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A diagnostic technique was published over 20 years ago on imaging compact-range reflectors by focusing plane-polar field-probe data. At that time, only synthesized data had been evaluated. Since then, a few reflectors have exhibited performance lower than expected, and this technique has been successfully employed to improve that performance based on their measured data. This paper reviews the technique and discusses the results of processing those measured data sets. The technique produces an image of the estimated field amplitudes at the reflector surface that do not contribute to the desired quiet-zone plane wave. Point sources, line sources, and deformations over an area have all been successfully identified, often outside the projected circular boundary of the field-probe data. All measurements to date have used very coarse angular spacing with acceptable degradation in image quality.
There are several applications in which knowledge of the location of the phase center of an antenna, and its twodimensional variation, is an important feature of its use. A simple example occurs when a broad-beam antenna is used as a feed for a reflector, where the center of the spherical phase fronts should always lie at the focal point of the paraboloidal surface. Here, the ability to determine the phase center of the feed from knowledge of its far-field phase/amplitude pattern is critical to the reflector's design. Previously published methods process a single cut of data at a time, yielding 2D lateral and longitudinal phase-center offsets. Eand H-plane cuts are thus processed separately, and will, in general, yield different answers for the longitudinal offset. The technique presented here can process either one line cut at a time or a full Theta-Phi raster. In addition, multiple frequencies can be processed to determine the average 3D phase-center offset. The technique can merely report the phase-center location, or it can also adjust the measured phases to relocate the origin to the computed phase center. Example results from measured data on multiple antenna types are presented.
Antenna test engineers are faced with testing increasingly complex antenna systems, one of these being the AESA (Active Electronically Steered Array) antennas used for cell communications, jammers, and radars. Often these antennas have integrated electronics and RF components that are an intricate part of the antenna, and as a result must be tested with the waveforms generated by the antenna itself. One cannot simply inject an unmodulated continuous wave signal. These antennas require new measurement techniques which are compatible with their broadband waveforms. The reference channel of a measurement receiver can be used to collapse the spectrum of the modulated signal into a single CW measurement. Done properly all the energy in the signal is captured with noise and interference being dispersed, resulting in no loss of DR (dynamic range) over a CW measurement. A receiver employing this technique can capture all the energy in modulated and pulsed signals wielding wide dynamic range measurements. Phased locked loops (PLL) are not used as they can preclude such measurements. A measurement receiver that uses a digital correlator to collapse the spectrum of modulated and pulsed signals will be presented. This paper will describe the technique used to do this and show measured results on example broadband signals.
More complex antennas with higher transmit power levels are being tested in compact range environments. AESA's and other phased array antennas can transmit significant power levels from a relatively small volume. Without consideration of the impact of the transmitted power levels for a given test article, human and facility safety could be at risk. This paper addresses designing a test chamber in light of these power handling considerations for high power antennas on two fronts: 1) A methodology is presented to determine the power levels seen by surfaces in the chamber that are covered with absorber material and 2) Calculating the power levels seen at the compact range feed due to the focusing effect of the compact range itself. A test case is presented to show the application of the methods.
Compact range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signature. This paper investigates strategies for improving compact range measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational finite-difference time-domain (FDTD) model at VHF/UHF frequencies to determine baseline performance and develop trade rules for more advanced designs. Physical optics (PO) models were used to analyze the final design at the frequencies of interest.
Multi Input Multi Output (MIMO) antenna systems are needed to meet the increasing demands of users in wireless systems. MIMO technology has been used to improve the capacity of wireless systems; however, designers have faced challenges to reduce antenna-size and increase the isolation between the antennas in MIMO systems. In this paper, a compact MIMO antenna array platform is proposed for LTE MIMO and Handset applications. The proposed array was designed to operate at the 2.6GHz Long-Term Evolution (LTE) band for wireless communication systems. The proposed array consists of four compact patch antennas on a dielectric substrate with total dimension of 12.5x6.25x1.27mm3. Modification of the ground plane along with the systematic placement and orientation of the antenna elements on top of the substrate play a key role to reduce mutual coupling, which normally degrades the performance of MIMO antenna arrays. The performance of this MIMO antenna array has been evaluated through simulations and measurements of the scattering parameters [S] and radiation patterns. The minimum gain of a single antenna with all the other three elements terminated in 50O loads is 1.49dBi, while the isolation is over 25dB between all the MIMO antennas located in the array structure. The measured results suggest that the antenna is well suited for LTE MIMO applications as well as handset antennas.
Measurement of rotating components in cluttered, high-temperature jet engine environments is challenging for conventional wired sensors. We have developed a passive wireless sensing (PaWS) system to measure strain, temperature, and other parameters in high-temperature, harsh environments such as jet engines. PaWS utilizes piezoelectric surface acoustic wave (SAW) devices which are conformal, require no batteries, and can operate at temperatures up to 300 C. PaWS were fabricated to operate at frequencies between 2.4 and 2.5 GHz and installed on test specimens representative of engine components. These specimens underwent strain of over 4000 microstrain at ambient and elevated temperatures. A miniature patch antenna was developed to incorporate a package whose total dimensions are smaller than 25x30x2 mm. An RF interrogation system (RFIS) was developed for simultaneous measurement of multiple sensors. Post-processing software has been developed to convert data from multiple sensors into calibrated strain values. Calibrated measurements were made with the RFIS using both wired and wireless methods in a laboratory and in a jet engine test cell. Sensor measurements
A new material validation and verification standard is designed to imitate the behavior of a lossy dielectric absorber. This standard is constructed from well-characterized, low-loss materials in a manner that ensures manufacturing repeatability. The performance of this standard is verified with S-parameter and permittivity measurements in a free space focused beam system and with finite difference time domain simulations. A sensitivity analysis, based on a series of simulations, is presented to quantify the uncertainty in the measured S-parameters due to dimensional and alignment variations from the ideal design values.
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.
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.