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In the last decades radar imaging techniques have been widely studied. Electromagnetic imaging is a very promising technique for many practical application domains (medical, surveillance, localization …). As an example, many RCS imaging systems have been developed for compact range indoor RCS measurement layouts. In this paper, a preliminary comparison of near field RCS images from Multiple Input Multiple Output (MIMO) arrays and monostatic radar is presented. The main objective of this study is to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays (ex. back projection) in order to develop real-time imaging applications based on MIMO array systems. The study was conducted with a one-dimensional MIMO array composed of 14 transmitting and receiving antennas. The goal of the optimization is to obtain radar images as similar as possible to those from monostatic radar. This paper presents the experimental layout, the imaging algorithms and the experimental results. As a conclusion, the imaging capabilities of MIMO arrays are discussed.
Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.
A widely used time-gating technique can be effectively implemented in near-field (NF) antenna measurements to significantly improve the measurement accuracy. In particular, it can be implemented to reduce or remove the effects of the following measurement errors [1]: -multiple environmental reflections and leakage in outdoor or indoor NF ranges -edge diffraction effects on measurement accuracy of low gain antennas on a ground plane [3] In addition, reflectivity in the range can be precisely localized, separated and quantified by using the time – gating procedure with only one addition (a subtraction operation) added to the standard near-field to far-field (NF – FF) transformation algorithms. In this paper a step by step procedure is described which includes acquisition of near-field data, transformation of the raw near-field data from the frequency to the time domain, definition of the correct time gate, transformation of the gated time domain data back to the frequency domain, and the transformation of the time gated near-field data to the far-field. The time gated results, as already shown in [2], provides for more accurate far-field patterns. In this paper it is shown how the 3D reflectivity/multiple reflections in the measurement chamber or outdoor range can be determined by subtracting the time gated results from the un-gated data. This technique is illustrated through use of several measurement examples. It is demonstrated that the time gated method has a clear physical explanation, and, in contrast with other techniques [4,5] is less consuming (does not require mechanical AUT precise offset installation, additional measurement and processing time) and allows for a better localization and quantization of the sources of unwanted radiation. Therefore, this technique is a straightforward one and is much easier to implement. The main disadvantage cited by critics regarding use of the time gating technique is the narrow frequency bandwidth used in many NF measurements. However, it is shown, and illustrated by the examples, that the technique can be effectively implemented in NF systems with a standard probe bandwidth of 1.5:1 and an AUT having a bandwidth as low as 5% to 10%.
A method to reduce the noise power in far-field pattern without modifying the desired signal is proposed. Therefore, an important signal-to-noise ratio improvement may be achieved. The method is used when the antenna measurement is performed in planar near-field, where the recorded data are assumed to be corrupted with white Gaussian and space-stationary noise, because of the receiver additive noise. Back-propagating the measured field from the scan plane to the antenna under test (AUT) plane, the noise remains white Gaussian and space-stationary, whereas the desired field is theoretically concentrated in the aperture antenna. Thanks to this fact, a spatial filtering may be applied, cancelling the field which is located out of the AUT dimensions and which is only composed by noise. Next, a planar field to far-field transformation is carried out, achieving a great improvement compared to the pattern obtained directly from the measurement. To verify the effectiveness of the method, two examples will be presented using both simulated and measured near-field data.
This paper presents a hybrid analysis algorithm, which is used at Radiation Group (UPM) to carry out the design of a conformal serrated-edge reflector for the mm-Wave compact range UPM facility. Main features of this algorithm involve its capability of handling conformal serrated rim parabolic reflectors, accuracy and computational efficiency.
Impedance and gain measurements for electrically small antennas represent a great challenge due to influences of the feeding cable. The leaking current along the cable and scattering effects are two main issues caused by the feed line. In this paper, a novel cable-free antenna impedance and gain measurement technique for electrically small antennas is proposed. The antenna properties are extracted by measuring the signal scattered by the antenna under test (AUT), when it is loaded with three known loads. The tech-nique is based on a rigorous electromagnetic model where the probe and AUT are represented in terms of spherical wave expansions (SWEs), and the propaga-tion is accounted for by a transmission formula. In this paper the measurement results by the proposed technique will be presented for several AUTs, includ-ing a standard gain horn antenna, a monopole an-tenna, and an electrically small loop antenna. A com-parison of measurement results by using the proposed method and by using other methods will be presented.
General Purpose Graphical Processor Units (GPGPUs) provide increased processing capability for applications with a high degree of data parallelism. In the past the few years, GPGPUs have become readily available in the commercial market, and off-the-shelf programming tools (e.g. CUDA from the NVIDIA Corporation and Jacket from Accelereyes, LLC) have made them more accessible to the technical community. SAR and ISAR imaging algorithms are inherently computationally intensive. In order to overcome performance limitations of CPUs and traditional DSPs, simplified, computationally-efficient algorithms are often used, but at the expense of the phase information available within the raw data. We have demonstrated that GPGPU acceleration of SAR/ISAR processing has greatly improved processing times of a less-efficient (but more flexible) algorithm, making its use more practical. We have shown that GPGPUs can provide performance improvement in excess of 30X for a backprojection-based SAR/ISAR imaging technique.
In this paper, application of a modified Wheeler cap method for the radiation efficiency measurement of balanced electrically small antennas is presented. It is shown that the limitations on the cavity dimension can be overcome and thus measurement in a large cavity is possible. The cavity loss is investigated, and a modi-fied radiation efficiency formula that includes the cavity loss is introduced. Moreover, a modification of the technique is proposed that involves the antenna working complex environment inside the Wheeler Cap and thus makes possible measurement of an antenna close to a hand or head phantom. The measurement procedures are described and the key features of the technique are discussed. The results of simulations and measurements by the proposed method are pre-sented and compared.
Operating microwave receivers with remote mixers in a system requires the LO power to be flat over broadband frequencies. In large systems, this is difficult to attain due to long RF cables. Most systems require significant engineering to ensure the LO power level to the mixer is adequate. To help understand the problem, commonly used techniques have been evaluated while recommending a particular approach. Operating over a small fundamental frequency range with harmonic mixing has the advantage of lower RF cable insertion loss but results in high mixer conversion loss. Using negative slope equalizers and amplifiers, RF cable slope and attenuation can be sufficiently combated. However, this requires extensive system engineering and customization to match cable losses, thereby making it expensive. The approach is also designed to only work with a certain set of RF cables. A more viable approach includes independently controlling the attenuators and amplifiers for the signal and reference channels which can be configured to provide optimal LO power to the respective mixer. A simple setup file configures components in each channel to adapt to any set of RF cables. Positive experimental results of implementing this technique in different configurations are presented.
M rejection curve was described. The steps to generate this rejection curve consist simply of (1) translating the coordinate origin of the measured pattern to a new location (2) performing a spherical modal analysis of the pattern, and (3) taking the total power in the lowest order mode as a measure of the strength of the radiation source at that location. Stepwise repetition of this process then generates the IsoFilterTM rejection curve. The basis for the process of generation was an empirical recipe for which no theoretical basis was presented. In this paper we relate the rejection curve to conventional electromagnetic theory. We begin with the general free space Green's function assuming a general distribution of current sources, and show how one may plausibly describe the IsoFilterTM rejection curve, and how it operates to reveal an arbitrary source distribution.
Making measurements on an outdoor range can be challenging for many reasons, including test article size, weather, and undesired electromagnetic effects. The challenges this paper addresses are those associated with the dense spectral environment in which measurements must often be made. Signals from external emitters must be prevented from causing interference with the measurement, and the outdoor range must not cause interference with other nearby systems. These criteria oppose each other in that if range transmit power is increased sufficiently to limit the effects of interference on the measurement, the range may cause interference to other systems. If low power is used in the range to avoid causing interference to others, the external emitter may make measurements on the range difficult to impossible. This paper demonstrates how, by using a sensitive receiver with high selectivity, one can make measurements right in the band of the interferer. By changing how the signal is processed, measurement capability is enhanced.
The cloud profiling radar (CPR) for the Earth, clouds, aerosols and radiation explorer (EarthCARE) mission has been jointly developed by JAXA and NICT in Japan. The development of CPR has required several technical challenges from the aspects of hardware designing, manufacturing and testing, because very large antenna reflector of 2.5m diameter with high surface accuracy, high pointing accuracy and high thermal stability had been required to realize the first space-borne W-band Doppler radar. In order to verify the RF design, we have just begun to perform antenna pattern measurement by using a CPR Engineering Model (EM). For this RF testing, we introduced a Near-Field Measurement (NFM) system with necessary capabilities for high accuracy measurement. This paper will present the summary of preliminary test results of the CPR EM antenna and the other technical efforts being taken for the antenna pattern measurement.
Computational Electromagnetics (CEM) Techniques have found wide use in scattering analysis of structures due to the fact that they require less cost and time than doing physical measurements. Numerical methods both in the time and frequency domain such as the Finite Integration Technique (FIT) [1], Method of Moments (MoM) [2], Multilevel Fast Multipole Method (MLFMM) [3], Transmission Line Method (TLM) [4] and Finite Element Method (FEM), have been known to provide accurate results for Bi-static as well as Mono-static Radar Cross Section (RCS) analysis in general but their practical applicability to specific types of structures is frequently misunderstood thus leading to mistrust in the results obtained. A result comparison between the different techniques is typically the best way of gaining trust in the results obtained, however this involves the general principle of result convergence which must be achieved for each individual solution technique. Using one of the standard benchmark radar targets which is the Cone-sphere [5], a comprehensive description of how to achieve result convergence for each technique will be presented and the final results will be shown to agree with published measured results [7, 8]. This target will be used in different configurations (with and without a slot) as well as coated with Radar Absorbent Material (RAM).
There are only a handful of commercially available antenna calibration laboratories in the US that are accredited to ISO-17025. Satimo has been operating an accredited laboratory in Atlanta since 2005 and an accurate and documented evaluation of measurement uncertainty has been a key element of the accreditation process. In order to develop the budgets, the parameters affecting the accuracy of the antenna measurement must be well understood. There are several references [2-9] that outline the method for preparing a measurement uncertainty budget, but few encompass the unique attributes associated with antenna measurements. Many of the papers published on the subject of the measurement uncertainty for antenna measurements address the characterization of a specific error term associated with an uncertainty budget, but few describe all of the terms contributing to the error budget nor how to practically determine their values. The intent of this paper is to outline and briefly describe the derivation of the uncertainty terms that contribute to the overall error budget for antenna measurements. Topics that will be discussed include: the uncertainty types, how to obtain or derive the error term for each uncertainty type, the distributions associated with each uncertainty type, the determination of the confidence level and coverage factor, how to combine the error terms as the references listed above are not in agreement on the method for combining the terms.
A Near-Field/Far-Field (NFFF) transformation for characterizing planar aperture antennas from plane-polar scanning data is presented. The method recasts the measurement problem as a linear operator one, and solves it as a Singular Value Optimization. The field sample positions are chosen to provide the minimum number of NF samples optimizing the singular value dymamics of the relevant operator. The available a priori information on the AUT is accommodated to limit the number of parameters needed for the characterization and the transformation is performed by a regularized Singular Value Decomposition (SVD) approach. Experimental results show the effectiveness of the technique in reducing the number of required samples.
The two most common test methods used to evaluate wireless test chambers are the Ripple Test Method standardized by CTIA - The Wireless Association and the Field Sensor Method standardized by the 3rd Generation Partnership Project (3GPP). Both methods sample the magnitude of the illuminating field at fixed spatial points in the Test Zone to determine the magnitude of the ripple in the test zone. This ripple data is then statistically processed to determine the expected measurement uncertainty attributable to chamber reflections at a given frequency. The strengths and weaknesses of each of these evaluation methods are discussed in detail. Test results using both methods to evaluate a single chamber are presented. A third wireless test chamber evaluation method is also described. In this method a series of Total Radiated Power (TRP) measurements are made on an antenna with the antenna positioned at various spatial locations in the test zone. If measured with a perfect plane wave, each TRP measurement should produce the same result regardless of the spatial location of the antenna. Variations in measured TRP relate directly to measurement uncertainty caused by deviations of the incident wave from a perfect plane wave.
Both mathematical simulations and experimental results have shown that it is possible to make accurate over-the-air measurements of wireless devices at much shorter range lengths than those indicated by the far-field criteria of 2D2/.. This paper describes a small shielded anechoic chamber designed to minimize the cost and floor space requirements of over-the-air measurements while at the same time providing measurement uncertainties that are comparable to larger chambers whose design is based on the far-field criteria. The design trade-offs are presented and the construction of the chamber described. The chamber was evaluated at different wireless frequency bands using the ripple test procedure from the CTIA Test Plan for Mobile Station Over The Air Performance. Total Radiated Power measurements were also made on gain standard dipoles to determine the uncertainty in integrated measurements. These measurement results are presented.
The overall measurement system details are presented, along with mechanical accuracies achieved for the scanner system. Details of the chamber and host facility are described. Finally, the paper concludes with measurements of a UHF-band Standard Gain Horn using the system. The challenges and benefits of such a system will be highlighted.
We report on the initial phase of our study to assess the electromagnetic interference and electromagnetic compatibility measurement and calibration procedures at the National Institute of Standards and Technology. We are developing a measurement-based uncertainty analysis of calibrations and measurements in the anechoic chamber. We intend to characterize all sources of uncertainty, which include power and probe-response measurements, noise, nonlinearity, polarization e.ects, multiple re.ections in the chamber, drift, and probe-position and probe-orientation errors. We present simple and repeatable measurement procedures that can be used to determine each individual source of uncertainty, which then are combined by means of root-sum-squares to state the overall measurement or calibration uncertainty in the anechoic chamber. We report on work in progress and future plans to characterize other EMI/EMC facilities at NIST.
This paper discusses design & measurement techniques for two choke horn antennas used as reflector feeds for space applications. Firstly, highly efficient ultra wideband (UWB) choke horn antenna with high beamwidth & linear polarization will be discussed. This choke horn exhibits excellent VSWR characteristics as antenna shows an incredible ultra wide bandwidth (UWB) of at least 5GHz ranging from 7.5GHz – 12.5GHz with an incredible VSWR< 1.2 for the entire frequency range. Wide beamwidth of approximately 45 degrees have been achieved at gain of 13.5dB. Secondly, wideband dual-fed circularly polarized choke horn antenna will be discussed. In this case, 90-degree phase shifter/ power coupler is being used to feed the antenna in order to keep antenna circularly polarized making polarization independent of matching issues. Both designs meet the ruggedness, strength & reliability, durability, weight, corrosion resistance, temperature variation requirements in space environment. Design Simulation & Optimization for horn antenna has been done on Finite Element Method (FEM) based software ‘Ansoft HFSS v11’. Absolute gain measurement techniques have been employed for the measurement of antenna gain.