R
R
S
commonly used symbol for source
impedance.
R
T
commonly used symbol for transfor-
mation ratio.
R-ALOHA
See
reservation ALOHA
.
R
L
Typical symbol for load resistance.
Rabi frequency
the characteristic cou-
pling strength between a near-resonant elec-
tromagnetic field and two states of a quan-
tum mechanical system. For example, the
Rabi frequency of an electric dipole allowed
transition is equal to
µE/hbar, where µ is the
electric dipole moment and
E is the maxi-
mum electric field amplitude. In a strongly
driven 2-level system, the Rabi frequency is
equal to the rate at which population oscil-
lates between the ground and excited states.
race condition
a situation where multiple
processes access and manipulate shared data
with the outcome dependent on the relative
timing of these processes.
raceway
a channel within a building
which holds bare or insulated conductors.
radar
an instrument that transmits elec-
tromagnetic waves and receives properties of
the reflected electromagnetic wave from the
target, which can be used to determine the
nature and distance to the target. Radar is an
acronym that stands for radio detection and
ranging.
radar cross section (RCS)
a measure of
the reflective strength of a radar target; usu-
ally represented by the symbol
s, measured
in square meters, and defined as 4
π times the
ratio of the power per unit solid angle scat-
tered in a specific direction of the power unit
area in a plane wave incident on the scatterer
from a specified direction.
RADHAZ
radiation hazards to personnel
as defined in ANSI/C95.1-1991 IEEE Stan-
dard Safety Levels with Respect to Human
Exposure to Radio Frequency Electromag-
netic Fields, 3 kHz to 300 GHz.
radial basis function network
a fully
connected feedforward network with a sin-
gle hidden layer of neurons each of which
computes a nonlinear decreasing function of
the distance between its received input and
a “center point.” This function is generally
bell-shaped and has a different center point
for each neuron. The center points and the
widths of the bell shapes are learned from
training data. The input weights usually have
fixed values and may be prescribed on the
basis of prior knowledge. The outputs have
linear characteristics, and their weights are
computed during training.
radial intensity histogram
a histogram
of average intensities for a round object in
circular bands centered at the center of the
object, with radial distance as the running in-
dex. Such histograms are easily constructed,
and, suitably normalised, form the basis for
scrutinizing round objects for defects, and
for measuring radius and radial distances of
cylindrically symmetrical features.
radial system
a network of straight wires
or other conductors radiating from the base
of a vertical monopole antenna that simulates
the presence of a highly conducting ground
plane beneath the antenna.
Typically, ra-
dial wires are approximately a quarter wave-
length long and are arranged to have equian-
gular spacing between them. The radial wire
ends at the base of the monopole are electri-
cally bonded together and to one conductor
of the feedline.
c
2000 by CRC Press LLC