Showing papers in "IEEE Transactions on Antennas and Propagation in 1976"

A model for non-Rayleigh sea echo

Abstract: A mathematical model for non-Rayleigh microwave sea echo is developed which describes explicitly the dependence of statistical properties of the radar cross section on the area of sea surface illuminated by the radar. In addition to the first probability distribution of the scattered radiation, its temporal and spatial correlation functions are also considered. It is shown that, in general, these correlation functions decay on at least two scales, the second, non-Rayleigh, contributions being strongly dependent on the properties of a "single scatterer." Predictions of the model are found to be in qualitative agreement with existing experimental data. A new class of probability distributions, the " K -distributions," is introduced, which may prove useful for fitting such data.

Adaptive arrays

Abstract: A method for adaptively optimizing the signal-to-noise ratio of an array antenna is presented. Optimum element weights are derived for a prescribed environment and a given signal direction. The derivation is extended to the optimization of a "generalized" signal-to-noise ratio which permits specification of preferred weights for the normal quiescent environment. The relation of the adaptive array to sidelobe cancellation is shown, and a real-time adaptive implementation is discussed. For illustration, the performance of an adaptive linear array is presented for various jammer configurations.

Adaptive arrays with main beam constraints

Abstract: Initial applications of adaptive array theory to the radar sidelobe jamming problem ignored the problem of incidental cancellation of the desired signal returns. In more recent applications, longer transmitted waveforms have combined with returns from extended clutter and/or strong targets to create a more serious signal cancellation problem. There are several ways in which the adaptive processor can be constrained from responding to desired main lobe target returns while maintaining good cancellation of interference in the sidelobes. This paper examines the major techniques for constraining the response of the adaptive processor, including methods of controlling the response of the array in the absence of external interference. Time domain and frequency domain techniques are discussed. The majority of the discussion is devoted to angle domain techniques such as pilot signals, preadaption spacial filtering, and control loop spatial filtering. Analysis is presented showing the relationship between these techniques. Finally, examples are given showing the effects of these constraints as well as control of the quiescent array pattern.

A generalized network formulation for aperture problems

Abstract: A general formulation for aperture problems is given in terms of the method of moments. It applies to any two regions isolated except for coupling through the aperture. The aperture characteristics are expressed in terms of two aperture admittance matrices, one for each region. The admittance matrix for one region is independent of the other region, and hence can be used for any problem involving that region and aperture. The solution can be represented by two generalized n -port networks connected in parallel with current sources. The current sources are related to the tangential magnetic field which exists over the aperture region when the aperture is closed by an electric conductor. Formulas for fields (linear functionals) and power (quadratic functionals) are given in terms of the admittance matrices.

A comparison of adaptive algorithms based on the methods of steepest descent and random search

Abstract: This paper compares the performance characteristics of three algorithms useful in adjusting the parameters of adaptive systems: the differential (DSD) and least-mean-square (LMS) algorithms, both based on the method of steepest descent, and the linear random search (LRS) algorithm, based on a random search procedure derived from the Darwinian concept of "natural selection." The LRS algorithm is presented here for the first time. Analytical expressions are developed that define the relationship between rate of adaptation and "misadjustment," a dimensionless measure of the difference between actual and optimal performance due to noise in the adaptive process. For a fixed rate of adaptation it is shown that the LMS algorithm, which is the most efficient, has a misadjustment proportional to the number of adaptive parameters, while the DSD and LRS algorithms have misadjustments proportional to the square of the number of adaptive parameters. The expressions developed are verified by computer simulations that demonstrate the application of the three algorithms to system modeling problems, of the LMS algorithm to the cancelling of broadband interference in the sidelobes of a receiving antenna array, and of the DSD and LRS algorithms to the phase control of a transmitting antenna array. The second application introduces a new method of constrained adaptive beamforming whose performance is not significantly affected by element nonuniformity. The third application represents a class of problems to which the LMS algorithm in the basic form described in this paper is not applicable.

Linearly polarized microstrip antennas

Abstract: An equivalent network for square and rectangular shaped microstrip radiating elements is derived. In order to simplify the problem the radiating element is considered as two slots separated by a transmission line of low characteristic impedance. The slots are characterized by their radiation pattern, directivity, and equivalent admittance. A design procedure for open circuit halfwave resonators and for arrays of such resonators is given. Finally, some antennas in the X band are designed and measured.

Decoupling and descattering networks for antennas

Abstract: The possibilities of connecting a lossless network between input ports and antenna ports such that there is no coupling and scattering between the antennas are discussed. A necessary condition for complete decoupling and descattering is power orthogonality between the patterns of the individual antennas. Numerical and experimental results are presented for monopole antennas.

Adaptive arrays in airborne MTI radar

Abstract: The ability of an AMTI (airborn moving target indication) radar to reject clutter is often seriously degraded by the motion of the radar. An adaptive receiving array can compensate for platform motion and provide excellent AMTI performance. Scattering from aircraft structure can also distort antenna patterns and reduce AMTI capability. The technique described here can adapt the element weights to compensate for near-field scatterers and element excitation errors. Another important advantage is the ability to null out discrete active interference sources without significantly degrading AMTI performance.

A uniform asymptotic theory of electromagnetic diffraction by a curved wedge

Abstract: Diffraction of an arbitrary electromagnetic optical field by a conducting curved wedge is considered. The diffracted field according to Keller's geometrical theory of diffraction (GTD) can be expressed in a particularly simple form by making use of rotations of the incident and reflected fields about the edge. In this manner only a single scalar diffraction coefficient is involved. Near to shadow boundaries where the GTD solution is not valid, a uniform theory based on the Ansatz of Lewis, Boersma, and Ahluwalia is described. The dominant terms, to the order of k^{-1/2} included, are used to compute the field exactly on the shadow boundaries. In contrast with the uniform theory of Kouyoumjian and Pathak, some extra terms occur: one depends on the edge curvature and wedge angle; another on the angular rate of change of the incident or reflected field at the point of observation.

An adaptive antenna array under directional constraint

Abstract: The concept of the adaptive system working on the principle of minimizing the output power under the constrained response to specified directions is introduced. Theoretical analysis and computer simulation experiments on this directional constraint system are also presented. By comparison with the computer experiment, the differential equation approach is shown to be useful to predict the behavior of the system. The effect of error in setting the constraint direction is investigated, and it is demonstrated that the system is not very sensitive to such error. This margin can be made even greater by adopting the double directional constraints system. This system is also analyzed theoretically and experimented by computer simulation. History of output power components during adaptation is shown, which enables the comprehension of the behavior of the system.

Microwave holographic metrology of large reflector antennas

Abstract: A microwave holographic technique for the determination of amplitude and phase of the principal and cross-polarized aperture fields of large reflector antennas is described. The hologram formation process utilizes the elevation over azimuth scanning system normally associated with these antennas, and, in this respect, appears to be unique among other proposed methods of field probing. The present work describes the means used to obtain vital information on the antenna structure such as E - and H -plane phase centers of the feed, and rms values of the reflector surface profile errors. Accurate prediction of E - and H -plane radiation patterns in the near- and far-field is also demonstrated.

Adaptive sidelobe nulling using digitally controlled phase-shifters

Abstract: Adaptive null-steering arrays which use phase-only weights are examined. The optimal phase weightings are developed and the mathematical equations are reduced to a simplified computational form. Approximations for low sidelobe arrays allow more efficient computations and demonstrate the decomposition of the antenna pattern into a sum of patterns which place gain towards each directional emitter. Computer simulations and breadboard array measurements substantiate the mathematical development and indicate the practicality of the method.

Application of the unimoment method to electromagnetic scattering of dielectric cylinders

Abstract: The unimoment method is applied to calculate the scattered fields of dielectric cylinders of arbitrary cross section or of inhomogeneous material. The basic technique of the method is the use of the finite element methods inside a mathematical circle, which encloses the inhomogeneous body. The fields outside are expanded in the usual cylindrical harmonics. The interior and exterior problems are then coupled at the circle. The versatility is increased greatly by introducing the method of "inhomogeneous element." The advantage of the proposed method is the simplicity and efficiency in programming. The validity of the computer program has been verified by comparing results with calculations from other methods for i) an off-centered circular cylinder, ii) two circular cylinders, and iii) a circular cylindrical shell.

Partial adaptivity for the large array

Abstract: Most studies and experimental work in the past on the subject of adaptive arrays have been concerned with systems with, at most, a few tens of adaptive elements. In the study of the problems facing the next generation of ballistic missile defense radars, it has become clear that jamming poses a very serious threat. These radar systems typically contain as many as 3000 to 5000 elements per array face. The adaptive array technique is a likely candidate to aid in countering this threat. This paper examines the problem of configuring such a large array to provide partial adaptivity. A method of analysis is presented and the results of simulations and analysis are presented showing the effects of configuration, the effects of practical component tolerances, and the transient response characteristics of several candidate systems.

Remote sensing of atmospheric water content from satellites using microwave radiometry

Abstract: Analysis is presented which substantiates the high correlation achieved in relating integrated water vapor and liquid water to brightness temperatures at frequencies near the 22.235 GHz water vapor line. The influence of atmospheric and surface variability is shown to be minimal over low emissivity sea surfaces. Determination of atmospheric water content using regression techniques is shown to follow directly from radiation transfer theory. Satellite data from the Nimbus-E Microwave Spectrometer (NEMS) aboard Nimbus-5 are compared with radiosonde water vapor measurements and cloud images recorded by the Temperature Humidity Infrared Radiometer aboard Nimbus 5.

Mutual coupling, gain and directivity of an array of two identical antennas

Abstract: It is shown that there is an optimum nonzero value of mutual coupling and passive reflection coefficient that is a function of only the embedded element pattern and element spacing. Optimum is defined as yielding maximum array gain. For closely spaced arrays, the optimum mutual coupling and passive reflection coefficient are quite large, but tend to cancel in the active mode to produce a low active reflection coefficient. A smaller value of mutual coupling would give lower gain than the optimum case. This again demonstrates that mutual coupling is a natural and desirable effect in array behavior. It is well known that it is possible to achieve array superdirectivity, in the sense that the directivity of an array of " N " elements is greater than " N " times the embedded element directivity; however, in this case the mutual coupling and passive reflection produce an active reflection coefficient that results in an array gain lower than " N " times the embedded element gain. In fact, higher superdirectivity always implies lower gain, relative to embedded element gain.

Analysis of antennas and scatterers with nonlinear loads

Abstract: Two methods for analyzing antennas or scatterers having nonlinear resistive loads are discussed. The first is a direct time-domain integral equation approach, whereas the second involves the use of frequency domain data to compute the time-dependent currents and voltages across the nonlinear load. Both transient and time-harmonic excitations are considered in the sample problems illustrated here which involve a center-loaded linear antenna. Although it is difficult to do so with the direct time-domain approach, the second method of analysis may be readily applied to an arbitrary antenna whose frequency response is either measurable or calculable.

A meteorological model for rain-induced cross polarization

Abstract: After a brief summary of the existing theory on microwave attenuation and cross polarization by rain and of the supporting experimental evidence, the paper gives a physical explanation of raindrop canting. It is shown that vertical windgradients are a cause of canting. A model for the dependence of windspeed on height is then employed to calculate actual values of canting angle and to investigate the dependence on windgradients, height above terrain, and the type of terrain. A generalization of the expressions for cross polarization as a function of raindrop canting angle is given, which allows actual cross polarization calculations to be made with the model. Results for sample cases on horizontal and slant paths are given to show the relative importance of various parameters.

Radar imaging from ramp response signatures

Abstract: A new approach to synthetic image generation of radar targets from electromagnetic scattering measurements is described and illustrated. Complex harmonic samples of the backscattered response at ten frequencies lying in the low resonance range of the target response spectrum are used as input data for this approach. A periodic ramp response waveform synthesized from these data is shown to be correlated to the cross-sectional area versus distance along the line-of-sight, or profile function, of the targets measured. A technique using profile function data at three orthogonal target look angles for the mathematical specification of a three-dimensional "approximate limiting surface" is described. Visual images simulating an isometric view of the surface are then constructed. Images produced from model measurements of several target shapes are presented to illustrate some characteristics of this imaging process.

Explorations in fixed and adaptive resolution at GE and SURC

Abstract: This paper recounts the invention and development of adaptive processing at General Electric and the Syracuse University Research Corporation, giving heuristic explanations of the behavior of adaptive systems and discussing the relation between fixed and adaptive resolution.

Frequency spectra for the geometric representation of wavefront distortions due to atmospheric turbulence

Abstract: The wavefronts of a point source of radiation that is immersed in a turbulent atmosphere become distorted and deformed as they propagate outward. The phase surfaces incident on an unobscured collecting aperture therefore will not be perfectly spherical, but instead will be aberrated in some way. We have expanded this phase profile in a set of orthonormal functions closely related to the Zernike polynomials and then determined the respective power spectra of the time dependent expansion coefficients. The results show, for all spectra, a sharp cutoff of f^{-11/3} for frequencies f > ( u/2R) , where u is the wind velocity transverse to the point source and the receiver, and R is the aperture radius. In particular, for the tilt component expansion coefficients, this result differs from the usual phase difference power spectra previously used by many workers.

Forward scattering from square cylinders in the resonance region with application to aperture blockage

Abstract: The relationship between the induced field ratio (IFR) of a cylinder and aperture blocking of a constant-phase aperture by cylindrical struts is discussed. An analytical technique is presented whereby the IFR of rectangular cylinders can be calculated using the method-of-moments with internal constraint points. An experimental technique using a forward-scattering range is used to measure the IFR's of square and circular cylinders in an anechoic chamber. These experimental results are compared with the theory, and their implications on aperture blocking losses and boresight cross polarization are discussed.

Electromagnetic excitation of a wire through an aperture-perforated conducting screen

Abstract: Integro-differential equations are formulated for the general problem of a finite-length wire excited through an arbitrarily shaped aperture in a conducting screen. The wire is assumed to be electrically thin and perfectly conducting, and it is arbitrarily oriented behind the perfectly conducting screen of infinite extent. A known, specified incident field illuminates the perforated-screen/wire structure. The integro-differential equations fully account for the coupling between the wire and the aperture/screen. They are specialized to the case of the wire parallel to the screen with the aperture a narrow slot of general length. These special equations are solved numerically and data are presented for wire currents and aperture fields under selected conditions of wire/slot lengths and orientation. Data indicative of the coupling between the wire and slot are presented.

The input admittance of the rectangular cavity-backed slot antenna

Abstract: The input admittance of the rectangular cavity-backed slot antenna is investigated. The slot is assumed narrow so that the voltage distribution in its aperture is sinusoidal. Equations which represent the input admittance of this slot, backed by a rectangular cavity in which a single propagating wave is assumed to exist, are given. Calculations based on these representations are compared to available measured data. As the depth of the cavity increased the resonant frequency decreased and the bandwidth became narrower. Input admittance curves as a function of electrical slot length are also presented for several size cavities.

A note on SAR imagery of the ocean

Abstract: An example of SAR imagery of the ocean surface including the Gulf Stream Boundary reported by Moskowitz [1] is used to discuss potential mechanisms of SAR image formation of the ocean. It is pointed out that images in addition to those due to modulation of surface scatterer strength may result from organized scatterer motions such as those due to currents or wave orbital velocities. The modulation of scattering cross section by large waves is expected to depend on the magnitude and direction of the wind, making the relationship between image strengths and wave amplitudes rather complicated. It is suggested that the effect of wave orbital accelerations upon azimuthal focus might provide a more direct measure of wave amplitude.

Efficient numerical techniques for solving Pocklington's equation and their relationships to other methods

Abstract: It is shown that testing Pocklington's equation with piecewise sinusoidal functions yields an integro-difference equation whose numerical solution is identical to that of the point-matched Hallen's equation when a common set of basis functions is used with each. For any choice of basis functions, the integro-difference equation has the simple kernel, the fast convergence, the simplicity of point-matching, and the adequate treatment of rapidly varying incident fields, but none of the additional unknowns normally associated with Hallen's equation. Furthermore, for the special choice of piecewise sinusoids as the basis functions, the method reduces to Richmond's piecewise sinusoidal reaction matching technique, or Galerkin's method. It is also shown that testing with piecewise linear (triangle) functions yields an integro-difference equation whose solution converges asymptotically at the same rate as that of Hallen's equation. The resulting equation is essentially that obtained by approximating the second derivative in Pocklington's equation by its finite difference equivalent. The authors suggest a simple and highly efficient method for solving Pocklington's equation. This approach is contrasted to the point-matched solution of Pocklington's equation and the reasons for the poor convergence of the latter are examined.

Design of line source antennas for difference patterns with sidelobes of individually arbitrary heights

Abstract: A design method is presented for the determination of continuous line sources which will yield sum patterns consisting of a narrow main beam and sidelobes whose individual heights can be adjusted to any arbitrary specification. The method is based on a generalized Taylor pattern which is perturbed in successive iterations until the desired pattern results. For practical cases the convergence is rapid, and an economical computer program of general applicability has been written which will terminate when the desired pattern has been achieved within a specified tolerance, at which point a final pattern and aperture distribution are printed out. The theory is illustrated by several cases of practical interest.

Synthesis of nonuniformly spaced arrays using a general nonlinear minimax optimisation method

Abstract: Antenna patterns can be synthesized using a new nonlinear minimax optimization method with sure convergence properties. Not requiring derivatives, the proposed method is general and easy to use so that it might be applied to a wide variety of nonlinear synthesis problems for which analytical solutions are not known. To test the algorithm a group of test problems for which exact analytical solutions are known has been considered, namely, optimization of Dolph-Chebyshev arrays by spacing variation. The method is further applied to find the element positions in nonuniformly spaced linear arrays with uniform excitation that produce minimized (equal) sidelobe levels, and comparisons are made with conventional Dolph-Chebyshev arrays.

Statistical rainstorm models: Their theoretical and physical foundations

Abstract: Precipitation has become a serious source of attenuation as higher frequencies are being employed for microwave communications. System performance is strongly influenced by the quantity and character of precipitation that occurs over the links of the system. Rain appears to be the precipitation form that accounts for most of the serious attenuation occurrences. It is also for rain that the Scattering theory is most complete. Rain attenuation can be accurately predicted if the drop-size distribution along the propagation path is known. The drop spectrum determines as well the rainfall rate, radar reflectivity, and microwave emission of rain. Consequently, it is possible to make estimates of rain attenuation through indirect measurements by raingauge, radar, and radiometer. Recent experiments have confirmed that these estimates are sufficiently accurate for practical purposes. From propagation experiments and studies of the fine-scale structure of rain, data are becoming available on the horizontal extent of heavy rain areas and the way this structure influences system performance. These data have been used to formulate statistical raincell models that permit prediction of the performance of single-path and path-diversity systems. The current status of raincell models is reviewed and suggestions for future research are offered.

Adaptive arrays for communication systems: An overview of research at the Ohio State University

Abstract: Adaptive array research at The Ohio State University is presented in a summary form. The main objective of the research was to assess the capabilities of adaptive arrays for communication systems in real environments. Consequently the theoretical work was often corroborated by experimental implementation and testing of proposed techniques. Specifically, three systems in which adaptive arrays were successfully incorporated are described, a single channel spread spectrum communication system, a TDMA satellite communication system, and a remote sensor communication system. Numerous other adaptive array studies are briefly summarized.