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This paper shall discuss a method for measuring the current distribution – in both magnitude and phase - along the length of a floating antenna operating on the surface of the ocean. The method makes use of a novel toroidal current sensing device and balun arrangement, with a vector network analyzer serving as the measurement instrument. The current data obtained using this method can then be used to compute the far-field pattern of the antenna, both at the horizon and overhead, in a manner similar to near-field scanning of aperture antennas. This new method has significant advantages over the conventional far-field method of measurement in terms of accuracy, time, and cost, and can also be used to determine the realized gain of the antenna. Measured and theoretical data shall be presented on example antennas to illustrate the process of measuring the current distribution as well as computation of the far-field pattern.
An optical diffraction technique was developed for performing far field transformations of spherical near field data. The first goal of the development was to promote a better physical understanding of the phenomena of spherical near field transformation. Along the way, the limitations of this type of measurement became associated with real, optical physics, thus providing new considerations that might not be easily derived from the traditional, multi-pole expansion of spherical waves. In addition, new applications of spherical near field measurements are suggested by this approach. Specifically, the optical method allows for better understanding of the necessity and application of probe compensation. An optical transform eliminates the need for axially symmetric probes. Perhaps more importantly, this understanding leads to new considerations toward the applicability of single scan, spherical transforms which may lead to significant increases to the effective lengths of far field ranges. The purpose of this paper is to present a conceptual foundation to the spherical transformation of near field data by optical means and the immediate, associated benefits.
The equivalent source approach [ 1-4] has been presented recently as an advanced antenna diagnostics tool. The equivalent source approach is a true 3D approach as opposed to traditional methods based on plane wave expansion using hemispherical field information. This method is therefore highly suitable for diagnostics on low and medium directivity antenna and even allows the possibility to isolate, identify and filter unwanted effects close to the antenna from the measurements such as cable interactions. Spatial filtering is used in spherical near field measurements through the truncation of the mode spectrum. From knowledge of the minimum sphere enclosing the Antenna Under Test (AUT) the minimum number of spherical modes needed to represent the antenna can be determined and information resident in the higher order modes are eliminated as associated to sources outside the spatial domain of the source. Due to the nature of the spherical waves, spatial filtering truncation cannot be performed too close to the physical minimum sphere enclosing the antenna. Since the equivalent current approach is based on an accurate reconstruction of the physical currents on an object slightly larger than the physical antenna, a more acute spatial filtering can be performed. This paper discusses the advantages that can be obtained from the 3D spatial filtering on spherical near field antenna measurements.
The spherical near-field antenna measurement system StarLab has recently evolved to cover the entire frequency band from 800MHz to 18GHz [1, 2, 3]. The system is based on patented probe array technology and the Advanced Modulated Scattering Technique (A-MST) [4, 5]. The StarLab system combines the speed advantages of a probe array with mechanical rotation in elevation, thereby allowing for unlimited angular resolution over the full 3D sphere. By adding a linear scanner, the StarLab system can be converted to a compact cylindrical near-field measurement system (Starlab BTS), as shown in Figure 1. This system is particularly well-suited for measurements of base stations and other sectorial type array antennas. This paper describes the innovative design aspects of the StarLab portable antenna measurement system, with emphasis on the cylindrical near-field measurement capabilities. The system validation testing has been performed with comparative measurement campaigns, including both multiprobe and other traditional (single probe) measurement facilities at customer locations.
A stand-alone commercial program, performing advanced electromagnetic processing of measured data, is being developed by TICRA. The program reads the measured field and computes the extreme near field or the currents on the antenna surface. From the inspection of the extreme near field or currents, the program will solve typical antenna diagnostics problems, such as identification of array element failure and antenna surface errors, but also allow artificial removal of undesired contributions, such as currents on cables and fixtures, thereby saving valuable time and resources in the antenna design and validation process. The program will be based on two field reconstruction techniques, the SWE-PWE presented at AMTA in 2007, and a new and more accurate inverse higher-order Method of Moments (INV-MoM). The paper will illustrate the theory behind the two techniques and present numerical cases with simulated data.
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.
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.
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.
Spherical near-field to far-field transformation techniques allow for the reconstruction of the complete radiation pattern of the antenna under test (AUT) from the knowledge of the tangential electric field over a spherical surface [1-2]. However, in practical spherical near field measurements there are zones on the measurement sphere where data are either not available or less reliable. When the spherical wave coefficients (SWC) are calculated from incomplete near-field data by setting to zero the unknown samples, the abrupt discontinuity in the field values at the edge of the scan area may lead to erroneously large values of the higher-order spherical harmonic coefficients. Different solutions have been proposed to circumvent this problem [3-4] and have been demonstrated effective for small truncation areas [3]. In this paper a novel approach is proposed for the reduction of the truncation error in spherical near-field measurements. The method is based on a proper filtering of the SWC in accordance with the extent of the minimum sphere enclosing the AUT. More specifically, it consists in iteratively imposing the matching of the near-field with the measured samples and performing a spectral filtering in the spherical harmonics domain, based on the knowledge of the physical extent of the AUT [5-8]. The procedure has been tested on synthetic as well as measured near-field data and has proved to be effective and stable against measurement errors. The approach has shown to be effective even for increasing truncation areas.
Spherical near-field to far-field transformation techniques allow for the reconstruction of the complete radiation pattern of the antenna under test (AUT) from the knowledge of the tangential electric field over a spherical surface [1-2]. However, in practical spherical near field measurements there are zones on the measurement sphere where data are either not available or less reliable. When the spherical wave coefficients (SWC) are calculated from incomplete near-field data by setting to zero the unknown samples, the abrupt discontinuity in the field values at the edge of the scan area may lead to erroneously large values of the higher-order spherical harmonic coefficients. Different solutions have been proposed to circumvent this problem [3-4] and have been demonstrated effective for small truncation areas [3]. In this paper a novel approach is proposed for the reduction of the truncation error in spherical near-field measurements. The method is based on a proper filtering of the SWC in accordance with the extent of the minimum sphere enclosing the AUT. More specifically, it consists in iteratively imposing the matching of the near-field with the measured samples and performing a spectral filtering in the spherical harmonics domain, based on the knowledge of the physical extent of the AUT [5-8]. The procedure has been tested on synthetic as well as measured near-field data and has proved to be effective and stable against measurement errors. The approach has shown to be effective even for increasing truncation areas.
Millimeter-wave measurements on spherical near-field scanning systems present a number of technical challenges to be overcome to guarantee accurate measurements are achieved. This paper will focus on the affect of mechanical alignment errors of the spherical rotator system on the antenna’s measured performance. Methods of precision alignment will be reviewed. Sensitivity to induced mechanical alignment errors and their affect on various antenna parameters will be shown and discussed. Correction methods for residual alignment errors will also be described. The study includes 38 and 48 GHz data on the Alphasat EM model offset reflector antenna measured by TeS in Tito, Italy on a NSI-700S-60 Spherical Nearfield system, as well as a 40 GHz waveguide array antenna measured by NSI on a similar NSI-700S-60 Spherical Nearfield System at its factory in Torrance, CA, USA.
A bi-lateral comparison of power gain for a V-band (50 to 75 GHz) Cassegrain antenna and a standard gain horn antenna has been performed between KRISS and NIST. Measurement parameters for this comparison are the power gain and complex reflection coefficient of the traveling standards, and their measurement frequencies are 65 GHz for the high-gain antenna and 50, 65, and 75 GHz for the horn antenna. All participants used the planar near-field scanning method for characterizing the high-gain antenna and the three-antenna extrapolation technique for characterizing the horn antenna. This paper summarizes the comparison and its measurement results with uncertainties. Generally, the agreement between results in all measurements is within the uncertainty of each participant except for the gain result of the horn antenna at 65 GHz.
This paper analyzes the applicability of near field scanners into existing compact test ranges. The analysis is motivated by creating multi-purpose test chambers having the advantages of both, near field systems and compact test ranges. This contribution comprises the discussion of near field scanners at several positions inside a typical compact test range. A ray tracing analysis is presented taking these positions into account in the assessment of near field errors due to multi-path reflections. It is presented how reflections from the absorbers and reflectors are differently impacting near field measurements of low, medium and high gain antennas. The impact is quantified in terms of error levels used in common near field error budgets. It is shown that the combined approach is realizable for specific configurations only.
The Global Precipitation Measurement (GPM) mission is a satellite based Earth science mission that will study the global precipitation from rain, ice and snow. A critical part of this satellite is the multi-frequency radiometer system that covers frequencies up to 183 GHz. Beam pointing and beam efficiency must be measured very accurately to calibrate the radiometer response. This paper will focus on the measurements of the offset reflector antenna operating up to 183 GHz using a Nearfield Systems Inc. (NSI) planar near-field measurement system and the special challenges that this presents. Results will be presented and the uncertainty in beam pointing will be discussed.
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.
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.
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.