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576 radial ground • radioactive
radial ground An earth connection composed of
radials buried in the ground.
radial lead A lead (pigtail) attached perpendicular
to the axis of a component, such as a resistor or
capacitor.
radian Abbreviation, rad. The angle at the center of
a circle subtended by an arc whose length is
equal to the radius. Equal to approximately
57.2958 degrees.
radiance The radiant flux emitted by an object. Radiance
is measured in terms of the amount of energy
contained in a unit solid angle (steradian)
with the source at the apex.
radians-to-degrees conversion The conversion of
radian angular measure into degrees. To change
radians to degrees, multiply the number of degrees
by 57.2958. Thus, 0.7854 radian = 45 degrees.
Compare DEGREES-TO-RADIANS CON-
VERSION.
radiant efficiency The ratio of the radiant energy
emitted by a source to the energy consumed by
the source. The radiant energy is generally specified
within a certain range of wavelengths. An example
is the incandescent light bulb, which has
relatively low radiant efficiency in the visible
spectrum between about 750 and 390 nanometers.
radiant energy 1. Any form of energy emitted by a
source and propagated through space as an electromagnetic
disturbance. Included are radio
waves, infrared, visible light, ultraviolet, X rays,
and gamma rays. 2. Electromagnetic disturbances
at infrared and shorter wavelengths.
radiant flux The rate at which radiant energy is
emitted.
radiation 1. The emission of energy or particles
(e.g., waves from an antenna, X rays from an X-
ray tube, energy from a radioactive material, heat
from a body, etc.). 2. Radio waves, infrared, visible
light, ultraviolet, X rays, or gamma rays.
3. Ionizing emissions from radioactive substances
(e.g., alpha particles, beta particles, neutrons,
gamma rays, etc.).
radiation angle The horizontal or vertical angle at
which electromagnetic waves are radiated from
an antenna. Measured between the central axis of
the main lobe and the horizon, or between the
central axis of the main lobe and geographic
north.
radiation belts See VAN ALLEN RADIATION
BELTS.
radiation counter An instrument used for determining
the intensity of atomic-particle radiation,
X rays, or gamma rays. It operates by means of
ionization of a gas in a sealed tube.
radiation field The portion of the electromagnetic
field that is propagated by a radiator, as opposed
to the induction field.
radiation intensity For a directional radio transmitting
antenna, the radiated power per steradian
in a given direction.
Antenna
Average terrain
(horizontal)
radiation angle
Maximum
radiation
Angle
radiation loss Loss of energy through radiation
from a conductor. Also see LOSS.
radiation pattern A graphical representation of
the intensity of the electromagnetic field in various
directions from a radiator, such as a transmitting
antenna. It is usually shown in either the
horizontal plane or a specific vertical plane containing
the antenna. Also see LOBE.
radiation physics The study of radiation and its
effects on matter. Radiation physics is especially
concerned with ionizing radiation, but it can involve
any kind of particle or electromagnetic energy.
radiation pressure Pressure exerted on a surface
by impinging electromagnetic radiation.
radiation resistance The inherent resistance at
the feed point of a resonant radio antenna.
radiation sickness General physiological symptoms
resulting from a short-term overdose of
X rays, gamma rays, or atomic-particle radiation.
radiator 1. The element of an antenna from which
radio energy is directly radiated, as opposed to
the transmission line, lead-in, reflector, or director.
2. See LOUDSPEAKER.
radio 1. Wireless electrical communication, i.e., by
means of electromagnetic waves. 2. See RADIO
RECEIVER. 3. See RADIO TRANSCEIVER. 4. See
RADIO TRANSMITTER. 5. To communicate by radio.
radio- 1. A prefix meaning “pertaining to wireless
electrical communication.” Examples: radiotelephone
and radiotelegraph. 2. A prefix meaning
“using radio waves.” Examples: radiosonde, radiolocator,
and radiothermics. 3. A prefix meaning
“pertaining to using or possessing radioactivity,”
or “pertaining to X rays.” Examples: radiograph,
radioisotope, and radiologist.
radioactive Having the property of emitting alpha,
beta, and (sometimes) gamma rays as the result
of nuclear disintegration. Also see HALF-LIFE.