DIRECTIONAL ANTENNAAn antenna that radiates most effectively in only one direction .
DIRECTIONAL COUPLERA device that samples the energy traveling in a waveguide in one direction
DIRECTIVITYThe ability of an antenna to radiate or receive more energy in some directions than in
others. The degree of sharpness of the antenna beam   .
DIRECTLY HEATED CATHODEA 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 .
DIRECTORThe parasitic element of an array that reinforces energy coming from the driver element
DIRECT SHORTSame as SHORT CIRCUIT .
DISCRETE COMPONENTSIndividual transistors, diodes, resistors, capacitors, and inductors .
DISCRIMINATORA circuit in which amplitude variations are derived in response to phase or frequency
DISPERSIONThe refraction of light waves that causes the different frequencies to bend at slightly
different angles .
DISPLACEMENT CURRENTThe current that appears to flow through a capacitor .
DISTILLED WATERWater that has been purified through a process of evaporation and condensation
DISTORTIONAny unwanted change between an input signal and output signal  .
DISTRIBUTED CONSTANTSThe constants of inductance, capacitance, and resistance in a
transmission line. They are spread along the entire length of the line and cannot be distinguished
DISTRIBUTIVE LAWIn 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) .
DOMAIN THEORYA 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 .
DOMINANT MODEThe easiest mode to produce in a waveguide, and the most efficient mode in terms
of energy transfer .
DONORAn impurity that can make a semiconductor material an N-type by donating extra "free"
electrons to the conduction band .
DONOR IMPURITYSee PENTAVALENT IMPURITY .
DOORKNOB TUBEAn 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 .