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Near-field measurements on antennas require magnitude and phase information dependent on the antenna position to support the near-field to far-field transformations. Modern active antennas are often integrated into frequency converters with embedded local oscillators (LO). For example, devices ranging from small 5G transceivers to large satellite payloads often need to be tested with the antenna integrated into the overall solution. There is no access to the embedded LO signal in these systems. The unknown phase of the embedded LO masks or corrupts the near-field phase measurement of the integrated antenna under test. A novel solution to this challenge is presented based on a new Vector Network Analyzer (VNA) platform. The system utilizes two stimulus signals (a measurement signal and a pilot signal) to characterize the antenna under test which is integrated into the frequency convertor. The pilot signal captures the phase information of the embedded LO, allowing the measurement signal to capture the antenna's magnitude and phase pattern as the antenna under test is moved within the near-field region.
Recently, lack of configurable radio environments which requires new signals to be created for data transmission results in increased power consumption has been identified as one of the major challenges in wireless communications. This challenge is argued to be preventing wireless network operators from realizing intelligent, sensing and computing platforms. On the other hand, programmable, frequency-selective surfaces, smart reflectarrays or mirrors, embedding arrays of low-cost antennas or coating surfaces with reconfigurable meta-surfaces are considered to be potential solutions. Highly reconfigurable reflectarrays provide electronic beam steering, as the patch elements can be tuned and modified for real time beam steering. Using tunable materials and lumped components are the most widely used techniques for emerging reconfigurable reflectarrays. Devices using tunable materials such as liquid crystals, ferroelectrics, and graphene have been proposed. The major hurdles in developing highly reconfigurable reflectarrays include keeping reflection losses as low as possible, reflection phase range as wide as possible, and being able to achieve wide beam widths such that all the patch elements can receive the incident signals from distant feeds. We propose an ultra-reconfigurable device based on the insulator-to-metal transition property of VO2. A VO2 layer is placed on a high-density micro-heater matrix consisting of pixels that can be switched on via electronic control. Controlling the pixels in this manner, heat can be transferred to the selected areas of the VO2 layer and convert to highly conductive metallic phase. This technique allows dynamically changing the shape of the reflection antenna surface with high speed. We numerically investigated the heat activated switching and RF reflection characteristics of a reflectarray designed for potential 5G applications operating in 32-86 GHz. It consists of heating pixels with the size of 615 x 615 um which can generate metallic VO2 patches or arbitrary shapes with the same spatial resolution. Our analyses resulted in large phase range of ~360 deg and low loss between -0.13 dB to -0.48 dB. The proposed device can serve as a novel platform for ultra-reconfigurable reflectarrays and metasurfaces for various of RF applications in wide a spectral range.
With growing communications, nowadays there are increasingly sophisticated antenna systems with associated electronics aboard aircrafts. Placement of antennas for various systems is a challenge due to the coupling between the antennas and the resulting co-site interference. Advances in electromagnetic (EM) simulations have significantly improved the design process to assess coupling between various antennas resulting in reduced testing time and costs. While it is ideal to use the actual simulation models of antennas during the design process, system designers normally do not have access to the simulation models (or CAD models) of the antennas for various reasons, such as antenna vendor confidentiality etc. It then becomes essential that actual antenna simulation models to be replaced with accurate representation using equivalent sources, such as near fields, far fields etc. In this paper, antenna coupling, and co-site interference calculations are demonstrated using equivalent antenna with near field sources representation installed on electrically large platform such as a commercial airplane. Advanced hybrid EM solutions using both full wave solvers, such as Method of Moments (MoM) and asymptotic solvers such as Large Element Physical Optics (LE-PO) are used for computationally efficient simulations. In this paper, we present a series of case studies using different configurations of equivalent antenna representations mounted on electrically large platform (aircraft). To illustrate the process, near field patterns are computed for a monocone antenna at 1 GHz. The real antennas are replaced by the equivalent near field sources and the antenna coupling is computed when installed on an electrically large platform. LE-PO method is used for electrically large platform to simulate this model for all the configurations. Equivalent antenna representation coupling results are in good agreement with the S-Parameter values obtained using real antenna mounted on the electrically large platform. A detailed analysis on Out of Band coupling are also performed for all these configurations and will be presented.
Over the years many methods have been developed and used for measuring permittivity and permeability of materials. The most widely used methods are: 1) free-space techniques; 2) cavity perturbation techniques; and 3) transmission line of waveguide methods. Each technique has its own advantages and limitations. The free-space methods are employed when the material is available in a big sheet form. These measurements are less accurate because of unwanted reflections from surrounding objects, difficulty in launching a plane wave in a limited space, and unwanted diffraction from the edges of the sample. The resonant cavity measurement or cavity perturbation techniques are more accurate. Recently "epsilon-near-zero (ENZ) metamaterials have received much attention for several interesting phenomena like super-coupling, transparency and cloaking devices and pattern reshaping at microwave and optical frequencies. The rapid growth and excitement of ENZ materials was due to their ability to achieve very long wavelength in zero permittivity material, allowing propagation in a static-like manner. This paper presents the evaluation of complex dielectric permittivity and magnetic permeability of materials using planar ENZ tunnel structure with substrate integrated waveguide technology. The changes in resonance frequency and quality factor are related to the dielectric permittivity and magnetic permeability properties of the sample through Cavity Perturbation Technique. ENZ tunnel structure has very high sensitivity, which yields more accurate results when compared to other techniques, such as perturbation of conventional cavities. Design, optimization, and simulation of the ENZ tunnel structure at microwave frequencies is presented. Simulations are performed on various dielectric and magnetic samples using the cavity perturbation technique of the ENZ tunnel structure and validated with measured data.
To build a reliable wireless communication for biomedical implant devices, many researches on the implantable antenna design have been performed to satisfy various requirements and constraints such as antenna shape, biocompatibility, miniaturization, and broad operation frequency. In addition to the wireless communication, it is also important to stably transfer wireless power required for the implanted devices to work continuously in a human body because normal battery cannot be a permanent power source inside a human body. For a wireless power transfer system applicable to implantable medical devices, two techniques based on a different theoretical background has been considered: Inductive coupling in near-field region and antenna coupling in far-field region. The inductive coupling is more advantage in terms of low specific absorption rate, but it requires high Q which limits the operation bandwidth. However, the antenna coupling is a critical limitation in transmitting power due to SAR (Specific Absorption Rate) although the antenna is well designed to achieve high power coupling. In this work, inductive coupling based WPT system is designed inside the implanted antenna not to require additional space and thickness. The implanted antenna is designed with a rectangular shaped PIFA (Planar Inverted F Antenna) using the medical implant communication service (MICS) band (402-405MHz) recommended by the Federal Communications Commission (FCC). Also, the WPT system is designed to operate at the industrial scientific medical (ISM) band, 13.56MHz, to easily implement a resonant coil with high ??-factor. Furthermore, the mutual effects between the WPT coil and the antenna are studied in terms of impedance, efficiencies, and SAR. Finally, the fact that how much maximum power can be delivered from the outside to the designed WPT system indicates that magnetic coupling is more promising than the antenna coupling for implantable medical devices.
The 18-term NIST error model is a common tool for analyzing potential sources of error in antenna measurements. One of the error terms to be considered describes the phase errors occurring in a measurement system. However, this quantity plays a rather negligible role for conventional ranges, such as roll-over-azimuth positioning systems. In particular, the contribution caused by flexing cables is normally insignificant. This results from the fixed installation of the cables or the decoupling of the movement at important points using rotary joints. Current developments in the field of antenna measurement technology focus, among other things, on performing measurements using industrial robot arms. These are characterized by their high flexibility regarding the various measurement sequences, such as planar, cylindrical or spherical measurements. However, it is to be expected that the high freedom of positioning possibilities will introduce additional phase uncertainties, since the RF cables in the cable carrier chain of the robot arm itself are often not decoupled. Instead, a single cable is used for each signal path, which follows the movements of the robot. The robot-based measurement system at the Institute of High Frequency Technology at RWTH Aachen University has been designed for frequencies above 60 GHz, where phase stability is a challenging task. Depending on the setup, it may even be required to pass Intermediate Frequency (IF) signals on the same cable as the Local Oscillator (LO) signals. This results in different test cases for the phase deviations depending on the frequency range of the IF (279 MHz) and LO (typically 10 GHz to 18 GHz) signals. Additional factors such as the measurement path of the robot or the position of the linear axis must also be taken into account. Therefore, a thorough analysis of the phase uncertainties caused due to flexing cables is of outstanding importance for robot-based measurement systems.
One of the main disadvantages of Spherical Near-Field (SNF) measurements is their acquisition time. This is due to the need of sampling a whole sphere around the Antenna Under Test (AUT) to perform the Near-Field-to-Far-Field Transformation (NFFFT). A step of the NFFFT is to decompose the measured signal in each one of the spherical waves it consists of, thus retrieving the Spherical Mode Coefficients (SMCs) associated to the AUT. Under typical measurement conditions, the SMCs of most physical AUTs prove sparse, i.e., most of their terms are zero or neglectable. Using this assumption, the system of linear equations with the SMCs as variables can be solved with fewer equations, that is, fewer measurement samples. This is done by applying an l1-minimization solver, following classical methodology from the field of compressed sensing. However, the location of the measurement points that generate non-redundant equations is not trivial. In typical compressed-sensing applications, a random sampling matrix is taken. Since a random matrix is inefficient for the acquisition with mechanical roll-over-azimuth positioner systems, a recent approach is to take an equidistant distribution of points on elevation and to calculate their corresponding pair on azimuth that delivers the minimum coherence of the sampling matrix. However, the number of sampling points M required for a successful reconstruction depends on the sparsity level of the SMCs of the unknown AUT, making its choice critical and based on a pessimistic approach. A method for the adaptive choice of M is suggested. After the acquisition of a starting set of M_0 measurement points, chosen using phase transition diagrams, the SMCs are estimated online with few iterations of an l1-minimization algorithm. Afterwards, further points are acquired, and the SMCs are estimated again using them. Following the evolution and the decrease of the variation between estimates, it is possible to truncate the measurement at a point where a successful reconstruction is guaranteed. The method for the construction of a minimum-coherent sampling matrix for adaptive acquisition and the truncation criteria for a specific accuracy are discussed with a focus onimplementation, and supported with numerical experiments, performed with measurementdata.
While the use of antennas in automobiles was earlier confined to AM and FM radio, most vehicles that we see today employ antennas additionally for satellite navigation, remote keyless entry etc. More number of antennas are likely to be needed in the future in vehicles for the purposes of internet and video browsing, collision avoidance radar systems, and for communication either between vehicles or between vehicles and infrastructure. Over the years, many planar monopole antennas that utilized patch slotting techniques have provided the desired multiband operation. But the choice of shape, size and position of slots in most of these works has been experimental and no generalized approach has been adopted. Fractal antennas, more particularly, those using the self-similar Sierpinski gasket geometries facilitate log periodic multiband behaviour. Although, the planar monopole configuration of this antenna geometry can be more useful candidate for most automotive applications, it has been found that the impedance mismatch at design frequency is a common problem in these antennas. We demonstrate a simple solution to this problem by considering an example of a third iterated gasket antenna of height 100 mm that has a geometric scale factor of 0.5. When the substrate of this antenna is loaded with a combination of SRR and strip wire on its rear side, a negative time delay gets added to the wave travelling inside the medium which results into the miniaturization of antenna. The CST MWS simulations clearly indicate this effect through the reduction of initial frequency resonances of 0.464 GHz, 1.568 GHz, 3.336 GHz and 6.2 GHz to 0.456 GHz, 1.312 GHz, 2.576 GHz and 5.032 GHz. The implementation of SRR-strip wire combination as a NFRP element in the antenna structure provided the impedance match at the reduced set of frequencies which is reflected in terms of improved return loss The loaded antenna structure provided gains of 2.44 dB, 4.74 dB, 5.7 dB and 6.2 dB respectively at the corresponding multiband frequencies. Based on such performance, the DNG-MTM loaded planar gasket monopole antennas of different heights can be expected to replace other planar monopole antennas for multiband automotive applications.
The planar wide-mesh scanning (PWMS) methodology is based on Non-redundant scanning schemes allowing faster measurements than classical Nyquist-compliant acquisitions based on denser, regular, equally spaced Near Field (NF) sampling. The methodology has no accuracy loss and has been validated at different bands and with different antennas [1]. The effectiveness of the PWMS technique has always been proven in error-free (or quasi-error-free) scenarios, assuming that possible errors introduced by the technique itself are independent of the typical source of measurement uncertainty. In this paper, we investigate for the first time the sensibility of the method wrt one of this error source included in the 18-terms lists [2], considered by the measurement community as an exhaustive list of the NF errors: X and Y probe positioning errors. Such errors are unknown and random and are associated to the mechanical vibrations and/or backlash of the system. The investigation has been done considering actual measurements of a multi-beam reflector antenna with approximately 35 dBi gain (MVG SR40 fed by two MVG SH5000 dual ridge horn). The AUT has been measured in planar geometry emulated by a 6-axis Staubli robot. The test was performed at 22-33 GHz. A set of measurements has been performed introducing a uniformly distributed random error in the range [0-1] mm, corresponding to ?/10 at 30 GHz. Errors are considered unknown. In the paper it will be shown that both in the classical and PWMS approaches the main beam is basically not affected by the introduced errors. The sidelobes are instead affected by such errors especially in the pattern cut where the beam is tilted. Such error levels obtained with the classical approach are comparable to those obtained with the PWMS approach, meaning that the latter is stable and against such type of perturbations.
NF-FF transformations have proven to be a convenient tool to accurately reconstruct the antenna pattern from NF measurements. In this framework, a very hot issue is the reduction of the time required to perform the measurements. To obtain a remarkable reduction of this time, nonredundant (NR) NF-FF transformations with planar spiral scannings have been developed in [1], by applying the NR representations of electromagnetic fields [2]. Optimal sampling interpolation (OSI) formulas have been used to efficiently reconstruct the massive NF data for the classical plane-rectangular (PR) NF-FF transformation from the NR spiral samples. The drastic measurement time-saving is due to the reduced number of needed NF samples acquired on fly, by adopting continuous and synchronized motions of the linear positioner of the probe and of the turntable of the AUT. However, such a time-saving is obtained at the expense of a nonuniform step of the spiral. Therefore, the linear positioner velocity is not constant, but must vary according to a not trivial law to trace the spiral, and this implies a complex control of the linear positioner. This work aims to develop an effective NF-FF transformation with planar spiral scanning for volumetric AUTs, wherein the spiral step is uniform and, hence, the linear positioner velocity becomes constant. To this end, the AUT is considered as enclosed in a sphere, the spiral is chosen in such a way that its step coincides with the sampling spacing needed to interpolate along a radial line according to the spatial band-limitation properties, and the NR representation along such a spiral is determined. Then, an OSI algorithm is developed to recover the NF data needed by the PR NF-FF transformation from the spiral samples. Numerical simulations assessing the accuracy of the developed NF-FF transformation will be shown.
Spherical near-field systems installed in shielded anechoic chambers are typically involved in modern automotive antenna measurements [1-3]. Such systems are often truncated at or close to the horizon to host the vehicle under test while limiting the size/cost of the chamber. The vehicle is usually placed on a metallic floor [4] or on a floor covered by absorbers [5]. The latter solution is intended to emulate a free space environment and is a key factor to perform accurate measurements down to 70 MHz. The availability of the free-space response also enables easy emulation of the car's behaviour over realistic grounds [6-7] while such emulations are more complex when a conductive ground is considered [8]. Conductive ground measurements also suffer from a strong interaction between the conductive floor and the measurement system and only in a limited number of situations such types of floor are a good approximation of realistic grounds (such as asphalts). However, the main advantage of conductive floor systems is the ease of accommodation of the vehicle under test which is simply parked in the center of the system. In absorber-based systems, instead, more time is generally needed to remove/place the absorber around the vehicle. Moreover, at low frequencies (70-400 MHz), large and bulky absorbers are normally used to ensure good reflectivity levels and the vehicle needs to be raised to avoid shadowing effect of absorbers. In this paper we investigate whether the measurement setup phase in absorber-based systems can be simplified by using smaller absorbers at low frequencies and/or not using them at all but considering conductive floors. The loss of accuracy in such scenarios will be studied considering a scaled vehicle and an implemented scaled automotive system where it is possible to access the full-spherical, real free-space scenario which is used as reference. The analysis is carried out considering (scaled) frequencies relevant to automotive applications in the 84-1500 MHz range. Two types of scaled absorbers, of different size and reflectivity, are considered to emulate the behaviour of the realistic full-scale 48-inch and 18-inch height absorbers. Measurements over metallic floor are included also in the analysis.
We present an antenna measurement methodology requiring a radically lower number of field samples than the standard Nyquist-based theory maintaining a comparable accuracy. simulations and partial knowledge of the geometry of the Antenna Under Test are combined to build a set of numerically defined expansion functions: the method uses basic knowledge of the antenna and the assumption that scattering from large surfaces can be predicted accurately by numerical tools; areas of the antenna such as feeding structures are treated as unknown and represented by equivalent electric and magnetic currents on a conformal surface. In this way, the complexity, and thus the number of unknowns, is dramatically reduced wrt the full problem for most antennas. The basis functions representing the full antenna are used to interpolate a radically reduced set of measured samples to a fine regular grid of Near Field (NF) samples in standard geometries. Regular NF to Far Field (FF) transformation techniques are then employed to determine the FF. The sampling reduction is evaluated compared to a regular sampling on standard Nyquist-complaint grids. The method can be employed in standard sampling ranges. In [1] asymptotic simulation tools were used to build the numerical basis. In this paper, methods based on Surface Integral Equations (SIEs) are used to compute currents and fields. The currents induced on the antenna structure by each elementary source are computed and used to evaluate the radiated field. Both electric and magnetic elementary sources are placed around the antenna and the SIE problems use a fast algorithm to evaluate matrix-vector products. The methodology is validated with planar and spherical acquisitions on a reflector antenna (MVG SR40) fed by a dual ridge horn SH4000 and in a multi-feed configurations (using several SH5000) at 18 and 30 GHz. Patterns obtained with down-sampled fast approach are compared to standard measurements. Down-sampling factors up to 8 are achieved maintaining very high correlation levels with standard techniques.
The problem of modelling a radiator or a scatterer using an equivalent radiator is of interest in a large number of applications as, for example, antenna synthesis [1], electromagnetic compatibility [2] and the design of echo generators [3]. Such a modelling problem requires determining the shape and the dimensions of a radiating surface capable to generate, in a certain region of space, an electromagnetic field close to that produced by the radiator/scatterer of interest. The purpose of this contribution is that of proposing, for a fixed equivalent radiator's shape, an approach for the solution of such dimensioning issue. The solution is new and, at the best of our knowledge, the dealt with problem has not been yet addressed throughout the literature. The proposed approach relies on the use of the Singular Value Decomposition (SVDs) of the operators linking the radiator/scatterer to the field on the region of interest, say D, and the equivalent radiating panel to the field on D, again. The singular functions of such operators corresponding to the most significant singular values represent the spaces to which the fields radiated by the primary radiator/scatterer and that radiated by the equivalent one essentially belong to, respectively. The approach consists into determining the dimensions of the equivalent radiator minimizing the error by which the field radiated on D by the equivalent radiator approximates the primary radiated/scattered one. The error is expressed as a hermitian, definite positive quadratic form so that the problem amounts to the maximization of its minimum eigenvalue. Numerical results will be presented for an equivalent, planar radiator of rectangular shape.
Spherical Near-Field (SNF) systems using a swing arm gantry configuration have been the go to solution for automotive measurement systems. Recent advances in the automotive industry have warranted a need for SNF systems with high mechanical positioning accuracy supporting measurements up to 40 GHz and beyond. This paper presents the design and implementation of a new swing arm gantry positioner having an 8-meter radius and a radial axis to support high frequency SNF measurements. We first define the relation of the gantry axis to the global coordinate system and discuss primary sources of errors. Next, a robust mechanical design is presented including design considerations and implementation. We then present errors measured using a tracking laser interferometer for probe position through the range of gantry axis travel. Static corrections for probe positioning errors are implemented in the control system using the radial axis. The resultant residual error for the swing arm gantry is then shown to have the accuracy required for high frequency SNF measurements.
The Austin RCS Benchmark Suite has recently been introduced to enable quantitative and objective comparison of computational systems for solving electromagnetic scattering problems, particularly, those relevant to aerospace applications. In the last year, five sets of problems were added to it: dielectric almonds (problem set III-B), mixed material almonds (III-C, III-D), perfectly electrically conducting (PEC) aircraft models (IV-A), and dielectric aircraft models (IV-B). For each problem set, a range of lengths and frequencies of interests are identified, interesting features are highlighted, and datasets containing reference results (from measurements, analytical methods, or numerical methods) are shared online. Although data from several radar cross section (RCS) measurement campaigns of non-metallic targets are available in the literature, these lack the information necessary to precisely model the materials, target geometries, and measurement setups, to quantify uncertainties in the data, and to identify appropriate directions for improving computational methods' performance. This limits their utility for benchmarking computational systems. This article presents an expansion of the Suite to include problems with more complex materials and reference results from a measurement campaign that attempted to ameliorate the deficiencies of existing datasets. Specifically, a set of thin-plate problems are added to the Austin RCS Benchmark Suite to increase material diversity. These consist of problem sets II-B: thin perfect-electrically conducting (PEC) plates, II-C: thin dielectric plates, II-D: thin magnetic radar absorbing material (MagRAM) plates, and II-E: thin MagRAM-coated PEC plates. Reference RCS data that enables validation, RCS measurement and material property uncertainty quantification, and benchmarking are also provided by conducting a simulation-supported measurement campaign in a compact range. To facilitate reproducibility, a popular low-loss dielectric material and a commercially available MagRAM were chosen for these problems: The dielectric material for problem set II-C is PolymaxTM polylactic acid (PLA). For problem sets II-D/E, the ARC Technologies' DD-13490 material is used. Thin plates were manufactured and their RCS were measured at Lockheed Martin's Rye Canyon Facility. The monostatic RCS measurement results and supporting simulation results are available online. Performance data for simulations as well as RCS measurement results with accompanying uncertainty will be presented for problem sets II-B/C/D/E at the conference.
Tapered Chambers are best suited for antenna pattern measurements at low frequencies. The advantage of such chambers over rectangular shaped chambers would be achieving a desired performance level in terms of field uniformity and ripple at the quiet zone due to the low reflectivity of the chamber. To achieve such performance using a rectangular shaped chamber could lead to design of larger rooms and associated significant cost. Hence, this paper tries to analyze the characteristics of the tapered chambers using the novel Fourier analysis characterization method. The Fourier analysis method was applied on the transverse and longitudinal planar scan data at the quiet zone of a tapered chamber. The analysis yield the performance of the chamber at different frequencies depicting the plane wave behavior at the low frequencies and breakpoint of the plane wave behavior with increase in frequency. It also shows the images or reflection hotspots formed at the throat of the tapered section at the higher frequencies. In addition, the longitudinal scan analysis portray the reflections from the back wall of the chamber. In conclusion, the known concepts and ideologies of the tapered chamber design are reexamined from a different perspective based on the analysis results.
There is a growing need for wearable RF modules integrated into clothing for medical sensing applications. Also, there is a concurrent interest for RF devices to collect energy and power body-worn devices, such as biosensors used to monitor vital signs. To do so, an approach is to integrate into garments, near-field wireless power transfer and harvesting RF circuits that can collect ambient RF energy from nearby Wi-Fi sources to charge biosensors. In this regard, resonant RF harvesting structures for near-field power transfer have been explored before and demonstrated close to 100% of power transfer efficiency (PTE) at close distances. However, this impressive efficiency can only be realized when there is perfect polarization and special alignment between transmitter and receiver. Herewith, we provide an approach that mitigates the efficiency challenges due to misalignment. With the above in mind, we propose a new class of resonators referred to as corrugated X resonators that are resilient to lateral, angular, and diagonal misalignments. The resonators operate at 500 MHz. Using these resonators, their PTE was found to range from 40% to 60% across 1 to 10-cm distances for broadside direction. In case of only lateral misalignment in the direction perpendicular to the feedline, the PTE is 70% across the same distances. Also, a PTE of up to 85% was achieved when the misalignment was only in the direction of the feedline. At the meeting, we will present the design and performance of the developed low-profile resonators RF surfaces. The evaluation is done when integrated into wearable textile surfaces under near-field illumination for RF harvesting.
With the advent of small-scale satellite technologies, a significant challenge faced by antenna engineers today is the design of high-gain and low-profile lens antennas. The conventional approach to lens design has been to shape the surfaces of dielectric materials to form homogeneous dielectric lenses. Alternatively, the refractive index of the lens can be varied throughout the lens body to form Gradient-Refractive Index (GRIN) lenses, which are now conceivable because of the advent of metamaterials. Optical ray paths are controlled inside the GRIN meta-lens rather than relying only on refraction at the air-lens interface. In particular, spherically symmetric lenses (such as the Luneburg lens) can achieve much greater control over the ray trajectories than homogeneous lenses since incoming electromagnetic waves can travel in a curved path within the volume of the lens. Due to spherical symmetry, however, the material that fills the volume of the spherically symmetric GRIN lens increases dramatically, and the weight becomes impractical for applications that require highly directive antennas. In order to overcome this disadvantage, the concepts of flat meta-lenses resulting into lightweight and slimmed alternatives to the spherically symmetric lenses have recently gained attention. In this work, we first present a methodology for the synthesis of the refractive index of flat GRIN lens antennas. Previous methods to obtain the material inhomogeneity relied on the assumption that the ray path is straight within the volume of the lens. These methods are approximate and applicable only for on-axis fed lenses. Contrary to the previous methods, the applied numerical synthesis algorithm based on Geometrical Optics and Particle Swarm Optimization is used to synthesize both on-axis and off-axis fed flat lenses with circularly symmetric aperture phase thus providing conically scanned beams for the off-axis designs. Then, metamaterial elements of variable sizes distributed on planar dielectric substrates are synthesized to form a multi-layered flat metamaterial lens and satisfy the required refractive index distribution. Simulation and near-field/far-field measured results of a proof-of-concept prototype of a multi-layered flat lens operating at 13.4 GHz are also presented. Additionally, microwave holographic approach is used to evaluate the goodness of the exit aperture amplitude and phase.
Phase uncertainty in antenna measurements introduces significant errors to the amplitude of the transformed pattern in Spherical Wave Expansion (SWE). To get a better understanding of the impact of phase errors, the measured phase error of a Low Noise Amplifier (LNA) is synthesized as a random phase error and subsequently added to the measured antenna patterns of three different antennas during the SWE. The resulting erroneous patterns are compared with the measured reference patterns and the error magnitude and probability distribution are studied. It is proven that the introduced errors to the transformed far-field patterns can be substantial. Furthermore, the relation between the antenna type and the error level and distribution is elaborated. The error level is different for the three antennas and the error level distribution is dependent on the mode spectra of the antennas.
The significant measurement standards in the antenna measurement community all present suggested error analysis strategies and recommendations. However, many of the factors in these analyses are static in nature in that they do not vary with antenna pattern signal level or they deal with specific points in the pattern, such as realized gain, side lobe magnitude error or a derived metric such as on-axis cross polarization. In addition, many of the constituent factors of the error methods are the result of analyses or special purpose data collections that may not be available for periodic measurement. The objective of this paper is to use only a few significant factors to analyze the error bounds in both magnitude and phase for a given antenna pattern, for all levels of the pattern. Most of the standards metrics are errors of amplitude. However, interest is increasing in determining phase errors and, hence, this methodology includes phase error analysis for all factors.