Showing papers on "Antenna array published in 1989"

The tapered slot antenna-a new integrated element for millimeter-wave applications

Abstract: Tapered slot antennas (TSAs) with a number of potential applications as single elements and focal plane arrays are discussed. TSAs are fabricated with photolithographic techniques and integrated in either hybrid or MMIC circuits with receiver or transmitter components. They offer considerably narrower beams than other integrated antenna elements and have high aperture efficiency and packing density as array elements. Both the circuit and radiation properties of TSAs are reviewed. Topics covered include: antenna beamwidth, directivity, and gain of single-element TSAs; their beam shape and the effect of different taper shapes; and the input impedance and the effects of using thick dielectrics. These characteristics are also given for TSA arrays as are the circuit properties of the array elements. Different array structures and their applications are also described. >

A study of microstrip array antennas with the feed network

Abstract: The radiation and losses in microstrip antennas with a corporate feed network are studied. A surface current approach is applied in which the electrical currents in the feed lines are modeled as in ideal transmission lines. The free-space radiation and the surface-wave excitation of typical segments in printed feed networks are studied. A four-element array antenna with its printed feed network is analyzed and predicted radiation patterns, directivity, and gain are presented and compared with experimental results. The gain and directivity of large arrays of 16, 64, 256 and 1024 elements are calculated and measurements in the frequency range of 10 to 35 GHz are reported. >

Phased array antenna calibration and pattern prediction using mutual coupling measurements

Abstract: A technique which utilized the inherent mutual coupling in an array to both calibrate and predict the radiation patterns of a phased-array antenna is investigated. The only restriction of the technique is that the ability to transmit and receive with pairs of the array elements is required. The theory associated with array mutual coupling and its relationship to both array calibration and array patterns is discussed. The design of a test bed phased-array antenna is covered. The mutual coupling technique (MCT) is used experimentally to calibrate the test array as well as to predict the array radiation patterns. It is shown that the results obtained by MCT are in good agreement with conventional far-field measurements. >

Design principles of sequentially fed, wide bandwidth, circularly polarised microstrip antennas

Abstract: The technique of sequentially rotating the feed position of a microstrip patch antenna to improve the circular polarisation bandwidth of an array of such elements is briefly reviewed in the wider context of other known antenna parameter staggering concepts. A first order analysis then shows how the sequential technique significantly improves the axial ratio bandwidth in the relation to the number of patches in the array and the feeder connections. Unwanted effects such as feeder phase errors, multiple reflections in feeders, the presence of higher order modes and mutual coupling are then considered. Computations embracing these effects and measurements illustrate the various properties and topological options available. Finally, the application of the technique is briefly illustrated for a single patch and an array of patches. In conclusion the design principles are summarised and feeder effects are identified as the main limitation when implementing the sequential technique.< >

Application of sequential feeding to wide bandwidth, circularly polarised microstrip patch arrays

Abstract: Sequential rotated feeding of circularly polarised microstrip arrays gives substantial improvements in performance and is an important new technique finding application in several current systems. Here, the application to microstrip arrays is examined. A detailed analysis of patch arrays with corporate feed networks incorporating non isolating splitters is described. Computed results for input VSWR and axial ratio for various array configurations show that, for best performance, the isolated patch VSWR and ellipticity on resonance should be optimised and that maximum bandwidth is obtained for clusters of three circularly polarised patches. Gain loss is found to be a significant bandwidth constraint resulting in an overall bandwidth not substantially wider than that of linearly polarised patches. It is concluded, however, that sequential rotation will allow use of lower Q patches leading to good array ellipticity and match over wider bandwidths than currently possible.< >

Bearing estimations with mutual coupling present

Abstract: Estimating angles of arrival of radiation sources using an antenna array with mutual coupling between elements is investigated. The presence of mutual coupling distorts phase vectors of radiation sources and the eigenstructure of the covariance matrix. Two approaches are proposed to counteract that distortion so that eigenstructure methods can perform well. The method described first eliminates the distortion of the signal component of the the covariance matrix and then applies spatial smoothing to decorrelate the coherence between sources. Computers simulation results show that this method successfully counteracts mutual coupling. >

Blind separation and capture of communication signals using a multitarget constant modulus beamformer

Abstract: A multitarget least-squares CMA (constant modulus algorithm) that blindly adapts a narrowband beamformer to separate and capture multiple communication signals by exploiting only the low modulus variation of the communication waveforms is presented. It is shown that the algorithm can detect and classify transient and nontransient communication signals on the basis of modulus variation and extract those signals with maximum attainable output SINR (signal-to-interference and -noise ratio) in environments containing strong transient or nontransient cochannel interference. This is demonstrated via computer simulation for a narrowband (25 kHz bandwidth) four-element antenna array in environments containing FSK (frequency shift keying) bursts received at low ( approximately 4 dB signal-to-white-noise ratio) power levels in the presence of severe ( approximately -50-dB signal-to-interference ratio) interference. It is shown that the algorithm can capture the FSK bursts in less than 2 ms, and maintain capture on the signals as other signals appear and disappear over the signal ON times. >

An adaptive nulling system for a narrow-band signal with a look-direction constraint utilizing the conjugate gradient method

Abstract: The problem of iterative adjustments of the weights of a phased array with a look-direction constraint in the presence of jammers is presented. The technique described uses the adaptive conjugate method instead of the popularly used method of steepest descent to eliminate the jammer components, thus minimizing the error between the received signal and the actual one. This iterative method minimized the L/sub 2/ norm (the mean square error) and is guaranteed to converge in a finite number of steps which is not available for the present techniques utilized to solve this problem. Improvement in rate of convergence is thus achieved at the expense of algorithmic complexity. >

Self-cohering large antenna arrays using the spatial correlation properties of radar clutter

Abstract: A technique for self-calibrating a large antenna array system in the absence of a beamforming point source is presented that uses the spatial correlation properties of radar clutter. The array could be real or synthetic. It is shown that if R(X), the spatial autocorrelation function of the field (as measured by adjacent element pairs), is ensured to be real and positive in the neighborhood of the origin, both periodic and aperiodic arrays can be synchronized, forming retrodirective beams pointing at the axis of symmetry of the radar transmitter, provided that the interelement spacing does not exceed some limit (the order of the size of the transmitting aperture). If the spatial autocorrelation function is complex but has a linear phase, it is shown that one can still synchronize both periodic and aperiodic arrays, while if the phase of R(X) is nonlinear, only periodic arrays can be synchronized. In both cases of complex R(X), a residual beam-pointing error occurs. Computer simulations and airborne sea clutter data are reported that verify the theory and practicality of the algorithm. >

Analysis of an infinite phased array of aperture coupled microstrip patches

Abstract: An analysis of an infinite array of aperture-coupled microstrip patch antennas is described; this type of element is well suited to integrated phased-array applications, offering several advantages over other array configurations. The solution uses the spectral-domain moment-method approach, and combines features of a previous solution of infinite arrays of probe-fed patches and a reciprocity analysis of single-aperture-coupled microstrip element. The theoretical analysis is described and data are presented for the active input impedance of several arrays. Experimental data from a waveguide simulator confirm the theory. >

Improved theoretical and experimental models for the coaxial colinear antenna

Abstract: An improved theoretical model of the coaxial colinear (COCO) antenna is presented that takes into account different element lengths and power transfers between the antenna and the transmission lines. The antenna equation now contains an exact kernel instead of an approximate kernel and a piecewise constant basis function is used instead of a piecewise linear function, yielding faster results. More points are used for better accuracy and yet faster computations. The linear systems of equations of the theoretical model are solved using a preconditioned conjugate gradient method. Uniform and tapered current distributions are obtained experimentally and theoretically on end-fed coaxial colinear antennas. There is reasonable agreement between theory and measurements. The gains of a few COCO antennas relative to equivalent lengths of half-wave dipoles are given. >

Gain of circularly polarised arrays composed of linearly polarised elements

Abstract: The gain of circularly polarized (CP) array antennas realized by proper phasing of sequentially rotated linearly polarized (LP) elements is compared to that of arrays using CP elements and demonstrated by calculations for microstrip patch elements. When element spacing is large and array size is small, the advantages of LP elements are offset by the significant reduction in gain due to high cross polarized lobes in the diagonal planes. For large arrays of closely spaced elements, this gain loss reduces to a negligible amount. However, for spacings above a critical value of about 0.7 wavelengths, unacceptably high gain losses will be incurred.

Systolic array processor for MVDR beamforming

Abstract: An efficient systolic array for computing the minimum variance distortionless response (MVDR) from an adaptive antenna array is described. It is fully pipelined and based on a numerically stable algorithm which requires O(p2 + Kp) arithmetic operations per sample time, where p is the number of antenna elements and K is the number of look direction constraints.

Frequency limitations on broad-band performance of shunt slot arrays

Abstract: Two methods are presented for the analysis of slot array performance as a function of frequency, taking mutual coupling into account. The first is a direct method, the other an iterative one. These are applied to a seven-slot resonant array and to a 21-slot traveling wave array. This analysis permits prediction of the bandwidth and provides comprehension of broadband limitations. The two examples presented show good agreement between theory and measurement. >

A theoretical and experimental study of mutual coupling in microstrip antenna arrays

Abstract: Each microstrip antenna element in the array is replaced by an equivalent magnetic current source distribution over a grounded dielectric slab derived from the electric field on the walls of the element as given by the cavity model. A dyadic Green's function is developed for a grounded dielectric slab, using the rectangular vector wave functions. The function is used to calculate the magnetic field due to the magnetic current distributions. The formula for mutual impedance based on the reaction concept is used to calculate the mutual coupling in arrays of antenna elements. Some measurements have been conducted for mutual coupling between two rectangular patches in the C-band. Calculated results are in excellent agreement with measurements, including those of other authors. >

Tcas bearing estimation receiver using a four element antenna.

Abstract: A direction finding receiving system for use on a protected aircraft in which a signal from an ATCRABS transponder on an intruder aircraft is received has been described incorporating a four element interferometer type antenna array, reversing switches, individual receivers, a phase detector and a processor for processing the outputs of the phase detector. The invention overcomes the problem of correcting for the difference in phase delay of signals passing through the receivers.

Acquisition of spread spectrum signals by an adaptive array

Abstract: Discrete-time processing techniques are discussed for the acquisition of direct-sequence spread-spectrum signals by an antenna array. Both constant data and random data modulation of the code are considered. The maximum-likelihood (ML) procedures for estimating the received code lag are described, assuming an unknown channel and interference signal of either known or unknown covariance. Analytic and simulation results for performance of the optimum estimators are presented. Simulation results for the ML data estimators are shown to be close to analytical predictions of bit error rate (BER) performance. The ML procedure for data demodulation is also described. >

Gain optimization of a near-field focusing array for hyperthermia applications

Abstract: A variation of the array gain optimization problem has arisen in the study of microwave arrays used for hyperthermia, the heating of biological tissue. For a given array configuration and arbitrary medium it is desired to maximize the power deposition at a prescribed focus in the near field of the array. The authors show how the optimum excitation can be found by solving an eigenvalue problem. Their optimum solution is compared with two other solutions, namely a closed-form solution which optimizes the power in one linear polarization of the radiated field, and a solution based on the popular conjugate-field scheme. >

Distribution network for phased array antennas

Abstract: A distribution network for a modified space-fed phased array antenna consists of a planar array of radiating slots distributed along a coplanar wall in each of an ensemble of parallel waveguides. This waveguide ensemble is fed or excited by an orthogonal waveguide or waveguides, through a row of slots in a wall common to the excitation waveguides and the parallel waveguide ensemble, one slot per waveguide. A predetermined amplitude distribution is achieved in the plane parallel to the axis of the exciting waveguide by adjusting the coupling value of each exciting slot, and in the orthogonal plane by adjusting the displacement of the radiating slots from the center line of the waveguides, and by adjusting slot width, length, and geometry. Such an array of slots is used to feed the inside face of a quasi-space-fed antenna array having identical, individual electronics modules.

Pattern synthesis based on adaptive array theory

Abstract: Pattern synthesis based on either optimization of signal-to-noise ratio or noise minimization is developed. The connection between adaptive arrays and adapted pattern shape is used in synthesizing linear array patterns with freedom to adjust both amplitude and phase. Phase-only pattern synthesis is described in connection with suppressing interference or clutter in proportions of space. A planar array example applied to a constant-amplitude solid-state phased array is discussed. >

A broad-band printed circuit hybrid ring power divider

Abstract: The design of a broadband ring power divider that provides substantial improvement in coupling and phase characteristics over a wide frequency range compared to that of a conventional hybrid ring is presented. It is shown that the new power divider design can be easily constructed using realizable impedances and is applicable to both equal and unequal power divisions. Experimental verification of the broadband power divider was achieved in a stripline configuration at Ku-band for an equal power divider and a 3-dB power divider. Close agreement was observed between theoretical and experimental results for both of the power dividers. The usable bandwidth of the broadband power divider is approximately 45%. It is concluded that this broadband power divider is particularly suited for a narrowband low-sidelobe antenna array beam-forming network, for it provides substantially improved amplitude and phase characteristics, a requirement for low-sidelobe array antennas, and good isolation between output ports. >

Transient electromagnetic method for detecting irregularities on conductive containers

Abstract: A transient electromagnetic method for detecting irregularities on container walls by measuring wall thickness The method utilizes a transmitting antenna and a separate receiving antenna arranged in a loop-loop configuration The transmitting antenna induces current into the container wall and the receiving antenna and the receiver detect the decay of the induced current, which is then analyzed to detect wall thickness A receiving antenna array of many receiving antennas is used to increase the spatial resolution Simultaneous measurement of the induced current by the receiving antennas reduces noise that is coherent across the array Use of a noise antenna located so as to be unaffected by the transmitting antenna further reduces noise The received signal from the receiving antenna array is displayed on a two-dimensional display in a spatial arrangement that corresponds to the spatial arrangement of the receiving antennas The display can be scrolled over the time range of the received signals to produce a moving image, so that irregular areas can be more easily detected than with a static display

Multi-beam adaptive base-station antennas for cellular land mobile radio systems

Abstract: The authors address the problem of meeting the proliferating demands for mobile telephony within the confinements of the limited radio spectrum allocated to these services. A multiple-beam adaptive base-station antenna is proposed as a major system component in an attempt to solve this problem. The approach is demonstrated by employing an antenna array to resolve the angular distribution of the mobile users as seen at the base-station site, and then using this information to direct beams towards the mobiles in both transmit and receive modes. The energy associated with each mobile is thus confined within the addressed volume, greatly reducing the number of co-channel cells. For a given performance criterion, this results in an increase in the spectral efficiency or capacity of the network. The concept is demonstrated using results generated by a computer simulation of a simplified mobile scenario. The realization of a fully adaptive base-station antenna test rig is discussed. >

Optical beam former for high frequency antenna arrays.

Abstract: A wideband, true time delay antenna beam-forming network (10) for millimeter wave phased array antennas is disclosed herein. The beam-forming network (10) of the present invention includes a laser (30) for generating a beam B of electromagnetic energy. The beam-forming network (10) further includes an electro-optic modulator (40) for modulating the beam B in response to an input signal S modulated about a first frequency fo. A first antenna array (80) generates a first electromagnetic field pattern P by radiating the modulated beam B' in a first direction. The present invention further includes a constrained lens (90) for receiving the first field pattern P and for emitting an antenna driver feed beam F in a second direction in response thereto. An antenna array driver (115) electro-magnetically coupled to the constrained lens (90) by the feed beam F provides a set of signals modulated about the first frequency fo to drive an antenna array (20).

Coherent interference suppression by an antenna array of arbitrary geometry

Abstract: An approach is investigated that can be applied to an array of arbitrary geometry for coherent and incoherent interference suppression. This approach consists of constructing a weight vector which minimizes the array output power subject to null constraint in the directions of the coherent interferers. In this way, the coherent interference is suppressed by the null constraint and the incoherent interference is suppressed by minimizing the array output. The approach requires the estimation of the angles of arrival of the coherent interferers. A simple bearing estimation method is described which can be applied to the scenario of one coherent interferer. The effect of error in bearing estimation on the array performance is discussed. The use of a high-order null constraint to counteract that error is discussed. >

Theoretical and experimental SAR distributions for interstitial dipole antenna arrays used in hyperthermia

Abstract: Theoretical predictions and experimental measurements of power deposition in muscle tissue phantoms are compared for various arrays of microwave dipole antennas used for hyperthermia cancer therapy. The antennas are linear coaxial dipoles which are inserted into small nylon catheters implanted in the tumor volume. The specific absorption rate (SAR) patterns for a 2-cm square array of four 915-MHz antennas are presented for both resonant and nonresonant dipoles. Arrays of dipoles with lengths much shorter than the resonant half-wavelength have a far more reactive input impedance and a much smaller absolute SAR magnitude in the array center than is seen for arrays of resonant dipoles, and the maximum SAR shifts from the array center to the antenna surfaces. The absolute length of the volume heated by the small-diameter antennas with the longer half-wavelength was longer than that of the larger-diameter antennas. SAR distributions for 4-cm square arrays of eight and nine antennas fed with equal amplitude and phase are also compared. It is shown that much of the array volume has a power deposition less than 25% of the maximum SAR and that the distribution is nonuniform for both the eight- and nine-antenna configurations. >

Three-dimensional electromagnetic power deposition in tumors using interstitial antenna arrays

Abstract: A three-dimensional model of a tumor of arbitrary shape subjected to the fields of an interstitial antenna array is developed to predict the EM power deposition in an inhomogeneous tumor-tissue medium. The volume integral equation for the imbedded tumor is developed and solved by the method of moments. The incident fields are calculated based on the available formulation of interstitial antennas in homogeneous media. The accuracy of the developed computer code was checked by comparing the results from the volume integral approach with the Mie solution for the special case of spherical tumors. Good comparison was obtained for tumors with properties approximately 25% different from those of the surrounding tissue. Comparisons of results from models of antenna arrays with and without imbedded tumors show significant differences in their predictions of the EM power deposition in the tumor. The developed inhomogeneous model was used to examine the feasibility of controlling the uniformity of the power deposition pattern in large tumors by adjusting the amplitude or relative phase between the array elements. >

Analysis and design of series-fed arrays of printed-dipoles proximity-coupled to a perpendicular microstripline

Abstract: An analysis of a printed dipole element and a generalized configuration of a series-fed array of such elements, electromagnetically coupled to a covered microstripline running perpendicularly under it in a substrate-superstrate configuration, is presented. The solution is based on the principle of reciprocity and is formulated using a rigorous method of moments and full-wave spectral-domain Green's functions for multilayer dielectric substrates. The dipole excitation is characterized by an equivalent impedance, and can be controlled by suitably selecting the offset of the dipole from the feed line. Mutual coupling between dipoles is included. Using the results of the element analysis, a series-fed array prototype has been successfully designed, built, and tested in a standing-wave configuration; the design details are described, and measured performances are evaluated using the results of the array analysis. Mutual coupling effects are found to be not detrimental for this configuration, but can be severe for other nonstanding-wave configurations. >

Separation and bearing estimation of co-channel signals

Abstract: The authors describe a technique for simultaneously estimating the bearing of and separating (copying) multiple cochannel signals received by a multielement antenna array. The method uses a combination of adaptive beamformers and signal cancelers to sequentially lock onto and remove emitters one by one from the element signals. Inherent to the technique is the generation of a direction vector for each of the isolated emitters. The technique requires neither prior knowledge of emitter bearings nor array calibration is required only to map direction vectors into bearing estimates. The technique is computationally simple and easily amenable to hardware implementation. For reasonably narrowband signals ( >

Fast acquisition of burst and transient signals using a predictive adaptive beamformer

Abstract: The author presents a simple prediction algorithm for blindly adapting a narrowband beamformer to rapidly acquire burst or transient signals by exploiting only the abrupt ON/OFF transitions of the transient waveforms, that is without using knowledge of the frequency or DOA (direction of arrival) of the signal of interest or knowledge of the manifold, geometry, or noise covariance of the array. It is shown via analysis and computer simulation that the algorithm is able to detect burst and transient signals, and to capture those signals with maximum attainable output SINR (signal-to-interference and -noise ratio) in environments containing severe stationary cochannel interference. In particular, it is shown that the algorithm can adapt a four-element antenna to blindly detect and capture FSK (frequency shift keying) bursts with 10-20-ms burst durations in less than 2 ms, even at low power levels ( approximately 4-dB signal-to-white-noise ratio) and in the presence of strong ( approximately 50-dB SIR) stationary interference. >