Dilatometry: a powerful tool for the study of defects in ultrafine-grained metals

Vacancies, dislocations, and interfaces are structural defects that are deliberately introduced into solids during grain refinement processes based on severe plastic deformation (SPD). Specific combinations of these defects determine the improved mechanical properties of the obtained ultrafine-grained materials. High-precision, non-equilibrium dilatometry, i.e., measurement of the irreversible macroscopic length change upon defect annealing, provides a powerful technique for the characterization and the study of the kinetics of these defects. It is applied to determine absolute concentrations of vacancies, to characterize dislocation processes, and to assess grain boundary excess volume in pure, FCC and BCC ultrafine-grained metals processed by SPD.     

Recrystallization kinetics of ultrafine-grained Ni studied by dilatometry

The release of excess volume upon recrystallization of ultrafine-grained Ni deformed by high-pressure torsion was measured with a high-precision difference-dilatometer employing constant heating rates in the range from 0.3 to 10 K min^?1. The kinetics of the recrystallization process was analyzed according to the Johnson–Mehl–Avrami–Kolmogorov theory adapted to the case of constant heating rates. An effective Avrami exponent of 2 and a value of 1.20 eV for the activation energy of recrystallization was determined. Analysis by the Kissinger method yielded the same result for the activation energy.     

Methods for null control and their effects on the radiation pattern

Several methods for synthesis of array antenna patterns with prescribed nulls are reviewed. Methods based on full amplitude/phase control at each array element and methods with a restricted number of degrees of freedom are compared, with attention focused on the characteristic features of the resultant patterns. These features are largely independent of any algorithm for achieving the nulls, and therefore they also provide a perspective on the performance of adaptive antenna systems, which employ these various control architectures.     

Mutual coupling compensation in small array antennas

A technique to compensate for mutual coupling in a small array is developed and experimentally verified. Mathematically, the compensation consists of a matrix multiplication performed on the received-signal vector. This, in effect, restores the signals as received by the isolated elements in the absence of mutual coupling. This technique is most practical for digital beamforming antennas where the matrix operation can be readily implemented     

Pattern synthesis for TechSat21 - a distributed space-based radar system

The TechSat21 space-based radar employs a cluster of free-floating satellites, each of which transmits its own orthogonal signal and receives all reflected signals. The satellites operate coherently at the X band. The cluster forms essentially a multielement interferometer, with a concomitantly large number of grating lobes and significant ground clutter. A novel technique for pattern synthesis in angle-frequency space is proposed, which exploits the double periodicities of the grating lobes in the angular domain and of the radar pulses in the frequency domain, and allows substantial gains in clutter suppression. Gains from 7 to 17 dB relative to the normal random, sparse array appear feasible.     

On antenna power pattern synthesis

Power pattern synthesis for linear antenna arrays is discussed without the usual phase constraint on the far field. Mathematically, the problem is to find the optimum nonnegative harmonic approximation to a given function. Two types of error criteria, defining the optimum sense, are considered.     

Simple method for pattern nulling by phase perturbation

A method of sidelobe hulling, which involves perturbing the array illumination phase only, is presented. The general nonlinear problem is linearized by assuming the perturbations to be small, and an analytic solution is derived. Illustrative examples of sinc and Chebyshev patterns with imposed nulls are given.     

Mutual coupling analysis of a finite planar waveguide array

This paper deals with mutual coupling in a finite planar array antenna, composed of open-ended circular waveguides in a ground plane. The element-by-element approach is used and the problem is formulated as an integral equation, by requiring the transverse electric and magnetic fields to be continuous across the apertures. The equation is then solved by the method of moments and the mutual coupling in a 127-element array is computed. Excellent agreement with measurements and with the active reflection coefficient for the corresponding infinite array is found. The presented method of coupling analysis can be considered as a supplement to the established periodic-structure approaches for infinite arrays and may be useful for the analysis of small or nonperiodic arrays.     

Analysis of a dual-frequency array technique

This paper describes the radiating properties of a new element for a dual-frequency phased array that operates over two frequency bands separated by about an octave. The analysis considers the array element excited by two waveguides for high-frequency operation, and includes an evaluation of the element radiation properties at both frequency bands. The analysis of radiation is carried out for the case ofH-plane scan, which is expected to be the most severe test of the element's scanning behavior. Computed results indicate that the structure radiates efficiently within the two bands and operates free from anomalous behavior over substantial scan angles. These results demonstrate the feasibility and practicality of the basic concept.     

Broadband FDTD analysis of infinite phased arrays using periodicboundary conditions

A new technique for the FDTD analysis of periodic infinite phased arrays is presented. Periodic boundary conditions are implemented which reduce the computational volume to that of a single unit cell. The technique applies to pulse scanning in oblique directions and is performed by continuously moving the unit cell in the scan plane