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 R from a specified direction. RADHAZ radiation hazards to personnel as defined in ANSI/C95.1-1991 IEEE Stan- RS commonly used symbol for source dard Safety Levels with Respect to Human impedance. Exposure to Radio Frequency Electromag- netic Fields, 3 kHz to 300 GHz. RT commonly used symbol for transfor- mation ratio. radial basis function network a fully R-ALOHA See reservation ALOHA. connected feedforward network with a sin- gle hidden layer of neurons each of which RL Typical symbol for load resistance. computes a nonlinear decreasing function of the distance between its received input and Rabi frequency the characteristic cou- a “center point.” This function is generally pling strength between a near-resonant elec- bell-shaped and has a different center point tromagnetic field and two states of a quan- for each neuron. The center points and the tum mechanical system. For example, the widths of the bell shapes are learned from Rabi frequency of an electric dipole allowed training data. The input weights usually have transition is equal to µE/hbar, where µ is the fixed values and may be prescribed on the electric dipole moment and E is the maxi- basis of prior knowledge. The outputs have mum electric field amplitude. In a strongly linear characteristics, and their weights are driven 2-level system, the Rabi frequency is computed during training. equal to the rate at which population oscil- lates between the ground and excited states. radial intensity histogram a histogram of average intensities for a round object in race condition a situation where multiple circular bands centered at the center of the processes access and manipulate shared data object, with radial distance as the running in- with the outcome dependent on the relative dex. Such histograms are easily constructed, timing of these processes. and, suitably normalised, form the basis for scrutinizing round objects for defects, and raceway a channel within a building for measuring radius and radial distances of which holds bare or insulated conductors. cylindrically symmetrical features. radar an instrument that transmits elec- tromagnetic waves and receives properties of radial system a network of straight wires the reflected electromagnetic wave from the or other conductors radiating from the base target, which can be used to determine the of a vertical monopole antenna that simulates nature and distance to the target. Radar is an the presence of a highly conducting ground acronym that stands for radio detection and plane beneath the antenna. Typically, ra- ranging. dial wires are approximately a quarter wave- length long and are arranged to have equian- radar cross section (RCS) a measure of gular spacing between them. The radial wire the reflective strength of a radar target; usu- ends at the base of the monopole are electri- ally represented by the symbol s, measured cally bonded together and to one conductor in square meters, and defined as 4π times the of the feedline. c 2000 by CRC Press LLC radiated emission an electromagnetic radiation efficiency in antenna theory, the field propagated through space. ratio of radiated power to the amount of input power to the antenna. Has a value between 0 radiating aperture a basic element of an and 1, inclusive. antenna that in itself is capable of effectively radiating or receiving radio waves. Typical radiation intensity in antenna theory, a example of a radiating apertures are a slot, far-field quantity that is a function of angle and horn antennas or a truncated metallic that gives the level of radiation in a specific waveguide. direction. Radiation intensity is the radial component of the time average Poynting vec- radiation the phenomenon by which tor with all the terms associated with the dis- sources generate energy, which propagates tance from the antenna normalized out. The away from them in the form of waves. units are watts per square radian. radiation pattern a plot of the far-field ra- radiation boundary condition (RBC) a diation as a function of the angle phi or theta boundary condition that is imposed to trun- while the other angle is held constant. A ra- cate the computational domain of a differen- diation pattern can be either polar or rectan- tial equation method so that it satisfies the gular, and can be either logarithmic or linear. Sommerfield radiation condition. radiative broadening a spectral line radiation condition the condition that broadening mechanism that arises due to specifies the field behavior at infinity; in fact, spontaneous decay of the excited state. for an unbounded domain it is necessary to specify the field behavior on the surface at radiative heat transfer the process by infinity. By assuming that all sources are which long-wave electromagnetic radiation contained in a finite region, only outgoing transports heat from a surface to its surround- waves must be present at large distances from ings. the sources; hence the field behavior at large distances from the sources must meet the radio astronomy the study of celestial ob- physical requirement that energy travel away jects based on the investigation of their radio from the source region. This requirement is frequency electromagnetic waves spectrum. the Sommerfield “radiation condition” and constitutes a boundary condition on the sur- radio frequency (RF) a general term used face at infinity. Let us denote with A any to refer to radio signals in the general fre- field component transverse to the radial dis- quency range from thousands of cycles per tance r and with k the free-space wavenum- second (kHz) to millions of cycles per second ber. The transverse field of a spherically (MHz). It is also often used generically and diverging wave in a homogeneous isotropic interchangeably with the term microwave to medium decays as 1/r at large distances r distinguish the high frequency AC portion of from the source region; locally, the spher- a circuit or signal from the DC bias signal ical wave behaves like a plane wave travel- or the downconverted intermediate frequency ling in the outward r direction. As such, each (IF) signal. field component transverse to r must behave like exp(−jkr)/r; this requirement may be radio frequency choke an inductance (a phrased mathematically as coil of wire) intended to present a very high impedance at radio frequencies. Such fre- ∂A quencies span the range of kilohertz to hun- lim r + jkA = 0 r→∞ ∂r dreds of megahertz. c 2000 by CRC Press LLC radio frequency integrated circuit (RFIC) radius of curvature one of the parame- integrated circuit designed to operate at ra- ters characterizing the reflecting surface of a dio frequencies as amplifiers, mixers, oscil- spherical mirror. lators, detectors or combinations of above. Typically, RFICs are configured for specific radix the base number in a number sys- application to operate as a complete RF sys- tem. Decimal (radix 10) and binary (radix 2) tem. are two example number systems. radio frequency interference (RFI) radix complement in a system that uses electromagnetic phenomenon that either di- binary (base 2) data negative numbers, can be rectly or indirectly contributes to degrada- represented as the two’s complement of the tion in the performance of a receiver or other positive number. This is also called a true RF system, synonymous with electromag- complement. netic interference. See also electromagnetic interference. Radon transform the Radon transform of a function f(x,y),r(d,φ) is its line integral along a line inclined at angle φ from the y radio horizon the maximum range, from axis and at a distance d from the origin. transmitter to receiver on Earth’s surface, of direct (line-of-sight) radio waves. This is greater than the optical horizon, because the radwaste a contraction for "radioactive radio waves follow a curved path as a result waste," usually referring to mildly radioac- of the continuous refraction it undergoes in tive sludge removed from the coolant in a the atmosphere. nuclear reactor. RAID See redundant array of inexpensive radio local loop See wireless local loop. disks. radio waves electromagnetic radiation RAKE receiver a receiver type in which suitable for radio transmission in the range the received signal from each (or a few domi- of frequencies from about 10 kHz to about nating) resolvable propagation ray is individ- 300 Mhz. ually demodulated and subsequently com- bined into one decision variable according radiography an imaging modality that to some criterion. RAKE receivers are com- uses an X-ray source and collimator to create monly used in wideband transmission sys- a projection image. The image intensity is tems such as spread-spectrum systems where proportional to the transmitted X-ray inten- the large bandwidth allows several rays to be sity. resolved, thus creating a diversity gain (fre- quency diversity). radiology the monitoring and control of RAM See random access memory. radioactivity, especially in regard to human exposure, in a nuclear power plant. RAM neuron a random access memory with n inputs and a single output. The in- radiometer a passive receiver that detects puts define 2n addresses and presentation of energy from a transmitting source or rera- a particular input vector allows the contents diated energy from a target. Typically, ra- of the 1-bit register at that address to be read, diometers are used in remote sensing appli- or to be written into. Training consists of cations. writing 1s or 0s into the 1-bit registers, as c 2000 by CRC Press LLC required for the various input vectors in the Raman–Nath diffraction in acousto- training set. optics, the regime where many diffraction or- ders exist due to the thinness of the acoustic RAMA See resource auction multiple grating relative to the acoustic wavelength in access.