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In this work we study the near-field antenna measurement using cubical surface scanning and related near-field to far-field (NF-FF) transformations. The cubical surface scanning is a fascinating idea because it can be realized using widely used planar scanning on six surfaces of a cube, and it provides the possibility to determine the complete 3-D pattern instead of the pattern in a limited angular region as in traditional planar scanning. The NF-FF transformation presented in this paper is based on spherical vector wave expansion (SWE). The most important issue of this paper is to introduce the azimuthal mode decomposition technique to be applied as a part of the NF-FF transformation allowing a reduction in the computational burden of the transformation.
When making antenna measurements, great care must be taken in order to obtain high quality data. This is especially true for near-field antenna measurements as a significant amount of mathematical post-processing is required in order that useful far-field data can be determined. However, it is often found that the integrity of these measurements can be compromised in a large part through range reflections, i.e. multipath [1]. For some time a technique named Mathematical Absorber Reflection Suppression (MARS) has been used to reduce range multi-path effects within spherical [2, 3], cylindrical [4, 5] and most recently planar [6, 7] near-field antenna measurement systems. This paper presents the results of a recent test campaign which yields further verification of the effectiveness of the technique together with a reformulation of the post-processing algorithm which, for the first time, utilises a rigorous spherical wave expansion based orthogonalisation and filtering technique.
Ultra wideband (UWB) systems use short pulses in order to achieve high data rate wireless communications and/or radar resolution. Thus, UWB antennas should be designed carefully, both in time and frequency domains, with the system performance in mind. Time domain characterization of an antenna can be performed first by measuring the frequency domain transfer function of a direct link consisting of two identical antennas. Then, the time domain response is obtained by post processing the frequency domain data using the Inverse Fast Fourier Transform (IFFT). This paper discusses frequency and time domain performance of four-arm equiangular and Archimedean spiral antennas operating in mode 2. The frequency domain transfer function is synthesized using complex far field information measured in a spherical near-field chamber from 2GHz to 12GHz. The synthesized approach is validated using simulation and direct link measurements. The quality of radiated pulses is evaluated in terms of fidelity factor over a full field of view, a task not trivial for the direct link measurements.
In this paper, a beam-steering computer design is explored for a large space-fed phased-array antenna. GTRI previously developed a beam-steering computer for a smaller phased-array antenna which accomplished spherical propagation focusing and multiple phase-only beam-broadening modes. In a subsequent effort, the beam-steering computer design was scaled for a large phased-array antenna to accomplish similar tasks. To verify the design, a series of far-field measurements was initiated to characterize the performance of the antenna by comparing with past measured near-field data and modeled results. One of the primary responsibilities of the beam-steering computer was the focusing of the spherical propagation wave front. A measurement technique is discussed to accomplish this focusing for the large space-fed phased-array antenna by correcting measurement errors in the spherical propagation routine of the beam-steering computer. Additional patterns were taken using the updated feed horn focal point for spherical propagation correction. By correcting the phase errors caused by spherical propagation defocusing in the original beam-steering computer, significantly better antenna performance was obtained, including higher peak gain, reduced nearby sidelobe levels, and removal of beam-pointing errors. Another important responsibility of the beam-steering computer was the ability to realize multiple antenna modes, including a focused pencil beam as well as defocused broadened-beam modes. A stochastic gradient descent algorithm was utilized to obtain several phase tapers to accomplish beam-broadening for the antenna modes. These modes were implemented in the beam-steering computer and tested on a far-field range. The antenna patterns were compared with modeled results and with previous measured data to ensure validity of the implementation.
Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
Pattern distortion due to finite range measurement of antennas in close proximity to electrically large metallic media is examined. The ubiquitous yet arbitrary 2D2/. distance requirement cannot be blindly applied to scenarios where antennas couple to nearby structures. A C-band standard gain horn antenna is analyzed near a circular metallic plate at 6 GHz using the commercial software FEKO. The near-fields are computed at various radii, which are set to multiples of D2/., where D is defined as the largest dimension of the complete structure. The radiating near-field patterns are normalized and compared to the far-field pattern. Results indicate that measurement at 2D2/. may not be necessary. Increasing fractions of D2/. results in a diminishing measurement error that may be tolerable, depending on the intended application.
– far-field transformation with cylindrical scanning are efficiently determined by using an optimal sampling interpolation algorithm. The comparison of the far-field patterns reconstructed from the acquired irregularly distributed measurements with those obtained from the data directly measured on the classical cylindrical grid assesses the effectiveness of the approach.
The Georgia Tech Research Institute (GTRI) analyzed a phased-array antenna for the purpose of testing phase-only defocusing methods. The array is defocused with the objective of broadening its beam at the cost of lower antenna gain. A design for the beam-steering computer is accomplished which adds the capability of focusing a beam, steering in azimuth and elevation, and performing beam defocusing using only element phase. Widening of the beam is accomplished using only 180° phase shifts in the elements, and it is compared with widening accomplished using gradual phase tapers. The antenna is measured in a near-field range to obtain amplitude and phase information as a function of each element in the array. Near-field testing of the antenna is also used to verify the capability of the beam-steering computer; two-dimensional antenna patterns and near-field hologram projections are compiled to prove this functionality. A software model is designed to mimic the behavior of the phased array antenna in its operational modes; it is also used to predict antenna gain and beamwidth prior to near-field testing. Measured and modeled antenna patterns are compared using focused and defocused modes. Metrics are performed on the near-field data to infer statistics of the individual phase shifters and on the computed far-field patterns to characterize the entire antenna. The defocusing methods under analysis are phase-only methods, due to the inability to control amplitude weighting of elements in this antenna. One method discussed uses only 180° shifting of elements in the antenna to achieve a desired beamwidth. This is compared with another method which gradually spoils the beam by applying a phase taper across the aperture. The results from near-field testing compare the defocusing methods and characterize the relationships between gain, beamwidth, and sidelobe levels for both defocusing methods.
Very often far field conditions are violated at high frequencies RCS measurements and in real life scenarios. People go to great lengths to carry out these measurements in the far field. They make large investments to build suitable compact ranges, or long outdoor ranges. Others make extensive efforts to correct the near field measurements to the far field values. This paper suggests that those elaborate measures are superfluous, as far as the total RCS is concerned. Although near field measurements clip the high peaks, they broaden their shoulders compensating for the loss. Simulations and actual measurements show that the accumulative distribution of RCS values in the near field is equal or slightly higher than the distribution of these values in the far field, until one looks for very high 90th percentiles. Thus, for detection and survivability estimates the near field measurements provide a close upper bound.
An Indian Defense Research and Development Organization (DRDO) laboratory has commissioned a state-of-the-art indoor far-field antenna test facility in 2009. This facility supports highly accurate measurement of a wide range of antenna types over 1.12–40 GHz. Owing to the heavy usage of this range, it was decided to enhance the existing facility to include a Hybrid Planar Near-Field facility for high speed accurate antenna measurements with minimal changes to the existing chamber configuration. The scanner is implemented as a highly innovative Hybrid T-type scanner, with a Y-axis that consists of a Linear Multi-Probe array and a traditional single probe configuration. The linear Multi-Probe array consists of two sets of dual polarized probes each one covering a sub set of the full frequency range. In particular one set covers the 1.0-6.0GHz band (operational from 400MHz) and the other set covers the 6.0 to 18.0GHz band. The traditional Single Probe configuration includes a set of Open Ended Waveguide Probes to facilitate an operational frequency range of 1.12 – 40.0GHz, as in the existing Far Field system. The Hybrid scanner is placed along the sidewall opposite the door, on the DUT positioner side. The major benefit of this layout is that there is no need to change the basic design of the chamber and it is built according to the original plans. When the chamber is used in the far-field mode, the tower is moved to the end of the horizontal axis in the direction of the corner of the chamber. The tower sides that face away from the chamber corner are covered with absorbing material to reduce reflections from the tower. Assuming that the chamber is intended to measure directional antennas, the existence of the tower behind and to the side of the AUT is expected to introduce minimal interference. The high speed linear Multi-Probe array has typical measurement speeds ranging between 5 and 15 minutes at 5 frequencies and 2 polarizations. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 6.0 package for both Single Probe & Multi Probe modes. A Real-Time Controller (RTC), accompanied by a 4-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
An Indian Defense Research and Development Organization (DRDO) laboratory has commissioned a state-of-the-art indoor far-field antenna test facility in 2009. This facility supports highly accurate measurement of a wide range of antenna types over 1.12–40 GHz. Owing to the heavy usage of this range, it was decided to enhance the existing facility to include a Hybrid Planar Near-Field facility for high speed accurate antenna measurements with minimal changes to the existing chamber configuration. The scanner is implemented as a highly innovative Hybrid T-type scanner, with a Y-axis that consists of a Linear Multi-Probe array and a traditional single probe configuration. The linear Multi-Probe array consists of two sets of dual polarized probes each one covering a sub set of the full frequency range. In particular one set covers the 1.0-6.0GHz band (operational from 400MHz) and the other set covers the 6.0 to 18.0GHz band. The traditional Single Probe configuration includes a set of Open Ended Waveguide Probes to facilitate an operational frequency range of 1.12 – 40.0GHz, as in the existing Far Field system. The Hybrid scanner is placed along the sidewall opposite the door, on the DUT positioner side. The major benefit of this layout is that there is no need to change the basic design of the chamber and it is built according to the original plans. When the chamber is used in the far-field mode, the tower is moved to the end of the horizontal axis in the direction of the corner of the chamber. The tower sides that face away from the chamber corner are covered with absorbing material to reduce reflections from the tower. Assuming that the chamber is intended to measure directional antennas, the existence of the tower behind and to the side of the AUT is expected to introduce minimal interference. The high speed linear Multi-Probe array has typical measurement speeds ranging between 5 and 15 minutes at 5 frequencies and 2 polarizations. Instrumentation is based on an Agilent PNA E8362B. Software is based on the MiDAS 6.0 package for both Single Probe & Multi Probe modes. A Real-Time Controller (RTC), accompanied by a 4-port RF switch, facilitates multi-port antenna measurements, with the possibility of interfacing to an active antenna.
Far field antenna measurements require specialized chambers, not very commonly available. The measurement process is inherently time consuming. If this time can be reduced it would increase the through put of the test chamber and would decrease the incurred expenses. This paper describes a novel adaptive far field measurement methodology for antennas, by varying the angular resolution and IF bandwidth in an adaptive manner. Different adaptive angular resolution techniques have been proposed and verified. Different type of antennas were measured with conventional antenna measurement methods and compared with proposed technique. It was observed that fine angular resolution can be achieved in main beam and first sidelobe levels with a little compromise on other side lobe and back lobe levels. The comparative results and their analysis are presented. On the average 20 to 40% measurement time is reduced with the proposed methodology. All measurements have been conducted in a CATR.
Near-field antenna measurements are accurate and common techniques to determine the radiation pattern of an antenna under test. The minimum near-field sampling rate is dictated by the electrical size of the antenna and usually equidistant sampling is applied for planar, cylindrical, and spherical measurements. Certain applications either rely on or benefit from near-field sampling on irregular grids. To handle irregular measurement grids near-field transformation algorithms like equivalent current methods or the multilevel fast multipole accelerated plane wave based technique are required which do not rely on regularly sampled data. In this contribution the plane wave based near-field transformation is applied to spherical, cylindrical, and “combined” near-field measurements employing irregular sampling grids. The performance is assessed by various simulated near-field measurement scenarios.
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.
Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3.. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a “Two Level GTD” approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also [6]).
Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3.. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a “Two Level GTD” approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also [6]).
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.