Amplitude Tapering Research Articles

Amplitude-only pattern synthesis of nonuniform linear arrays using a generalized pattern search optimization

In this work, synthesis of linear array geometry and excitation amplitudes is first formulated as a linearly constrained multiobjective optimization problem with the goals of minimum sidelobe level, null control and high directivity and then solved by a generalized pattern search (GPS) algorithm for the optimum element locations and excitation amplitudes. The constraints are imposed on the interelement spacing and dynamic range ratio of the amplitude tapering to reduce mutual coupling effects between the elements. GPS methods are newly discovered, derivative-free methods where the current iterate is updated by sampling the fitness function at a finite number of points along a suitable set of search directions to find a decrease in the function value. Thus, GPS methods can be exploited efficiently in solving optimization problems without requiring any information about the gradient of the fitness function which may be even discontinuous, nondifferentiable, stochastic or highly nonlinear. Finally, four worked examples are presented that illustrate the use of the whole GPS synthesis method, and the optimization goal in each example is easily achieved. Furthermore the full-wave simulations of the synthesized arrays are also completed to examine the mutual coupling effects. Finally the results of the GPS algorithm are validated by comparing with results obtained using the genetic algorithm, and the results of the uniform and Dolph - Chebyshev arrays, having the same number of element and the same aperture length. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2011.

77 GHz Stepped Lens With Sectorial Radiation Pattern as Primary Feed of a Lens Based CATR

We describe the design, fabrication and measurements of an axisymmetric dielectric lens, featuring a sectorial radiation pattern at 77 GHz. It will be used as the primary feed of a lens-based compact antenna test range (CATR). Due to symmetry of revolution, the sectorial lens profile can be designed in one dimension by using phase only control. The phase variation is echoed on the lens depth. The resulting stepped lens is simulated using France Telecom Orange Labs SRSRD software (¿in-house¿ software developed for dielectric axisymmetric radiating structures) and measured in an anechoic chamber at 77 GHz. Two lenses were fabricated with different materials: PVC and polyurethane, respectively. Good agreements were obtained between simulations and measurements. Less than 0.2 dB ripple in the central beam are obtained for the polyurethane lens although relatively high secondary lobes occur at 11°. Comparisons between the near field of a CATR illuminated by a small horn providing a uniform amplitude taper and the sectorial lens are conducted using numerical simulations. Results show that on-axis oscillations are reduced from 6 to 1 dB with the sectorial lens.

Exploration of Amplitude Tapering in Linear Phased Arrays with Pattern-Reconfigurable Elements

In classical periodic phased arrays, designers use the amplitude and phase of identical array elements to achieve desired characteristics such as sidelobe level, main beamwidth, scan angle, etc. A phased array composed of individual pattern-reconfigurable antenna elements possesses a third degree of freedom that may enhance or expand array performance. This work explores the functional benefits of utilizing radiation reconfigurable antennas with beam tilts in phased arrays, focusing on the effects of reconfigurability on sidelobe level, gain, and excitation dynamic range of amplitude tapered linear phased arrays. Simulated patterns from a broadside-to-tilted radiation reconfigurable antenna were used to calculate the overall array pattern via superposition for a linear, equally spaced array. Results indicate the concept has potential for lowering sidelobe levels, increasing system efficiency, and expanding array capability compared to that of a traditional planar array with fixed broadside elements.

Sunflower Array Antenna with Adjustable Density Taper

A deterministic procedure to design a nonperiodic planar array radiating a rotationally symmetric pencil beam pattern with an adjustable sidelobe level is proposed. The elements positions are derived by modifying the peculiar locations of the sunflower seeds in such a way that the corresponding spatial density fits a Taylor amplitude tapering law which guarantees the pattern requirements in terms of beamwidth and sidelobe level. Different configurations, based on a Voronoi cell spatial tessellation of the radiative aperture, are presented, having as a benchmark the requirements for a typical multibeam satellite antenna.

18–40-GHz Beam-Shaping/Steering Phased Antenna Array System Using Fermi Antenna

This paper concerns 18-40 GHz 1times 16 beam shaping and 1times 8 beam steering phased antenna arrays (PAAs) realized on a single low-cost printed circuit board substrate. The system consists of a wideband power divider with amplitude taper for sidelobe suppression, wideband microstrip-to-slotline transition, a low-cost true time piezoelectric transducer (PET)-controlled phase shifter, and wideband Fermi antennas with corrugations along the sides. A coplanar stripline is used under a PET-controlled phase shifter, which can generate 50% more phase shift compared to the perturbation on microstrip lines previously published. The systems are fabricated using electro-fine-forming microfabrication technology. Measured return loss is better than 10 dB from 18 to 40 GHz for both the beam-shaping and beam-steering PAAs. The beam-shaping PAA has a 12deg 3-dB beamwidth broadening range. The sidelobe ratios (SLRs) are 27, 23, and 20 dB at 20, 30, and 40 GHz, respectively, without perturbation. The SLRs are 20, 16, and 15 dB at 20, 30, and 40 GHz with maximum perturbation. The beam-steering PAA has a 36deg (-17deg to +19deg ) beam-scanning range measured at 30 GHz.

Quinary excitation method for pulse compression ultrasound measurements

A novel switched excitation method for linear frequency modulated excitation of ultrasonic transducers in pulse compression systems is presented that is simple to realise, yet provides reduced signal sidelobes at the output of the matched filter compared to bipolar pseudo-chirp excitation. Pulse compression signal sidelobes are reduced through the use of simple amplitude tapering at the beginning and end of the excitation duration. Amplitude tapering using switched excitation is realised through the use of intermediate voltage switching levels, half that of the main excitation voltages. In total five excitation voltages are used creating a quinary excitation system. The absence of analogue signal generation and power amplifiers renders the excitation method attractive for applications with requirements such as a high channel count or low cost per channel. A systematic study of switched linear frequency modulated excitation methods with simulated and laboratory based experimental verification is presented for 2.25MHz non-destructive testing immersion transducers. The signal to sidelobe noise level of compressed waveforms generated using quinary and bipolar pseudo-chirp excitation are investigated for transmission through a 0.5m water and kaolin slurry channel. Quinary linear frequency modulated excitation consistently reduces signal sidelobe power compared to bipolar excitation methods. Experimental results for transmission between two 2.25MHz transducers separated by a 0.5m channel of water and 5% kaolin suspension shows improvements in signal to sidelobe noise power in the order of 7–8dB. The reported quinary switched method for linear frequency modulated excitation provides improved performance compared to pseudo-chirp excitation without the need for high performance excitation amplifiers.

Designing Waveforms for Temporal Encoding Using a Frequency Sampling Method

In this paper a method for designing waveforms for temporal encoding in medical ultrasound imaging is described. The method is based on least squares optimization and is used to design nonlinear frequency modulated signals for synthetic transmit aperture imaging. By using the proposed design method, the amplitude spectrum of the transmitted waveform can be optimized, such that most of the energy is transmitted where the transducer has large amplification. To test the design method, a waveform was designed for a BK8804 linear array transducer. The resulting nonlinear frequency modulated waveform was compared to a linear frequency modulated signal with amplitude tapering, previously used in clinical studies for synthetic transmit aperture imaging. The latter had a relatively flat spectrum which implied that the waveform tried to excite all frequencies including ones with low amplification. The proposed waveform, on the other hand, was designed so that only frequencies where the transducer had a large amplification were excited. Hereby, unnecessary heating of the transducer could be avoided and the signal-to-noise ratio could be increased. The experimental ultrasound scanner RASMUS was used to evaluate the method experimentally. Due to the careful waveform design optimized for the transducer at hand, a theoretic gain in signal-to-noise ratio of 4.9 dB compared to the reference excitation was found, even though the energy of the nonlinear frequency modulated signal was 71% of the energy of the reference signal. This was supported by a signal-to-noise ratio measurement and comparison in penetration depth, where an increase of 1 cm was found in favor for the proposed waveform. Axial and lateral resolutions at full-width half-maximum were compared in a water phantom at depths of 42, 62, 82, and 102 mm. The axial resolutions of the nonlinear frequency modulated signal were 0.62, 0.69, 0.60, and 0.60 mm, respectively. The corresponding axial resolutions for the reference waveform were 0.58, 0.65, 0.62, and 0.60 mm, respectively. The compression properties of the matched filter (mismatched filter for the linear frequency modulated signal) were tested for both waveforms in simulation with respect to the Doppler frequency shift occurring when probing moving objects. It was concluded that the Doppler effect of moving targets does not significantly degrade the filtered output. Finally, in vivo measurements are shown for both methods, wherein the common carotid artery on a 27-year-old healthy male was scanned.

An efficient approach for the analysis of irregularly contoured planar phased arrays with tapered excitation in complex environments

An efficient uniform high‐frequency representation of the radiation from a large irregularly contoured planar tapered phased array, which is accurate throughout the near, intermediate, and far field zones, is presented. The planar array is regarded as the superposition of a sequence of parallel line arrays, and each one of them is represented in terms of equivalent continuous tapered lines via the Poisson summation formula. The amplitude tapering along the line is asymptotically approximated by a physically based, observation‐dependent linear combination of a small number (seven at most) of equiamplitude linearly phased excitations, which allows the reconstruction of the dominant contributions. After superimposing the asymptotic contributions of each line array, the total radiated field from the planar array is described in terms of rays emerging from the actual endpoints on the array rim and from a discrete sequence of points in linear loci on the surface of the array. As a consequence, high‐frequency techniques may effectively be used to describe the interaction of these rays with the surrounding environment.

S-band active phased array antenna with analog phase shifters using double-balanced mixers for mobile SATCOM vehicles

We propose an S-band active phased array antenna (APAA) with analog phase shifters (PSs) that use double-balanced mixers (DBMs). The quadrature phase shift keying modulator integrated circuits (ICs) used as the phase shifters can control both the amplitude and phase of the input signal continuously and independently by using the control voltages. The phase and amplitude errors during phase rotation were 0.5/spl deg/ rms (max=1.7/spl deg/) and 0.3 dB rms (max=0.7 dB), respectively. The range in which the amplitude could be linearly changed was 7 dB. The proposed antenna exhibited sufficient performance for mobile vehicles to handle satellite communications: beam-pointing resolution of 0.04/spl deg/, beam-pointing accuracy in the null pointing of 0.4/spl deg/ rms, and sidelobe suppression with an amplitude taper achieved by setting the amplitude of the PS using two DBMs. Comparison between the analog PS and a reflection-type four-bit digital PS showed that although the analog one is unsuitable for use in a passive PAA, it is suitable for use in an APAA.

Asymptotic high-frequency Green's function for a large rectangular planar periodic phased array of dipoles with weakly tapered excitation in two dimensions

This paper deals with the derivation and physical interpretation of a uniform high-frequency representation of the Green's function for a large planar rectangular phased array of dipoles with weakly varying excitation. Thereby, our earlier published results, valid for equiamplitude excitation, and those for tapered illumination in one dimension are extended to tapering along both dimensions, including dipole amplitudes tending to zero at the edges. As previously, the field obtained by direct summation over the contributions from the individual radiators is restructured into a double spectral integral whose high-frequency asymptotic reduction yields a series of propagating and evanescent Floquet waves (FWs) together with corresponding FW-modulated diffracted fields, which arise from FW scattering at the array edges and vertexes. To accommodate the weak amplitude tapering, new generalized periodicity-modulated edge and vertex slope transition functions are introduced, accompanied by a systematic procedure for their numerical evaluation. Special attention is given to the analysis and physical interpretation of the complex vertex diffracted ray fields. A sample calculation is included to demonstrate the accuracy of the asymptotic algorithm. The resulting array Green's function forms the basic building block for the full-wave analysis of planar weakly amplitude-tapered phased array antennas, and for the description of electromagnetic radiation and scattering from weakly amplitude-tapered rectangular periodic structures

Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.

Domain decomposition FDTD algorithm for the analysis of a new type of E-plane sectorial horn with aperture field distribution optimization

In this paper, a universal and efficient approach of domain decomposition finite-difference time-domain FDTD (DD-FDTD) is presented for the analysis of a new type of horn antenna-E-plane sectorial horn with field amplitude taper and phase correction in the aperture. The power fed into the horn is redistributed to achieve the optimal field amplitude distribution in the aperture, and meanwhile the field phase is corrected by metal lens. Compared with conventional E-plane sectorial horns, the new horn antenna takes the advantages of low sidelobe level, short physical length and wide flare angle etc. Moreover, the most important property of this horn is the weak coupling with each other when it is used as the element of a phased array antenna. The field analysis of such a horn antenna is an extremely complicated three-dimensional EM boundary value problem. The domain decomposition FDTD method is presented in this paper to break through the drawback. The whole horn is decomposed into several subdomains and the meshes are created in local coordinates. In the iteration procedure of FDTD, the data are exchanged between adjacent subdomains with overlapped meshes. The aperture field distribution, voltage standing-wave ratio and pattern calculated by the DD-FDTD method are in good agreement with the experimental data.

Effect of amplitude tapering and frequency-dependent phase errors on radiation characteristics of radial waveguide fed non-resonant array antenna

A high-gain low sidelobe level array antenna that is fed by a radial waveguide and designed to operate over a frequency range is considered. Assuming azimuthally symmetric excitation, the radiation pattern of the circular array is approximated by the space factor for the equivalent circular aperture and a simple and efficient tool is introduced for analysis of the array radiation pattern. The effect of the tapering, for both paraboloidal and bell-shaped aperture amplitude distributions, and the consequences of the phase errors, caused by the variation with frequency of the electrical lengths in the radial waveguide feed structure, on the gain and sidelobe level are studied in detail. Results obtained based on the proposed analysis are validated with respect to experimental data available in the literature. They are applicable to designs of arrays as well as aperture antennas.

Ring loading of annular sector radiating line antenna

A modified annular sector radiating line antenna has been designed to improve the efficiency of the basic Anserlin antenna by introducing smooth or serrated annular rings around the antenna. Design curves generated for the modified Anserlin antenna are utilised to produce a four-element series-fed linear array with 18 dB sidelobe taper at 2.1 GHz without the need for an amplitude tapering network.

Pattern characteristics of harmonic and intermodulation products in broadband active transmit arrays

A three-term power series model for the nonlinearity in the power amplifiers of an active array is used to explore the radiation characteristics of the harmonics and intermodulation products. This model leads to simple equations for predicting the beam-pointing direction and the aperture amplitude taper of the spurious signals. Cases of a single transmit signal, dual transmit signals within a single beam and dual beam operations are analyzed. This model shows that the second and third harmonics are co-boresighted with the fundamental signal but have squared and cubed aperture amplitude distributions. The third-order intermodulation products in general have beam-pointing directions offset from the fundamental signal beam(s). Graphical methods of locating the intermodulation beams in (u, v) space are outlined and illustrated by example.

Atlantic Equatorial Deep Jets: Space–Time Structure and Cross-Equatorial Fluxes

The so-called equatorial stacked jets are analyzed with ship-board observations and moored time series from the Atlantic Ocean. The features are identified and isolated by comparing vertical wavenumber spectra at the equator with those a few degrees from the equator. Mode-filtering gives clear views of the jets in meridional sections, the typical extent being ±1° in latitude. The vertical structure can be well described (explaining 82% of the variance) by N−1-stretched cosines, with a Gaussian amplitude tapering in the vertical. The stretched wavelengths are somewhat variable. Fitting jets of a fixed (stretched) wavelength to four moored sensors in the depth range 1300–1900 m, allows one to track the vertical phase of the jets with an rms error of 30°–45°. The resulting fit from a 20-month moored time series shows long periods of unchanging jet conditions and intermittent times of high variability. There is no significant vertical propagation on these timescales nor a seasonal reversal. Using a composite from many different experiments, interannual variability is visible, however. A possible mechanism for the stacked jets is inertial instability, resulting from background meridional shears at the equator. A condition is that the Ertel potential vorticity becomes zero somewhere, due to meridional asymmetries in the zonal flows. The ship-board observations show that this may be approximately fulfilled by the instantaneous zonal low-mode flows at various depths, resulting from an excess of zonal momentum south of the equator most of the time. Inertial instability should act to redistribute this zonal momentum, and our mooring data show indeed persistent northward momentum flux, but not at the depth levels expected. The momentum transport might suggest that the jets can also flux or mix other properties across the equator.

Multifrequency corrugated feed with groove discontinuityat input

The design of a multifrequency feed system for the microwave radiometer for Ocean Sat-II satellite covering 18.7, 23.8 and 36.5 GHz frequency bands is reported. A groove discontinuity is introduced at the input of the corrugated horn to optimise amplitude taper and the symmetry of primary patterns at the highest frequency without affecting the performance at lower frequency bands. Computed and measured radiation patterns are compared at all the frequencies.

Scanning properties of sequentially rotated linear arrays of circularly polarized patch radiators

AbstractThis paper presents a theoretical analysis of the effect of beam scanning and amplitude tapering on the cross‐pol level of sequentially rotated linear arrays composed of circularly polarized patch antenna elements. Arrays of eight elements are considered to illustrate the analysis. Several sequential phase distributions are analyzed in terms of the cross‐pol level cancellation effect over the antenna field of view. These arrangements are analyzed in terms of the location of the maximum cross‐pol peak levels and their values. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 343–350, 2001.

Design of a 5:1 bandwidth stripline notch array from FDTD analysis

A 5:1 bandwidth stripline notch array antenna is designed from parametric investigations of flare and feed dimensions. The finite-difference time-domain (FDTD) method is employed to perform the parametric studies. Both linear and planar single-polarization arrays are considered with half-wavelength element spacing at the highest frequency. The linear array elements depend upon E-plane element mutual coupling to achieve wideband behavior. Edge elements, which cannot benefit from full E-plane coupling, are shown to maintain good transmit performance with the application of amplitude tapering. The planar array is shown to have a scanability (active VSWR <2) averaging 51/spl deg/ off broadside in the E-plane and exceeding 60/spl deg/ in the H-plane. As an infinite planar array, the antenna is predicted to have a bandwidth exceeding 7:1 on broadside. Measurements are in good agreement with the computations.

A high-gain 58-GHz box-horn array antenna with suppressed grating lobes

A low-profile high-gain antenna array of box horns for the frequency band 57.2-58.2 GHz is presented. The antenna consists of 256 radiating elements divided into two subgroups of 128 elements fed by a rectangular waveguide feed network. The radiating elements are fed in parallel and the waveguides are connected with T-junctions. The matching of the T-junctions is improved with a matching pin and a splitter. Because of the waveguide feed network, the element spacing is larger than one wavelength, which causes grating lobes. The grating lobes and sidelobes in the H-plane have been suppressed by the use of a combination of subarrays, a special characteristic of the box horn, and an array amplitude tapering. The measured sidelobe levels in the H-plane are below -30 dB at angles larger than 8/spl deg/ from boresight. A gain higher than 35.7 dBi and a return loss higher than 14.4 dB have been measured for the antenna over the band 57.2-58.2 GHz.