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
