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We recently introduced large, lightweight, broadband plano-convex RF lens for close-range measurement of far-field antenna radiation pattern [1]. While the lens can drastically reduce the phase variation of the field across the transverse plane at a relatively short distance from the lens, the amplitude of the field in the same plane is affected by the diffraction from the circular edges of the lens, and to some extent by the transmitted field after internal reflections inside the lens. Furthermore, while the phase variation is minimal (within ±10°) and almost independent of the distance of the transverse plane from the lens, the field amplitude variation across the same plane increases with the distance of the plane from the lens. The amplitude variation reduces the useful size of the "quiet zone". To reduce the amplitude variation, we propose to incorporate "matching layers" around the lens. As we shall demonstrate in the paper, these matching layers help to reduce the aforementioned diffraction and internal reflections. As a result, the amplitude variation of the field across the transverse plane is reduced (to within ±1dB), thereby increasing the size of the "quiet zone". The matching layers are effective even for lenses as small as 6 in diameter.
A method for determining the anisotropic permittivity for arbitrarily-shaped, physically and electrically small material specimens with diagonal biaxial dielectric and magnetic anisotropy is described and representative measured results are presented. The method permits the extraction of the six complex tensor permittivity and permeability components from six or more independent reflection measurements on a single specimen in a shorted rectangular waveguide. The specimen need not fill either dimension of the waveguide cross-section and is permitted to be electrically short in the propagation direction. Extracted material parameters from a known specimen were used to demonstrate the method.
We propose a novel method to achieve a large quiet zone by means of an array of plano-convex lenses. As the lenses used are light-weight, they can be easily installed in an array. For this purpose, plano-convex lenses of radii are cut into squares with sides of v . These square lenses are placed next to each other with minimal separation between neighbouring lenses to create a large array of the desired shape. Lenses at the array edges or corners can be cut either to have straight edges or to retain their original rounded edges. The latter can be used to break up the otherwise straight edges of the lens array. Each lens has its own feed at its focus. For identical feeds, the resultant electromagnetic wave produced by the lens array will closely resemble a plane wave. This novel lens array provides a simple means to create a large quiet zone with customized cross-section (e.g. square or rectangular). Simulation results of the resultant electromagnetic field produced by a lens array will be presented. Specifically, the amplitude and phase distributions of the field at different distances from the lens will be shown. We will also present experimental results for a 1 2 lens array made of 1m square lenses.
A number of interesting applications of the equivalent current/source method (EQC) have been presented recently for antenna design and diagnostics. The Dual-Equation formulation has been proven to be superior in terms of accurately reconstructing sources on, or very near, the AUT structure and is the only formulation directly applicable for diagnostics [1]. The maximum antenna size that can be handled by this method is limited by memory and run time constraints due to the construction and solution of the linear system describing the problem. This paper reports the enhancement of the Dual- Equation formulation by the integration of the Fast Multipole Method, which allows dealing efficiently with large antennas.
Stray signals/scattering suppression techniques will be deployed to enhance measurements quality of a combined compact antenna test range (CATR) and spherical near-field (SNF) measurement facility. Spherical mode filtering and softgating techniques will be the focus of this paper. Using soft-gating the mutual effects between the CATR and SNF facilities will be shown and mitigated. The use of SNF decomposition to enhance the far-field measurements will be also shown. This contributes to a reduction of the costs arising from the need of absorbers to shield both facilities and cover the antenna's support structure.
This paper presents V-band radiation pattern characterization of both low- and high-directivity antennas. A fourarm micro-machined spiral antenna with monolithically integrated mode-forming network designed for dual circularlypolarized radiation represents the low-directivity antenna, while a standard gain horn is used for the highly directive antenna. All measurements were performed using an in-house NSI-700S- 30 system capable of spherical near-field measurements from 1-50 GHz and direct far-field measurements from 50-110 GHz. Complete comparisons of simulated, near- and far-field patterns show the feasibility of near-field measurements in V-band. Based on pattern comparison and measurement statistics conclusions are drawn about V-band near-field measurements.
Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction
Reconfigurable radar antennas with rapid, real-time control of the radiation pattern beamwidth provide expanded performance for many instrumentation radar applications, including RCS signature measurement and dynamic Time Space Position Information (TSPI) radar tracking applications. Adaptive adjustment of antenna radiation patterns was traditionally accomplished by electro-mechanically selecting predefined aperture dimensions that corresponded to desired beamwidths (e.g., ? ?/D). For an array antenna consisting of as few as 200 elements, beam shaping can be accomplished by adjusting the relative phase of individual array elements, a technique defined as beam spoiling or decollimation. This paper analyzes an operational radar antenna array incorporating reconfigurable beamwidth and beam shape through independent phase control of each subaperture. By adjusting the relative phase of radiating elements, the system can illuminate a programmable field of regard with full transmit power. For this array, the phase distributions across the elements map to a smaller "virtual aperture" displaced behind the physical array. Theoretical and measured results are presented to validate the reconfigurable array pattern control technique.
There are many ways to acquire the current location, global positioning system (GPS), triangulation, radar, and dead reckoning. Today GPS is the most reliable and accurate navigation technique when there is a clear, unobstructed view of the satellite constellation. However, when GPS is not available, another means of reliable navigation must be accomplished. The Air Force Institute of Technology (AFIT) random noise radar (RNR) is a possible solution to the indoor navigation problem. However, the current implementation of the RNR requires a large amount of data to be transfered between radar pulses. This research determined if using a template replay strategy has the same RNR performance as using an analog noise source. Using the template replay approach, each RNR node has a priori knowledge about the transmitted waveforms of other nodes and does not require the large data transfer between radar pulses. The analysis here revealed that modifications do not significantly alter RNR functionality. The analysis revealed that even at signalto- noise plus interference ratio (SNIR) equal to 0 dB, there are no parameters that can be reliably extracted other than transmitted signal bandwidth and transmitted template length; the transmitted message length was able to be extracted because the message was repeated over and over. If the message was not replayed the analysis showed that there would be no ability to extract parameters. Finally, by using the RNR to transmit digitally generated templates, digital communication is possible and the symbol error rate (SER) is traceable to simulated SER.
Whenever one antenna receives a signal from another antenna, standing waves are set up between the two antennas. These standing waves created by the mutual coupling of the two antennas introduce measurement uncertainty into over the air measurements of wireless devices. This paper reviews the causes of mutual coupling and provides calculations of the magnitude and phase of the standing waves created by the mutual coupling of the two antennas. Two methods for measuring the magnitude of the standing waves are presented. The magnitude of the standing waves set up between the measurement antenna and the device under test is measured for gain standard antennas, cell phone handsets and notebook computers.
In the present paper, a non-dedicated mass market GNSS antenna calibration method is discussed, with a special focus on the significant error component due to phase variations of receiving antennas in precise GNSS applications. Different calibration methods are compared from the literature; the indoor (anechoic chamber) calibration has been selected. The algorithm used to compute the mean Phase Center (PC) and its associated Phase Center Variation (PCV) for all angular directions is also described and has been validated on simulated canonical antennas. PC and PCV are then computed when four antennas are placed near the command unit of an unmanned aerial vehicle (UAV), which emulates the final application scenario. The impact of this structure is evaluated thanks to PCV cartographies. Two low-cost COTS antennas have been selected and their PCV maps are compared with regards to their geometry. Lastly, a reproducibility study based on the PCV characterization of ten copies of one of the selected COTS antennas concludes on the robustness of the PCV calibration.
A new computer aided technique to automatically design reflectarrays and subreflectarrays is presented. The technique is able to generate the unit cell geometry and the geometrical model of both antennas taking into account input parameters such as the unit cell type, the operating frequency, the focal length, the periodicity, the desired main beam radiating direction, etc. The characteristics of the reflecting elements are selected considering the spatial phase delay at each unit cell to achieve a progressive phase shift. The tool also computes and provides the phase curve of the unit cell. To validate the proposed method, an offset-fed reflectarray and a center-fed subreflectarray have been designed. Good results have been achieved.
A microstrip feed and patch antenna capable of physical reconfiguration during deployment can provide beam steering and operation at adjustable frequencies and polarizations. This reconfigurable structure uses z-axis deflection of small pixels with a sandwiched ground layer, substrate, and conducting top layer. The Pixel Addressable Reconfigurable Conformal Antenna (PARCA) technology enables millisecond reconfiguration of a microstrip structure on a pixel-by-pixel basis. Pixel fabrication and actuation methods developed using microelectromechanical systems (MEMS) techniques enabled smaller pixels (1 mm square), which allow higher frequencies than past prototypes. Previous pixels relied on a thick metallization to make contact between adjacent pixels and yielded a pixel mass too large for MEMS z-axis deflection. A new pixel with a thin metallization combined with a matrix of dot electrodes on a superstrate has been implemented. Pixels in the up position make DC contact with the dot electrodes and are coupled to adjacent up pixels. This configuration resulted in lighter pixels with better pixel to pixel coupling. A 10x10 matrix of pixels with a set of dot electrodes was measured with results comparable to a continuous patch antenna of similar dimensions. The dot electrode matrix enables DC contact between adjacent pixels and makes a significant difference in the performance.
In order to determine the permittivity of homogeneous dielectrics used in the production of microwave absorbing materials, it is necessary and sufficient to know /measure the complex reflectivity of thick dielectric bricks utilizing the materials [1]. If the permeability of the material is to be sought then, in addition, the knowledge of the complex transmission coefficient is necessary to determine unknown parameters. The coaxial lines like the one described in [2,3], as well as NRL arch systems, are widely used in the industry to characterize the performance ( reflectivity) of absorbing materials over a wide frequency range. In these systems, the conventional S11 or S21 methods of parameter extraction from the absorptive samples are utilized in conjunction with reference measurements from a metallic surface. The difference between the two measurements characterizes the reflectivity of the samples. In this paper, extension of the S11 and S21 methods to measure the permittivity and permeability of absorbing materials in conventional coaxial line and on NRL arches is described. The technique is based on measurements of thick absorptive bricks followed by signal processing with time - gating being utilized. The application is very useful for rapid dielectric property measurements of the raw materials prior to impregnation or full absorbing materials after impregnation prior to cutting the material into pyramidal or other shapes.
A low profile UWB VHF antenna with a maximum diameter of 60.96cm and height of 5.08cm is presented. The top of this antenna is made of a wide conducting plate top which is shorted to ground at one end and a 50-ohm coaxial feed at the other end. Specially selected and shaped ferrite bars are strategically placed between the top plate and ground plane for achieving good antenna performance for wide angle coverage in the upper hemisphere from 30 to 300 MHz. Minimal amount of ferrite was used to keep the antenna’s weight at below 9.07 kg. Simulated and measured results will be discussed.
The improved calibration model proposed in this paper is based on the traditional capacitance model which suffers from errors caused by the assumption that the capacitance is independent of frequency and the permittivity of the ambient medium under test. By analyzing the near-zone field of the coaxial opening, we introduce the new near-field capacitance to account for the dependency on the external permittivity. Simulation results show that the calibration error is significant reduced for low and moderate loss medium. And the calibration of the unknown coefficients simply requires the premeasurement of three known material including air, which provides convenience for the real field measurement. Measurement results obtained by a novel wideband in-situ coaxial probe are included to prove the accuracy improvement improved calibration model. by using this
The 3D reconstruction algorithm is applied to a slotted waveguide array measured at the DTU-ESA Spherical Near-Field Antenna Test Facility. One slot of the array is covered by conductive tape and an error is present in the array excitation. Results show the accuracy obtainable by the 3D reconstruction algorithm. Considerations on the measurement sampling, the obtainable spatial resolution, and the possibility of taking full advantage of the reconstruction geometry are provided.
In this paper the implementation of a voltage controlled voltage source (VCVS) in full-wave electromagnetic simulation using finite-difference time-domain (FDTD) is introduced. The VCVS is used to model an operational amplifier (op-amp) commonly used in microwave circuits. This new approach is verified with several numerical examples including circuits with VCVS and op-amp. Good agreement is obtained when the results of the developed FDTD code are compared with those based on analytical solution.
Two antenna diversity schemes are evaluated for use with the 2.45 GHz wireless o.-body communication between a receiver worn at the ear and a stationary transmitter. A receive diversity scheme is compared to a transmit diversity scheme in an indoor environment. It is found that the two diversity schemes can provide the same improvement of the channel fading. Therefore the transmit diversity scheme may be a viable option for systems such as Hearing Instruments (HI) that are subjected to strict space requirements at the receiver end.
The effect of the anatomical variation of the head on the ear-to-ear communication at 2.45 GHz has been investigated. Several anatomical characteristics of the head, such as the dimensions and the position of the ears, have been recorded for a group of 25 test persons. Active Packet Error Rate (PER) measurements have been made by the use of digital Hearing Instruments (HI) as small wireless platforms in both indoor and outdoor environments. Two fundamentally different antenna con.gurations are compared. It is found that there is an effect of the distances over-the-top, around-the-front and aroundthe-back on the PER, due to constructive and destructive interference between surface waves that propagate along the different paths. The effect is different for the two different antenna types.