Amplitude Tapering Research Articles

Adaptively Thinned Arrays

Array thinning places a low sidelobe amplitude taper on a large antenna array by making certain elements in a uniform array inactive. The active elements contribute to forming the beam, while the inactive elements are terminated in a matched load. Transmit/receive (T/R) modules often have switches that not only connect to the transmit or receive paths but also connect to a calibration port. In addition to changing between transmit, receive, and calibration modes, these switches can be used to alter the array thinning to either modify the sidelobe level or move a null. This paper describes an approach to interference suppression by adaptively thinning the array in order to reject sidelobe interference. Changing the random thinning can lower a sidelobe or place a null in the direction of interference.

Grating Lobe Suppression in Aperiodic Phased Array Antennas Composed of Periodic Subarrays with Large Element Spacing

The techniques of grating lobe (GL) suppression in planar phased array antennas composed of periodic subarrays with rectangular or triangular grids with large element spacing are considered. GL suppression is achieved using an aperiodic arrangement of subarrays. This approach provides a means to decrease GLs without a significant increase in structural complexity relative to periodic arrays; moreover, other antenna parameters do not essentially change compared with periodic arrays. Estimations of maximum GL suppression are presented. Antenna characteristics such as sidelobe level, beam efficiency, and aperture efficiency are also considered. The influences of element radiation pattern and amplitude tapering are analyzed. Examples of radiation patterns of such phased arrays are given.

Hybrid Approach for the Synthesis of Unequally Spaced Array Antennas With Sidelobes Reduction

A hybrid approach is presented to synthesize unequally spaced array antennas with low sidelobes. The exact locations of all the elements for unequally spaced array antennas can be quickly determined with the composite Taylor-Exponent configuration arrangement. Furthermore, an amplitude taper with tremendously simple configuration was designed to reduce the sidelobes for unequally spaced array antennas. Numerical results show that the proposed hybrid approach is effective for the modified synthesis of unequally spaced array antennas, with the sidelobes reduced to be lower than -30 dB.

Array Antennas With Jointly Optimized Elements Positions and Dimensions Part II: Planar Circular Arrays

The paper describes a deterministic algorithm based on simple analytical equations for designing planar circular sparse array antennas where the positions and the dimensions of the radiating elements are jointly exploited and optimized. The proposed solution permits addressing one of the main limitations associated to sparse arrays, i.e., their poor aperture efficiency especially when patterns with low sidelobe levels are obtained using only the density of the elements avoiding any amplitude tapering. The accuracy of the method is guaranteed by the fact that the achieved directivity differs from the target value, associated to a continuous reference aperture distribution, by a quantity asymptotically equal to the factor π/4 (corresponding to about -1.05 dB) representing the filling factor of a circular planar array organized in concentric rings populated by circular apertures. As required for several applications, the method includes the possibility to add a constraint on the maximum scanning angle with respect to the boresight direction.

Wideband Dielectric Lens Antenna With Stable Radiation Patterns Fed by Coherent Array of Connected Leaky Slots

A broadband reflector feed is presented, which consists of a dielectric extended hemispherical lens fed by a connected array of leaky-wave slots. The slot elements are coherently combined to generate directive radiation patterns that mostly illuminate the central part of the lens, the most efficient one. The array is capable of producing secondary patterns with almost constant -10 dB beamwitdh over a 3:1 bandwidth. This allows efficient illumination of the reflector over a wide frequency range. Performance is estimated in terms of amplitude taper and phase error losses at the reflector, yielding efficiency of about 80% over the entire 3:1 bandwidth. Although the envisaged applications are in the terahertz (THz) and mm-wave frequency bands, a low-frequency prototype demonstrator has been tested in the 4 to 12 GHz band, for experimental validation of the concept.

Performance Evaluation of Interference Bound Backhaul Links in High Capacity Wireless Networks

An innovative architecture employing wireless backhaul in hierarchical wireless networks is considered in this paper to provide high throughput densities. Multi-beam antennas, combined with aggressive frequency reuse have recently been exploited to increase the throughput density in densely deployed wireless networks. However, large number of narrow beams cause not only co-channel interference but also inter-cell interference. To mitigate this problem, amplitude tapering in multi-beam antennas is investigated in this paper. Results show that tapered beams significantly improve the carrier to interference ratio from 5 to 8 dB and consequently achieve downlink throughput density over 0.65 Gbps/km $$^2$$ 2 . An overall throughput density of 1.2 Gbps/km $$^2$$ 2 is expected when taking both up- and downlinks into account.

Design of low sidelobe Microstrip antenna array

A low side lobe level is obtained using Tschebyscheff amplitude tapering. To obtain amplitude tapering Wilkinson power divider is used. Two antenna structures are designed, first is conventional corporate feed array that operates at 5.5 GHz with a side lobe level of -12.7 dB. The second structure is designed through appropriate Tschebyscheff distribution. It results in a decrease in SLL to -15.4dB. The simulated results are presented and discussed.

Far-Zone Focusing Lenses Designed by Complex Coordinate Transformations

In this letter, a transformation optics approach involving complex coordinates is presented for designing far-zone focusing lenses fed by a single radiating electromagnetic source. In contrast to conventional coordinate transformation (CT) methods that tailor only the real parts of the material parameters, a spatial amplitude tapering function containing either gain or loss is introduced within the lens, which provides an extra degree of freedom in controlling both the amplitude and phase distributions at the aperture of the lens, resulting in a controllable far-field radiation pattern. The concept is validated by full-wave lens simulations with parametric studies carried out to investigate the impact of both the geometrical and material properties of the lens on the far-field patterns. It is then demonstrated that the lens designs can be further optimized to control the sidelobe levels (peak as well as minor lobes) while maintaining a relatively high gain. Finally, it is shown that the lens exhibits a far-field radiation pattern comparable to that of a linear array with uniform element spacing, but is unidirectional rather than bidirectional. By providing a means to control both the amplitude and phase distribution across the lens aperture, the proposed complex CT design approach provides a pathway to constructing directive emitting devices with radiation properties similar to conventional antenna arrays but with only a single feeding source.

Conical Horn: Gain and Amplitude Patterns

The conical horn radiation characteristics (gain, aperture phase distributions, loss factors, and amplitude patterns) are revisited based on the spherical and quadratic aperture phase distributions. Calculations of the conical horn gain, using over the aperture either the dominant -mode of a circular waveguide or the modal variations of a conical waveguide, are calculated using the aperture phase distribution represented by spherical and quadratic wavefronts. The gains are compared with the classical data by Gray and Schelkunoff, in an attempt to confirm their validity and accuracy and to determine whether they were derived based on spherical or quadratic phase distribution. In addition, more accurate loss factors, to account for amplitude and phase tapering over the conical horn aperture, are derived which improve the conical horn gain. New formulas for the design of optimum gain conical horns, based on the more accurate spherical aperture phase distribution, are derived and reported, and guidelines are provided for their utility. This paper also introduces a method to calculate accurately the far-zone amplitude patterns in the E and H planes, including those in the far side and back lobe regions, of a conical horn by utilizing geometrical optics and diffraction theory.

High-Frequency Radars: Beamforming Calibrations Using Ships as Reflectors*

Abstract Linear array antennas and beamforming techniques offer some advantages compared to direction finding using squared arrays. The azimuthal resolution depends on the number of antenna elements and their spacing. Assuming an ideal beam pattern and no amplitude taper across the aperture, 16 antennas in a linear array spaced at half the electromagnetic wavelength theoretically provide a beam resolution of 3.5° normal to the array, and up to twice that when the beam is steered within an azimuthal range of 60° from the direction normal to the array. However, miscalibrated phases among antenna elements, cables, and receivers (e.g., caused by service activities without recalibration) can cause errors in the beam-steering direction and distortions of the beam pattern, resulting in unreliable ocean surface current and wave estimations. The present work uses opportunistic ship echoes randomly received by oceanographic high-frequency radars to correct an unusual case of severe phase differences between receiver channels, leading to a dramatic improvement of the surface current patterns. The method proposed allows for simplified calibrations of phases to account for hardware-related changes without the need to conduct the regular calibration procedure and can be applied during postprocessing of datasets acquired with insufficient calibration.

A 79-GHz Resonant Laminated Waveguide Slotted Array Antenna Using Novel Shaped Slots in LTCC

This letter presents a laminated waveguide (LWG)-based slotted array antenna in LTCC. The introduced novel slot shape derived from a standard longitudinal offset slot provides a further degree of freedom for the antenna design by bending the outer sections of the slot. Based on the characteristics of a single slot, the design procedure of an antenna array with amplitude tapering at a constant slot offset is presented. As an example, design, realization, and measurement of a 12-element array at 79 GHz with Dolph-Chebyshev power distribution with a sidelobe level of -20 dB are presented.

Antenna Array for Passive Radar: Configuration Design and Adaptive Approaches to Disturbance Cancellation

We consider the selection of an antenna array configuration, composed of a small number of omnidirectional elements, to be exploited for passive radar sensors. Based on properly identified pattern characteristics and design criteria for practical applications, a suitable planar configuration is selected that allows both angular selectivity and direct signal attenuation. The selected configuration is further optimized in terms of sidelobe level by resorting to appropriate amplitude tapering. Moreover, three different approaches are investigated for antenna-based adaptive disturbance cancellation, and a comparative performance analysis is carried out. Simulation results show that an effective clutter suppression is obtained if the direct signal from the transmitter is attenuated by means of spatial adaptive cancellation, and the multipath echoes from stationary obstacles are removed by means of temporal adaptive cancellation. In particular, the approach based on the Sidelobe Canceller is shown to yield good performance while requiring a limited system complexity.

Grating Lobe Suppression for Distributed Digital Subarrays Using Virtual Filling

A receiving array processing method is presented that synthesizes the antenna response of a contiguous array from the outputs of physically separated arrays of subarrays. It involves filling the gaps between the subarrays with “virtual” elements, thus forming a contiguous array. Therefore, the synthesized pattern has no grating lobes, and amplitude tapering for sidelobe control can be applied. The contiguous array's response can be duplicated in a number of directions limited by the total number of elements. The directions of interest would normally be those of the desired signals (main beam) and interference or clutter (low sidelobes). The directions of arrival of the signals of interest must first be determined before synthesizing the contiguous array response. The modified matrix pencil method has been extended to handle the problem of multiple subarrays for both single and multiple snapshots. The performance of the synthesis method is examined as a function of signal-to-noise ratio (SNR) per element and various array parameters .

An Innovative Strategy for Synthesis of Uniformly Weighted Circular Aperture Antenna Array Based on the Weighting Density Method

Uniformly weighted planar array with a low peak sidelobe level is desired in many engineering applications. In this letter, an innovative strategy for synthesis of uniformly weighted circular aperture array in order to obtain a low peak sidelobe level is presented. This method is based on weighting density (WD) of an array, which refers to the average areal distribution of amplitude taper. By using WD of the reference array, the optimum radius or rings of desired array can be searched. Two types of uniformly weighted circular aperture arrays are applied with this method, and simulation results show a low peak sidelobe level as well as symmetric element distribution can be achieved.

Spatial and angular domain analysis of the near-field radiation of electrically large apertures with non-uniform field

According to the equivalence principle, the design of the compact range (CR) reflector can be transformed into the design of its aperture. Therefore the analysis of the CR performance variations caused by the mechanical and electrical defects can be transformed into the analysis of the aperture near-field in the spatial and angular domains. Based on a square aperture and a well-designed serrated-edge aperture, the near-field characteristics have been analysed when typical amplitude and phase non-uniformities have been induced in the aperture. The aperture field convolution method and plane wave spectrum method have been adopted. Simulation results have been presented and compared in detail. The width and height of the apertures are 150 wavelengths in free space. The amplitude and phase non-uniformities discussed include: amplitude taper, serrated edge, periodic amplitude, periodic phase, linear amplitude, linear phase, local amplitude mutation and local phase mutation. The conclusions provide references for CR design, testing, measurements and antenna tolerance analysis.

A tapered undulator experiment at the ELBE far infrared hybrid-resonator oscillator free electron laser

A tapered undulator experiment was carried out at the ELBE far-infrared free electron laser (FEL). The oscillator FEL makes use of a hybrid optical resonator. The main motivation was to see whether the presence of a dispersive medium in the form of a waveguide in the resonator has any effect on the outcome. The FEL saturated power and the wavelength shifts have been measured as a function of both positive as well as negative undulator field amplitude tapering. In contrast to the typical high-gain FELs where positive tapering proves beneficial for the output power we observed an improvement of performance at negative taper. During the same experiments we studied the characteristics of the detuning curves. The width of the curves indicates a maximum small signal gain for zero taper while the output peak power increases with negative taper. The saturated power output, the detuning curve characteristics, and the wavelength shifts agrees with the theoretical predictions. Details of the experiment are presented.

Low Sidelobe Phased Array Pattern Synthesis With Compensation for Errors Due to Quantized Tapering

Recently, the iterative Fourier transform (IFT) method has been introduced for the synthesis of low sidelobe patterns for array antennas with periodic element arrangement. The IFT method makes use of the property that for an array with periodic element spacing, an inverse Fourier transform relationship exists between the array factor and the element excitations. This property is used in an iterative way to derive the array element excitations from the prescribed array factor using a direct Fourier transform. With some simple additions, the same IFT technique is also applicable to mitigate the degradation of sidelobe performance of array antenna caused by quantization of its taper across the array when using discrete control devices. Numerical examples of low sidelobe pattern synthesis applying compensation for taper quantization errors are presented. The results refer to both phase as well as amplitude tapering of linear arrays of various sizes.

Microstrip Grid and Comb Array Antennas

The design of a high-gain microstrip grid array antenna (MGAA) for 24-GHz automotive radar sensor applications is first presented. An amplitude tapering technique utilizing variable line width on the individual radiating element is then applied to lower sidelobe level. Next, the MGAA is simplified to a microstrip comb array antenna (MCAA). The MCAA shows broader impedance bandwidth and lower cross-polarization radiation as compared with those of the MGAA. The MCAA is designed not as a travelling-wave but a standing-wave antenna. As a result, the match load and the reflection-cancelling structure can be avoided, which is important, especially in the millimeter-wave frequencies. Finally, an emphasis is given to 45 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> linearly-polarized MCAA because the radiation with the orthogonal polarization from cars coming from the opposite direction does not affect the radar operation.

Photonic Beamforming Based on Programmable Phase Shifters With Amplitude and Phase Control

A new optically controlled phased array beamforming technique based on phase modulations of wavelength-division-multiplexed (WDM) single sideband modulated light with carrier using a two-dimensional array liquid crystal on silicon (LCoS) pixels device, is presented. It realizes multiple wideband photonic microwave phase shifters, operates over a wide frequency range, and simultaneously integrates the array phase taper and the array amplitude taper within a single unit. The beamforming is software programmable. Experimental results demonstrate multiple independent microwave phase shifters that operate simultaneously over a 10- to 20-GHz frequency range, and results demonstrate beamsteering in a four-element optically controlled beam steering array at 18 GHz.

Sidelobe reduction of the low profile multi-subarray antenna by genetic algorithm

Abstract In this paper, the sidelobe of the low profile multi-subarray antenna for satcom on-the-move (SOTM) is reduced by genetic algorithm (GA). The characteristics of the antenna are analyzed theoretically. It is indicated that the sidelobe level of the antenna is difficult to be reduced by amplitude tapering for its dispersed framework. In order to suppress the sidelobe level of the antenna in the whole elevation scan range, the subarray distances are optimized by GA. Two types of the antennas, the fixed-space and the active-space multi-subarray antennas, are analyzed respectively. Then the effects of amplitude perturbation on sidelobe cancellation are discussed under two conditions, i.e., the initial subarray distances and the optimal subarray distances. Simulation results prove the validity of the theoretical analysis, show that the perturbation of subarray distances can suppress the sidelobe level effectively, and the perturbation of amplitude is almost ineffective in sidelobe reduction. The conclusions can be used as the reference to the design of the multi-subarray antenna and other antennas whose array calibrations are discontinuous.