Physical Aperture Research Articles

Improved Coprime Arrays With Reduced Mutual Coupling Based on the Concept of Difference and Sum Coarray

As is well known, the prototype coprime arrays consist of two collinear subarrays, whose physical sensor number is coprime to each other. Moreover, their larger inter-element separations and physical array apertures make them have less mutual coupling than uniform linear arrays. Even so, there is still mutual coupling existing in them, which has an adverse effect on the direction of arrival estimation. To reduce the effects of mutual coupling, several improved coprime configurations based on the concept of difference and sum coarray are proposed in this paper, which can be obtained through three operations. The first one is to shift the prototype coprime array to obtain the translational coprime arrays, which can help to reduce the redundancy between the difference coarray and sum coarray. Second, flip one of the subarrays in the translational coprime arrays with zero points as the symmetric center to get the flipped coprime arrays, which can expand the physical array aperture and inter-element separations further. Finally, move one of the symmetrically placed sensors in the flipped coprime arrays to zero points. Then, the proposed arrays are derived. They have almost the same consecutive degrees of freedom (DOFs) as the prototype coprime array but dramatically reduced mutual coupling due to their larger physical array apertures and inter-element separations, where the virtual arrays are constructed by their respective difference and sum coarray. In the end, the numerical simulation results confirm the superiority of proposed arrays.

Link Budget and Noise Calculator for Satellite Communication

MATLAB is widely used in all areas of applied mathematics, in education and research at universities, and in the industry. MATLAB stands for MATrixLABoratory and the software is built up around vectors and matrices. This makes the software particularly useful for linear algebra but MATLAB is also a great tool for solving algebraic and differential equations and for numerical integration. Among MATLAB software main function is to produce a calculator to calculate the Link Budget System and the noise of the system. A link budget is accounting of all of the gains and losses from the transmitter, through the medium used (i.e. free space, cable, waveguide, and fiber) to the receiver in a telecommunication system. The purpose of calculating link budget is to investigate the system performance tied to operation threshold, to get the minimum power, Cmin that should be received at the demodulator input in order for communication to work properly with reading typically of 10 dB.By using this design, all the parameter (i.e. Effective Isotropic Radiated Power (EIRP), Path loss/Free space path loss, Effective aperture, Flux density, Physical aperture, Transmit gain, Receive gain, Transmit power and Receive power) can be calculated. Lastly, the purpose of calculating noise in satellite link system is to get noise temperature ratio and noise density ratio. A noise temperature ratio is a figure of merit used to represent the quality of a satellite or an earth station receiver. While, noise density ratio is one of the most important and most often used parameters when evaluating a digital radio system.

Experimental Results of a 3-D Millimeter-Wave Compressive Reflector Antenna Imaging System

This letter presents the first experimental results of our three-dimensional (3D) millimeter-wave (mm-wave) Compressive-Reflector-Antenna (CRA) imaging system. In this prototype, the CRA is 3D-printed and coated with a metallic spray to easily introduce pseudo-random scatterers on the surface of a traditional reflector antenna (TRA). The CRA performs a pseudo random coding of the incident wavefront, thus adding spatial diversity in the imaging region and enabling the effective use of compressive sensing (CS) and imaging techniques. The CRA is fed with a multiple-input-multiple-output (MIMO) radar, which consists of four transmitting and four receiving ports. Consequently, the mechanical scanning parts and phase shifters, which are necessary in conventional physical or synthetic aperture arrays, are not needed in this system. Experimental results show the effectiveness of the prototype to perform a successful 3D reconstruction of a T-shaped metallic target.

Coherency structure exploitation in DOA estimation for non‐circular sources

This study addresses the problem of direction-of-arrival (DOA) estimation for non-circular (NC) sources in the presence of multipath propagation. Recent research reveals that the use of the association of incident signals, i.e. source association (SA), can provide a significant gain in reducing DOA estimation error, especially in the presence of multiple sources. Here, in order to make full use of both the properties of NC sources and the SA information, the multipath model with NC sources is studied, and then a simple combination algorithm is proposed to estimate the SA, based on which, the authors derive an iterative root estimator to refine a given coarse DOA estimation. Meanwhile, the stochastic Cramer-Rao bound taking these two factors into account is derived as well. The proposed method is search-free and maintains the array aperture which is up to twice of the physical array aperture. Numerical simulations have been provided to verify the effectiveness of the proposed method.

High gradient pulsed quadrupoles for novel accelerators and space charge limited beam transport

Novel acceleration schemes like plasma wake-field based accelerators demand for high gradient focusing elements to match the Twiss parameters in the plasma to the transport lattice of the conventional accelerator beamlines, with typically much higher beta-functions. There are multiple candidates for achieving high gradient focusing fields, each one having certain drawbacks. Permanent magnets are limited in tunability, plasma lenses might degrade the transverse beam quality significantly and conventional magnets cannot reach very high gradients and often cannot be placed in direct proximity of the plasma accelerator because of their size. In this paper we present design considerations and simulations on compact, high gradient, pulsed quadrupoles, that could be used e.g. for final focusing of space charge dominated bunches into a LWFA (Laser Wake-Field Accelerator) at SINBAD or other facilities with similar demands. The target design gradient is 200 T/m at a physical aperture on the order of 10 mm.

A Low-Profile Lightweight Circularly Polarized Rectenna Array Based on Coplanar Waveguide

A low-profile and lightweight circularly polarized (CP) rectenna array based on coplanar waveguide (CPW) is proposed in this letter. The CPW is adopted for enhanced bandwidths. A CPW rectifier, which consists of a matching network, a series diode, and a compact dc-pass filter, is directly integrated on the ground plane of the receiving antenna. The proposed rectenna is fabricated and measured. It features stable conversion efficiencies on three azimuthal planes. The maximum efficiency of 71.2% has been measured with a 400 Ω load at 5.8 GHz. A 4 × 4 rectenna array is also built for a large physical aperture and increased output power. The maximum conversion efficiency of 65.24% is measured with the array. Taking advantage of the immunity to misalignment of the CP receiving antenna, the proposed rectenna and array are suitable to be used in mobile applications for wireless power transmission.

A 5.8 GHz Circularly Polarized Rectenna With Harmonic Suppression and Rectenna Array for Wireless Power Transfer

A compact circularly polarized (CP) rectenna based on wide-slot antenna is designed at 5.8 GHz. The feedback wide-slot antenna is used for its broad band, easy-to-integrate circuitry, simple structure, and low sidelobes. The designed receiving antenna has a bandwidth of 31.8%. In order to minimize the profile of the rectenna, the rectifier is integrated on the back side of the substrate with a matching circuit and a dc-pass filter. It features stable conversion efficiency on three azimuthal planes. The maximum efficiency of 66.2% and an output dc voltage of 6.33 V have been measured with a 900 Ω load at the power density of 11.4 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A 5 × 5 rectenna array is built for large physical aperture and increased output power. The maximum conversion efficiency of 62% is observed with the array. Taking the advantage of immunity to misalignment of the CP antenna, the proposed rectenna and array are suitable to be used in mobile applications for wireless power transfer.

Using Image Analysis to Determine Ideal Aperture Conditions for Optimal Z-Scan Signal Quality

An image analysis technique that is insensitive to power fluctuations and beam steering issues, and can be used with non TEM00 Gaussian beams, is introduced to determine the nonlinear properties of materials using the well-known Z-scan experimental technique. Instead of measuring the transmission through or around a physical aperture, this method records the complete beam profile digitally at each step as the sample is translated. Various apertures of any desired shape are then applied to the recorded data set to allow accurate nonlinear characterization under identical experimental conditions which means that both open aperture and closed aperture measurements can be performed on the same data set. Through varying the aperture shape, the signal to noise and sensitivity of the Z-scan measurement can be optimized allowing weak nonlinearities, such as the nonlinear absorption of toluene to be measured. The technique is used to simultaneously perform closed aperture Z-scan or eclipsing aperture Z-scan measurements, with the signal dependence on aperture size agreeing well with that theoretically expected. A novel ring aperture is introduced that allows accurate determination of the laser beam radius without requiring a nonlinear reference sample to calibrate this important Z-scan parameter.

Exploiting platform motion for passive source localization with a co-prime sampled large aperture array.

Co-prime array geometries have received a great deal of attention due to their ability to discriminate O(MN) sources with only O(M + N) sensors. This has been demonstrated both theoretically and in simulation. However, there are many practical limitations that make it difficult to realize the enhanced degrees of freedom when applying co-prime geometries to real acoustic data taken on a horizontal line array. For instance, co-prime sampling leads to grating lobes that can obscure lower signal-to-noise-ratio acoustic signals making them difficult to detect. In this work, a synthetic aperture (SA) method is presented for filling in holes and increasing redundancy in the difference co-array by exploiting array motion. The SA method is applied to acoustic data collected off the Southeastern shore of Florida on a fixed large aperture horizontal array. Array motion is simulated by taking a co-prime sampled subarray and virtually moving it along the horizontal aperture of the fixed array. It is demonstrated that SA processing on real acoustic data results in reduced side-lobe and grating lobe levels compared to that of the physical co-prime aperture.

Interferometric Sounding Using a Metamaterial-Based Compressive Reflector Antenna

This paper describes a new coded interferometric system for sensing the physical temperature radiated from the Earth’s surface. The proposed system consists of a compressive reflector antenna (CRA) coated with metamaterial absorbers (MMAs). The CRA and the MMA are used to code the received electromagnetic (EM) field in space and in frequency, at the focal plane array. The MMA is modeled by an equivalent magnetodielectric medium having a definite thickness. A high-frequency method based on physical optics is used to build the sensing matrix of the system, and the inverse problem is solved using a Nesterov-based compressive sensing (CS) methodology. Numerical examples are carried out in order to reconstruct the physical temperature of the Earth’s surface. The performance of the proposed system is compared to that of the conventional interferometric system, GeoSTAR. Preliminary results show that the metamaterial-based CRA provides comparable performance to the GeoSTAR configuration with only half of the feeding elements while keeping the same physical aperture size for the two configurations.

Investigations into improving the detectability of self‐resonant RFID tags

This study describes an experimental investigation into improving the detectability of self-resonant structures. Passive self-resonant structures have been used as an alternative to active devices in many applications including radio-frequency identification (RFID) due to their long lifetime and low manufacturing cost. They can be used to store information in terms of the presence or the absence of resonance peaks at a given frequency window. This work investigates practical validation of gain enhancement techniques, aiming to improve the resonance characteristics of passive RFID tags to increase their read range and allow them to operate in highly lossy mediums. Two passive resonator designs are studied: a uniform cross-shaped resonator and a folded (gammadion) cross having a physical aperture 45% less than that of the uniform cross. Arrays of elements have been used to improve the detectability of the structure. Measurement settings have also been investigated in order to achieve higher levels of detectability.

Azimuth Location Deambiguity for SAR Ground Moving Targets via Coprime Adjacent Arrays

The ground moving target's radial velocity estimation based on the interferometric phase in a multichannel synthetic aperture radar (SAR) system suffers from 2π modulo folding, resulting in the target's azimuth location ambiguity in the SAR image. To address this problem, a novel coprime adjacent arrays SAR (CAA-SAR) is proposed in this paper. The two sparse uniform subarrays constituting the CAA are arranged adjacently with a conjunct element, then a virtual array can be obtained with much more virtual elements and smaller element spacing, which will help to solve the azimuth location ambiguity. After ground clutter suppression, multiple pixels’ samplings are utilized based on the MUSIC algorithm for estimating radial velocity as well as azimuth shift by exploiting all virtual elements of CAA-SAR. Compared with the existing nonuniform linear array SAR method based on Chinese remainder theorem, the CAA-SAR can obtain a better accuracy with the same number of elements in a larger sparse configuration, or use fewer elements to obtain an approximate accuracy in almost the same physical aperture. Compared with the coprime arrays SAR, the CAA-SAR has a better accuracy due to its larger number of unique virtual elements and longer physical aperture. Finally, some results of numerical experiments are provided to demonstrate the effectiveness of the proposed method.

A General Precipitation-limited LX–T–R Relation among Early-type Galaxies

Abstract The relation between X-ray luminosity (L X) and ambient gas temperature (T) among massive galactic systems is an important cornerstone of both observational cosmology and galaxy-evolution modeling. In the most massive galaxy clusters, the relation is determined primarily by cosmological structure formation. In less massive systems, it primarily reflects the feedback response to radiative cooling of circumgalactic gas. Here we present a simple but powerful model for the L X–T relation as a function of physical aperture R within which those measurements are made. The model is based on the precipitation framework for AGN feedback and assumes that the circumgalactic medium is precipitation-regulated at small radii and limited by cosmological structure formation at large radii. We compare this model with many different data sets and show that it successfully reproduces the slope and upper envelope of the L X–T–R relation over the temperature range from ∼0.2 keV through . Our findings strongly suggest that the feedback mechanisms responsible for regulating star formation in individual massive galaxies have much in common with the precipitation-triggered feedback that appears to regulate galaxy-cluster cores.

Practical Superdirectivity With Resonant Screened Apertures Motivated by a Poynting Streamlines Analysis

Fundamental limits on the degree of practical achievable superdirectivity for antennas include narrow bandwidth, large antenna loss, and sensitivity to excitation and fabrication errors. Superdirective antennas are commonly considered as transmitters, but thinking of the antenna as a receiver may help to understand and overcome some of these limits on practical superdirectivity. We directly model a superdirective antenna in receive mode by calculating streamlines of Poynting vector field near the receiving antennas. Superdirectivity is achieved by expanding the shape of the effective area beyond the antenna physical aperture area. The degree of superdirectivity is parameterized by an effective area expansion distance. The theory predicts that superdirectivity is practical for electrically small and middle size antennas and electrically large antennas with a large aspect ratio. With this motivation, a $1.5\lambda \times 2\lambda $ superdirective horn antenna with resonant screen structure in front of the antenna aperture is designed and fabricated. The superdirective horn achieves a measured antenna efficiency of 115% over a reasonable bandwidth and with moderate sensitivity to fabrication errors.

Highly Accurate Two-Dimensional Direction Finding Using Cumulant

This paper proposes a new two-dimensional direction finding algorithm for estimating multiple non-Gaussian signals using cumulants. This new algorithm: 1) employs an array geometry consisting of a nine-element non-uniformly spaced cross-shaped sub-array plus another arbitrarily-spaced sub-array; 2) defines five cumulant matrices, from which the signal direction cosines can be estimated by using parallel factor fitting, and hence, requiring no open-form two-dimensional searching and parameter pairing; 3) can achieve array aperture extension by jointly extending array physical aperture and constructing virtual sensors, without adding more sensors; and 4) can resolve $2(L - 4)$ signals with an array of $L$ sensors in total. Simulation results are presented to verify the efficacy of the proposed algorithm.

Unified coprime array with multi-period subarrays for direction-of-arrival estimation

The original coprime array is a sparse geometry with a pair of one-period subarrays, where the interelement spacing and number of sensors are coprime so that no elements overlap except at the zeroth position. In this paper, we unify the coprime array by extending two subarrays from one period to multiple periods. Each subarray can have an arbitrary number of periods and therefore more than one element of the unified structure coincide. The resulting configuration treats the nested array as well as the existing coprime structures, such as the original coprime array, the extended one and the coprime array with compressed inter-element spacing (CACIS) as its special cases. The new coprime array has a lower peak side-lobe level than the nested array and the CACIS. An improved structure based on the multi-period coprime array is further proposed to improve the degrees-of-freedom (DOF) of the unified geometry with one of the subarrays displaced by a certain number of spacings. Although the improved configuration does not reach the same DOF as the CACIS and the coprime array with displaced subarrays (CADiS), it can acquire a low peak side-lobe level with the DOF not being decreased too much. Different from CADiS, we focus on the analysis and proper choice of the displacement to acquire more consecutive lags. The properties of coarray of the proposed array design strategy are provided as well. Finally, the direction-of-arrival estimation, physical aperture and peak side-lobe level performance of the unified structures are examined through numerical simulations.

Planar Aperture Antenna With High Gain and High Aperture Efficiency for 60-GHz Applications

This paper presents two new planar aperture antennas (PAAs) with high gain and high aperture efficiency. Each antenna is composed of an opening cavity and differentially fed by the grounded coplanar waveguides at two parallel edges. In order to improve the gain of the original PAA, physical aperture has been extended and patches are introduced on the aperture to uniform the electric field to improve the aperture efficiency. Two corresponding prototypes are designed. Experimental results show that for the first prototype whose aperture is extended in the guided-wave propagation direction, the reflection coefficient can maintain less than −15 dB from 55 to 66 GHz. The maximum gain is 19.7 dBi at 63 GHz with the aperture efficiency of 66.03%, and the 3-dB gain bandwidth is 57–65 GHz. The second prototype, whose aperture is extended in the transverse direction, can achieve −15-dB impedance bandwidth from 56.4 to 66 GHz. The maximum gain is 20.3 dBi at 61 GHz with the aperture efficiency of 63.06%, and the 3-dB gain bandwidth is 57–64 GHz. Due to their outstanding performance and fabrication merits, the proposed PAAs are good candidates for the future cost-efficient fifth-generation wireless systems.

A Dual-Polarization $X$ -Band Traveling-Wave Antenna Panel for Small-Satellite Synthetic Aperture Radar

This paper investigated the possibility of using an $X$ -band traveling-wave antenna for application in synthetic aperture radar (SAR) on a small-satellite platform. A rectangular slotted parallel-plate antenna panel capable of radiating dual polarization over the same physical aperture was selected for this purpose. The design procedure, which provides for an optimal antenna pattern in a SAR application, is described. A meta-heuristic multiobjective optimization algorithm was applied to synthesize an appropriate far-field pattern, which enhances the quality of the SAR image. Subsequently, the optimization results were used to build a more realistic model of the antenna panel in electromagnetic simulation software. An antenna panel was fabricated, and its electrical characteristics were measured. The fabricated prototype weighed 1310 g and measurements showed nearly 50% aperture efficiency for both circular polarizations [34.6-dBic right-hand circular polarized (RHCP), 34.5-dBic left-hand circular polarized (LHCP), and peak gain at 9.65 GHz], with low side-lobes. A beam shift of 2.1° was observed between the RHCP and LHCP beams at the design center frequency and was later analyzed to be the result of an error in the modeling of the adhesive layer.

On the Incident Power on a Receiving Slender Antenna and the Optical Theorem in the Near Field

Incident power upon a slender scatterer or a receiving antenna is not well defined, since physical apertures of such structures have little meaning. However, an area over which the incident power is admitted can nonetheless be defined. For a definition that relies solely on the receive mode, we develop a near-field version of the optical theorem. When equated with the conventional far-field version, an equation for the admitting area appears. This equation is solved via both a full-wave simulation and a quasi-analytical way. The result is also supported by two transmit-mode assessments. It is shown that under this definition, the half-wavelength dipole can be replaced by an equivalent aperture antenna facing the direction of the incident wave.

Efficient method of two-dimensional DOA estimation for coherent signals

An effective decoherence method called the fourth-order cumulants-based improved Toeplitz matrices reconstruction (FOC-ITMR) is addressed for two-dimensional (2-D) direction-of-arrival (DOA) estimation of coherent signals. To avoid the loss of the array’s physical aperture, the FOC-ITMR method fully utilizes the information of received data from the whole two parallel uniform linear arrays (ULAs) and the changing reference element based on FOC. Compared with previous works, the proposed method can offer excellent decoherence performance in both white noise and color noise environments. In addition, the proposed algorithm can achieve automatic pair-matching without additional computation. The theoretical analysis and simulation results confirm the effectiveness of the proposed algorithm.