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Most conventional spherical near-field scanning systems require the antenna under test to rotate in one or two axes. This paper will describe a novel rolling arch near-field scanner that transports a microwave probe over a hyper-hemispherical surface in front of the antenna. This unique scanning system allows the antenna to remain stationary and is very useful for cases where motion of the antenna is undesirable, due to its sensitivity to gravitational forces, need for convenient access, or special control lines or cooling equipment. This allows testing of stationary antennas over wide angles with accuracies and speeds that historically were only available from planar near-field systems. The probe is precisely positioned in space by a high precision structure augmented by dynamic motion compensation. The scanner can complete a hyper-hemispherical multi-beam, multi-frequency antenna measurement set of up to eight feet in diameter in less than one hour. The design challenges and chosen techniques for addressing these challenges will be reviewed and summarized in the paper.
Wireless sensors network desires a vertically-polarized, omnidirectional antenna. In the case where the sensor network was deployed close to ground, the communication range of the network degrades. To quantify the communication range near the ground, several field measurements have been reported with monopole and microstrip patch antennas in the literature. In this study, a low-profile antenna design suitable for wireless sensor network is introduced. Its performance near a metallic ground plane is measured and simulated.
Over-the-air (OTA) performance testing of wireless devices has become a major component of product qualification for today's wireless networks. The methodologies used currently expand on traditional passive antenna pattern measurement techniques and operate under the fundamental assumption that the radiation pattern of the DUT does not change for the duration of the test. Emerging wireless technologies such as MIMO that use multiple antennas and adaptive algorithms for communication violate that assumption. In addition, the enhanced feature set and higher complexity of these newer technologies means that there are other performance criteria to be evaluated beyond just radiated power and sensitivity. A new OTA test system has been developed that can simulate complex multipath systems in a fully anechoic environment. This technology allows evaluating the performance of the DUT in a host of simulated real-world environments. Existing spatial channel models used for conducted testing of these radios can be easily adapted to OTA testing. This paper describes the details of such a system, including requirements for calibration and validation, as well as showing typical test results and available metrics.
A well designed absorber configuration is a key factor for precise antenna measurements. Unfortunately, even a scanner covered with pyramidal absorbers can cause reflections that could degrade the measurement accuracy. A novel scanner absorber configuration using bent absorbers is presented in this paper. Another problem is that in most cases it is necessary to remove the absorbing material at the scanner to change the antenna under test. The absorbers covering the scanner suffer abrasion caused by the frequent manual movement. For this reason it was also the intention to find a faster and easier solution which also preserves the absorbing material. The new and the old absorber layout were benchmarked using a number of spherical nearfield measurements as well as time domain reflection measurements with a broadband probe antenna. A comparison of the results is also shown in this paper.
ABSTRACT A modern method is presented to measure small antenna efficiencies by implementing a variant of the WheelerCap method. As antennas become smaller in size, antenna efficiency typically decreases either because of matching functions or reductions in the antenna radiation resistance. It is important to know while designing small antennas, how much efficiency reduction can be tolerated before a particular design needs changing. Measuring antenna efficiency by integrating radiation functions is not a trivial task and prone to measurement errors. The modern method presented uses a plastic sphere which is internally coated with a highly conductive metallic paint, having low resistivity values (less than 0.1ohms per square), and useful to implement the Wheeler Cap measurements. The measurements are conducted with the use of a modern Agilent Vector Network Analyzer (VNA) which is calibrated to the antenna port (which includes any antenna matching networks). The apparatus was used to measure small “planar” antennas thus producing extremely good results. The paper presents the methodology used for the development of the apparatus and the measurement results.
Anechoic chambers simulate open air test conditions and are advantageous for testing avionics systems in a secure, quiet electromagnetic environment. The 412th Electronic Warfare Group’s Benefield Anechoic Facility (BAF), located at Edwards AFB, California was designed for testing systems in the radio frequency (RF) range from 500 MHz to 18 GHz. For frequencies below 500 MHz, the installed radar absorbent material (RAM) does not effectively absorb incident RF energy, thereby allowing undesired RF scattering off the chamber’s floor, ceiling, and walls. This leads directly to measurement inaccuracies and uncertainty in test data, which must be quantified for error analysis purposes. In order to meet the desired measurement accuracy goals of antenna pattern and isolation measurement tests below 500 MHz, RF scattering must be mitigated. BAF personnel developed a test methodology based on hardware gating, range tuning and improved RAM setup, to improve chamber measurement performance down to 100 MHz. Characterizing the chamber using this methodology is essential to understanding test zone performance and thus increases confidence in the data. This paper describes the test methodology used and how the test zone was characterized with resulting data.
We introduce a new type of planar near-field measurement technique for testing small antennas which, heretofore, have been traditionally tested via spherical or cylindrical scanning methods. Field acquisition in both these procedures is compromised to a certain extent by the fact that probe movement induces change in relative geometry with respect to, and thus interaction with, the anechoic chamber enclosure. Moreover, obstructing equipment, such as antenna pedestals, may significantly impede, or even reduce the available angular scope of any given scan. Our proposed procedure, by contrast, minimizes both the residual interaction contaminant and the threat of obstruction. We have in mind here a variant, a hybrid version of planar scanning wherein, on the one hand, we limit severely the size of the acquisition rectangle (and thus minimize the contaminating influence of a variable probe/chamber interaction), while, on the other, we really do collect near-field data throughout a complete range of solid angle around any candidate AUT, front, back, above, below, and on both sides. Such completeness is achieved through the mere stratagem of undertaking six independent planar scans with the AUT suitably rotated so as to expose to measurement, one by one, each of the faces of an enclosing virtual box. In the meanwhile, the inevitable AUT pedestal per se remains immobile and removed from any occupancy conflict with the scanned probe. We have accordingly named our new planar near-field data acquisition scheme the “Boxed Near-Field Measurement Procedure.” With subsequent use of our Field Mapping Algorithm (FMA), elsewhere reported, we obtain the entire field exactly, everywhere, both interior and exterior to the surrounding (virtual) box. In particular, we achieve enhanced accuracy in the far-field patterns of primary interest by virtue of the completeness of data acquisition and its relative freedom from spurious contamination. The angular completeness of data acquisition conferred by our procedure extends in principle to antennas of arbitrary size, provided, of course, that due provision is made for the necessary scope of measurement rectangles. The benefits are seen to be especially valuable in the case of narrow-beam antennas, whose back lobe pattern details, usually deemed as inaccessible and hence automatically forfeited during conventional (i.e., utilizing a “onefaced box,” in our new way of thinking) planar near-field testing, are thrust now into full view. Our new, full-enclosure planar acquisition technique as now described has been verified by analytic examples, as well as by hardware measurements, with excellent results evident throughout, as we are about to demonstrate.
Outdoor far- field antenna test ranges have declined in popularity due to the advent of alternative test methods, e.g., Near-Field Antenna ranges and Compact Antenna Test Ranges. They are also costly to maintain. A natural consequence of that trend is that far-field ranges are either shut down or rendered dormant for long periods of time. The latter was the situation for the NGAS (Northrop Grumman Aerospace Systems) CTS 10K Far-Field range. The Far-Field was an outdoor range with a 10,000’ range length, open transmit site and radome enclosed receive site. It had been dormant for 7 years and was needed for a unique test before the test site was vacated completely. This paper provides a brief description of the range, the upgrades made to address equipment obsolescence and the checkout process to ensure that the range would meet performance requirements. The range needed to operate from 100 MHz to 18 GHz. Therefore, range diagnostics were performed at various frequency points and swept measurements also executed. A Range Readiness report was created and presented internally. Elements of that report are shared in this paper.
A novel approach using Artificial Neural network (ANN) is proposed to identify the faulty elements present in a nonuniform linear array. The input to the neural network is amplitude of radiation pattern and output of neural network is the location of faulty elements. In this work, ANN is implemented with three algorithms; feed forward back propagation neural network, Radial Basis Function neural network (RBF) and Probabilistic neural network and their performance is compared. The network is trained with some of the possible faulty radiation patterns and tested with various measurement errors. It is proved that the method gives a high success rate.
In previous papers [1]-[4] we have presented formulations for the circular and linear near field-to-far field RCS transformations (CNFFFT and LNFFFT, respectively). These formulations assumed that the target did not have significant extent above or below a central (waterline) plane, and that the circular or linear near field scans lied in this waterline plane. In this paper, the CNFFFT and LNFFFT formulations are generalized to scans that lie in a plane parallel to and above or below the waterline plane. These scans correspond to conical or great circle RCS cuts, respectively, in the far field at elevation angles other than 90°. We will show that the generalization can be accomplished by modifying just the frequency domain processing steps that are common to both algorithms, while leaving the spatial processing portions (apart from a minor variable redefinition) unchanged. The paper focuses on the mathematical derivation and numerical implementation of the algorithms; examples of numerical and experiment results are deferred to future papers.
Reflections in antenna test ranges can often be the largest source of measurement error within the error budget of a given facility [1]. Previously, a technique named Mathematical Absorber Reflection Suppression (MARS) has been used with considerable success in reducing range multi-path effects in spherical near-field antenna measurements [2, 3, 4, 5]. Whilst the technique presented herein is also a general purpose measurement and post-processing technique; uniquely, this technique is applicable to cylindrical near-field antenna test ranges. Here, the post-processing involves the analysis of the cylindrical mode spectrum of the measured field data which is then combined with a filtering process to suppress undesirable scattered signals.
Abstract— Spherical near-.eld scanning measurements are used to determine the incident electromagnetic .elds within a test volume. Resultsfrom twoindependent approaches are comparedandfound to agree within estimated uncertainties.
Inverse equivalent current method has recently gained popularity in the applications of near-field far-field (NFFF) transformations especially when near-field (NF) measurements are carried out on irregular measurement grids around the arbitrarily shaped object under test. Usually low order (LO) Rao-Wilton-Glisson (RWG) basis functions or even point based low order basis functions are used for the discretization of the unknown surface current densities on the triangular discretization elements. Better accuracies are achievable when equal number of higher order (HO) basis functions is employed to represent unknown surface current densities. Nearly-orthogonal hierarchical vector basis functions complete to full first order with respect to the curl space are therefore utilized for the discretization of inverse equivalent surface currents defined on flat triangular domains. Various numerical examples are presented and comparison is made with the results of LO discretization.
Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.
This paper examines the inter-relatedness between the polarization vector of the linearly polarized feed/probe, the analysis coordinate system used for the DL-to-CP transformation, and how the test antenna generates its circular polarization response. The measurement of the performance characteristics of circularly polarized antennas is often accomplished using a linearly polarized feed/probe to measure horizontal and vertical polarization components. The measured orthogonal linearly polarized components are combined using a mathematical technique to transform them to the orthogonal right-hand and left-hand circular polarization components of the test antenna. One of the difficulties in using this technique is insuring the proper orientation of the feed/probe for the measurement, the analysis coordinate systems used for the DL-to-CP transformation algorithm. Another is understanding the manner in which the circular polarization is being generated by the antenna under test. These factors are inherently related, and without proper care the wrong answer can easily be calculated.
The back wall is an important element in a high performance tapered or compact range anechoic chamber operating at VHF/UHF frequencies, as by design it is intended to absorb the non-intercepted portion of the incident plane wave containing the majority of the power transmitted by the chamber illuminator. Back wall reflections may interfere with the direct illumination signal and thus influence the test zone performance. Consequently, in order to ensure that the overall test zone reflectivity specification is met, the reflectivity produced by the back wall should be better than the reflectivity specified for the test zone. The conventional approach used to achieve good reflectivity is to apply high performance, high quality absorbing materials to the back wall. Further improvement of up to 10 dB can be achieved if a Chebyshev absorber layout is implemented [1, 2]. This layout consists of high performance absorbing pyramids of different heights, and assumes that the performance does not depend on a metallic backing plate. This approach is expensive, and presents technical challenges due to the complexity involved in the design and manufacturing of the absorbing material. In addition, installation and maintenance is an issue for such large absorbers. In this paper an alternative approach is presented which is based on an implementation of a shaped back wall as, for example, suggested in [3-5], and use of lighter, lower grade absorbing materials whose performance essentially depends on reflections from the metallic backing wall. This type of design can be optimized at the lowest operating frequency, if the back wall and absorber front face reflections cancel each other. Different back wall shapes are considered for a tapered chamber configuration, and the test zone reflectivity produced by a flat, inverted “open book” and a pyramidal back wall are evaluated and compared at VHF frequencies using a 3D EM transient solver [6].
During lightning strikes buildings and other structures can act as imperfect Faraday Cages, enabling electromagnetic fields to be developed inside the facilities. Some equipment stored inside these facilities may unfortunately act as antenna systems. It is important to have techniques developed to analyze how much voltage, current, or energy dissipation may be developed over valuable components. In this discussion we will demonstrate the modeling techniques used to accurately analyze a generic missile type weapons system as it goes through different stages of assembly. As work is performed on weapons systems detonator cables can become exposed. These cables will form different monopole and loop type antenna systems that must be analyzed to determine the voltages developed over the detonator regions. Due to the low frequencies of lightning pulses, a lumped element circuit model can be developed to help analyze the different antenna configurations. We will show an example of how numerical modeling can be used to develop the lumped element circuit models used to calculate voltage, current, or energy dissipated over the detonator region of a generic missile type weapons system.
The primary purpose of a chamber for Far–Field (FF) antenna measurements is to create a test zone surrounding the AUT, where the electric field is to be as uniform as possible, and multiple reflections are kept to a minimum. It is well known, that typical rectangular anechoic chambers for Far–Field (FF) antenna measurements are subject to increased reflectivity from specular regions on the side walls, floor and ceiling. The reflectivity further increases if a larger test zone and, consequently, longer source antenna/ AUT separation is required. The alternative to a rectangular chamber, which can be implemented to reduce the reflectivity, could be a chamber with a shaped interior, where the side walls are to be shaped based on GTD/GO principles so that the reflections are diverted out of the test zone. Even more reflectivity suppression is expected, if, in addition, wedge absorbers are used throughout the specular region or entire wall with a smoothly varied wedge orientation chosen according to GTD principles. The combination of two approaches constitutes a chamber design method termed a “Two – Level GTD”. The chamber shape and wedge orientation for delivering reduced reflectivity in the test zone are not unique. According to a “Two -Level GTD” a plurality of solutions exists and can be practically implemented. Freedom in choosing these parameters can be utilized to satisfy the additional requirements for the chamber design to reduce RCS clutter and/or secondary reflections in the chamber. In this paper the method validity is confirmed based on comparison of various chamber designs performed using 3D EM analysis tools.
ESA’s Compact Antenna Test Range at ESTEC has been relocated which has given the chance to improve the alignment of the reflectors. Based on measure-ments of the reflector surfaces the best-fit positions and orientations of the reflectors have been deter-mined. It turned out that the choice of parameters to describe the reflectors and their position had impor-tant impact on the optimization process: The parame-ters shall – as far as possible – be orthogonal in the sense that a change in one parameter must not influ-ence the final value of the other parameters.
Transmitting equipment may interfere with sensitive electronic equipment even if both are in compliance with regulatory standards. This paper examines the potential for electromagnetic interference (EMI) between Federal Communications Commission (FCC) Part 15.247 for Ultra-High Frequency (UHF) emitters and immunity compliant sensitive equipment. At close ranges, the electromagnetic (EM) fields from these UHF emitters may exceed minimum standard immunity testing levels. This does not imply that interference will occur, but that the device may not be qualified to operate in the EM environment near the emitter. UHF Radio Frequency Identification (RFID) emitters are deployed regularly in locations where sensitive electronic equipment is in use. Recent studies have indicated that an interference potential can exist between some UHF emitters and medical, commercial and military systems. This paper estimates the range at which FCC compliant devices may pose a risk to industrial, consumer and medical devices and compares it to previously published data.