FEED POINT— The point on an antenna at which the RF cable that carries the signal from the transmitter is connected. FOT  (FREQUENCY  OF  OPTIMUM  TRANS- MISSION)— The most reliable frequency for propagation at a specific time. FREQUENCY—  The number of complete cycles per unit of time. FREQUENCY  DIVERSITY—  The method  in  which  the information  signal  is  transmitted  and  received  on  two separate  radio  frequencies  simultaneously  to  take advantage of the fact that fading does not occur simultaneously on different frequencies. FSK (FREQUENCY-SHIFT KEYING)— The process of shifting the incident carrier above and below the carrier frequency to correspond to the marks and spaces of a teleprinter   signal. G GAIN— An increase in signal strength. GIGAHERTZ (GHz)— A unit of frequency equal to 1000 megahertz. GROUND— A term used to denote a common electrical point of zero potential. GROUND-PLANE  ANTENNA—  A type of antenna that uses  aground  plane  (a  metallic  surface)  as  a  simulated ground to produce low-angle radiation. H HALF-WAVE  DIPOLE  ANTENNA—  A common type of  half-wave  antenna  made  from  a  straight  piece  of  wire cut in half. Each half operates at a quarter of the wavelength It is normally omnidirectional with no gain. HERTZ (Hz)— A unit of frequency equal to one cycle per second. HERTZ ANTENNA— An ungrounded half-wave antenna that is installed some distance above ground and positioned either vertically or horizontally. HF (HIGH FREQUENCY)— The band of frequencies from 3MHz to 30MHz. I IMPEDANCE— The total opposition to the flow of alternating   current. INCIDENT WAVE— The RF energy that travels from the transmitter to the antenna for radiation. INDUCTION  FIELD—  The  electromagnetic  field produced around an antenna when current and voltage are present on the antenna. K KILOHERTZ  (kHz)— A unit of frequency equal to 1000 hertz. L LEASAT—  Leased  satellite. LF (LOW FREQUENCY)— The band of frequencies from 30kHz  to  300kHz. LUF (LOWEST USABLE FREQUENCY)— The lowest frequency that can be used at a specific time for ionospheric propagation of radio waves between two specified   points. M MAGNETIC  FIELD—  One of the fields produced when current flows through a conductor or an antenna. MARCONI ANTENNA— A quarter-wave antenna that is operated with one end grounded; it is positioned perpendicular to the Earth. MEGAHERTZ  (MHz)—  A unit of frequency equal to l,000,000  hertz. MF (MEDIUM FREQUENCY)— The band of frequencies from 300kHz to 3MHz. MIRROR IMAGE— The part of the radiated signal of a quarter-wave  antenna  (Marconi  antenna)  appearing  to come from an underground image of the real antenna. This image is also called ground reflection. MODULATED  WAVE—  The  wave  that  results  after  the information from the modulating signal is impressed onto the carrier signal. The wave that is transmitted MODULATION—   The  process   of  adding,  or superimposing, information on an RF carrier wave. MUF (MAXIMUM  USABLE  FREQUENCY)—  The highest operating frequency that can be used at a specific time for successful radio communications between two points. AI-2