIRECT SHORT—Same as SHORT CIRCUIT [3]. DISCRETE COMPONENTS—Individual transistors, diodes, resistors, capacitors, and inductors [14]. DISCRIMINATOR—A circuit in which amplitude variations are derived in response to phase or frequency variations [18]. DISPERSION—The refraction of light waves that causes the different frequencies to bend at slightly different angles [10]. DISPLACEMENT CURRENT—The current that appears to flow through a capacitor [2]. DISTILLED WATER—Water that has been purified through a process of evaporation and condensation [18]. DISTORTION—Any unwanted change between an input signal and output signal [6] [8]. DISTRIBUTED CONSTANTS—The constants of inductance, capacitance, and resistance in a transmission line. They are spread along the entire length of the line and cannot be distinguished separately [10]. DISTRIBUTIVE LAW—In Boolean algebra the law which states that if a group of terms connected by like operators contains the same variable, the variable may be removed from the terms and associated with them by the appropriate sign of operation (for example, A(B + C) = AB + AC) [13]. DOMAIN THEORY—A theory of magnetism based upon the electron-spin principle. Spinning electrons have a magnetic field. If more electrons spin in one direction than another, the atom is magnetized [1]. DOMINANT MODE—The easiest mode to produce in a waveguide, and the most efficient mode in terms of energy transfer [11]. DONOR—An impurity that can make a semiconductor material an N-type by donating extra "free" electrons to the conduction band [7]. DONOR IMPURITY—See PENTAVALENT IMPURITY [7]. DOORKNOB TUBE—An electron tube that is similar to the acorn tube but larger. The doorknob tube is designed to operate, at high power, in the uhf frequencies [6].