Synthesized-reference-wave holography for determining antenna radiation characteristics

The far-field radar cross section (RCS) measurement of a large scatterer is generally difficult due to the limitation of available measurement range. One way to avoid this difficulty is to adopt a scale model by compact range measurement. However, there are some problems in this measurement: it is labour intensive and expensive to construct the scale model, and the frequency characteristics of the surface impedance of the scatterer may not be reproduced at shorter wavelength measurements, so real target measurement is necessary to obtain accurate data. There exists a near-field antenna measurement technique, however a direct application of this technique for RCS measurement fails because the RCS depends on the locations of both the source and the observation points. Falconer (1988) has overcome the above difficulty and successfully estimated the far-field RCS by the near-field RCS measurement. We have also proposed a far-field RCS prediction algorithm which uses cylindrical or planar near-field RCS data. However these algorithms are applicable only for the monostatic RCS. We report novel prediction algorithms of the far-field bistatic scattering cross section (SCS) by extending the proposed algorithms. These are presented in cases of spherical, cylindrical and planar scanning. We also show simulation results of the far-field SCS prediction in order to validate our methods.

Using symplectic relation and reaction concept, we propose a planar near-field antenna measurement method. A generalized probe compensation equation is deduced to obtain the probe correction formulation. To verify our approach, a reflector antenna with <TEX>$1{\times}2$</TEX> horn array is fabricated and measured in the near-field measurement facility. The near-field measurement results are compared with the physical optics (PO) simulation. The results of measurement and simulation agree very well near to the mainbeam.

Distributed Fiber-optic Sensing (DFOS) provides real-time data acquisition, monitoring and diagnostics for well stimulation and well spacing assessment. These include measurements of Distributed Acoustic Sensing (DAS) with high frequency acoustics in treatment wells, and low frequency strain/temperature sensing in offset monitor ones. The goal of this integrated study is to show the value of multi-well fiber sensing for real time fracturing diagnostics and stimulation optimization. By integrating near field injection to far field strain responses we assess overall reservoir development. The availability of fibers on both the treatment well and a nearby observation well allows us to investigate the near-wellbore injection profile and the far-field strain fracture propagation. Quantitative strain levels clearly respond to the effects of well distance, location and treatment well stimulation design. Monitoring well strain measurements of fracture density and triggered stimulated span were logged and compared to acoustic signals in the nearfield stage by stage. DAS interpretation was conducted during the treatment of each stage indicating the effectiveness and efficiency of the completion design. Results show that this is a very effective tool to better understand the performance of the fracturing treatment by digital transformation using DAS data. In addition, acoustic and strain measurements also validated its diagnostic capability for real-time operation monitoring. In this presentation we show how the near-field acoustic and far-field strain measurements allow for better understanding of the completion efficiency. This is by assessing the far field response to quantified DAS injected signals in the treatment. This analysis takes advantage of fiber installation on both the treatment and nearby monitor well. The fluid and proppant allocations in the near field were performed on the treatment well using relative acoustic intensities. Meanwhile, the fracture propagation induced strain change is recorded by the offset fiber well. Using this fiber data reveals dominant clusters and stage bias from near-field injection profile. Simultaneously the far-field identified fracture counts from strain further enable a geomechanical assessment of the stimulated reservoir and assess the effectiveness of the completion design. Multiple DAS fiber equipped wells not only provide single diagnostic tool for each of the fiber well, but also demonstrate significant integrated assessment of the stimulation effectiveness, completion efficiency, well interaction, and reservoir description. Availability of near and far field measurements constitutes an important tool to assess properties of the reservoir. Here we show how different vantage points can help illuminate a fracturing program in unconventional reservoirs.

In this study, photonics-based near-field measurement and far-field characterization in a 300-GHz band are demonstrated using an electrooptic (EO) sensor with planar scanning. The field to be measured is up-converted to the optical domain (1550 nm) at the EO sensor and delivered to the measurement system with optical fiber. The typical phase drift of the system is 0.46° for the one-dimensional measurement time of 13 s, which is smaller than the standard deviation of the phase measurement of 1.2° for this time scale. The far-field patterns of a horn antenna calculated from the measured near-field distribution are compared with that measured with the direct far-field measurement system using a vector network analyzer. For the angular related parameters, the accuracy of the results obtained by our near-field measurement are comparable to that of those obtained by direct far-field measurements. The sidelobe level discrepancy (approximately 1 dB) between the results obtained based on our near-field measurement and those from the direct far-field measurements are attributed to the excess noise of the probe correction data. We believe that photonics-based near-field measurements with spherical EO probe scanning will pave the way for the characterization of high-gain antennas at the 300-GHz band.

In recent years, more and more attentions have been paid to the method of obtaining the electromagnetic scattering characteristics of targets by near-field measurement. This approach generally uses a 2-D planar scanning frame to achieve 2-D scattering center distribution image acquisition under a specific attitude angle of the equipment. Compared with the traditional RCS near-field measurement system, the vehicle-mounted radar RCS near-field scanning measurement system is not limited by the hardware conditions such as lack of turntable/guideway and low scattering metal bracket at the test site, and has the advantages of high mobility and low cost in the scattering characteristics test of full-size targets. But the omnidirectional SAR mode with the vehicle-mounted radar platform moving around the target in one week is adopted directly for testing, the direct imaging image will produce serious defocus phenomenon due to the limitation of the accuracy of the platform movement on the ground. So the motion error compensation is necessary. In this paper, we propose a focusing method for near-field scanning test imaging based on differential GPS data correction, using the platform motion data recorded by differential GPS to compensate the phase of the target echo signal, so as to improve the focusing effect of radar images. The practical effect of the proposed method is verified by conducting experiments with two types of linear/circular platform motion patterns.

We propose a method for determining the radiation pattern of an elongated source by near-field pressure measurements. The method is based on parameterization of the transducer by a set of spherical sources. The parameters (spherical-harmonic amplitudes determined by solving the inverse problem) are used to calculate the transducer radiation pattern. The main attention is paid to the case where the transducer has an elongated shape. The results of numerical simulations are reported.

The problem of far-field pattern estimation from near-field data measurements through determination of currents distribution on the aperture plane is considered. Extension minimum method is applied for currents estimation problem, which outperforms standard regularization technique, when there is a priori information about finiteness of unknown support of currents on aperture plane. Fourier transform applied afterwards to determine far-field pattern. Arc-shaped scanning geometry is used for numerical simulation. Results are presented for one-dimensional aperture case.

A method to estimate the current distribution by measuring the near-field distribution has been proposed and applied to the Yagi-Uda antenna. The optimum parameters to scan the near field, such as the positions for measuring the near field, the area of scanning plane, and the number of measuring points for near field measurement have been determined by using the condition number. Numerically estimated results are presented and compared with the real data, confirming the validity of the method.

The acoustic holography (AH) method with single measuring plane has been well known as the conventional method and can be implemented by far field measurement with simple instruments. However, the noise source resolution of the AH is not sufficient. In order to improve the resolution in the noise source identification, several kinds of the acoustic holography methods have been so far proposed. For example, the near field acoustic holography (NAH) can provide high and accurate resolution of the holography by the nearfield measurement. However, the nearfield measurement within one wave length is sometimes impossible in the actual circumstances. The Acoustic Double Holography (A D H) proposed in this paper is a simplified approach with higher resolution of the noise source locations than that of the conventional AH methods. The ADH method basically uses dual measuring planes and does not require nearfield measurement. The sound pressure data detected on the rear plane are transformed into the virtual pressure data on the front plane taking into account of the distance between the plane and the object. Comparing the virtual pressure data with the actual data measured on the front plane, resolution on holography can be improved significantly. Computer simulation and an experiment with two loud speakers were executed in order to confirm the fundamental feature of the proposed method. Several advantages on the method with respect to resolution over the conventional AH method were discussed. Furthermore, the ADH measurement was carried out on running engine under the full load operation. Through these results, the highly noise radiating areas on the engine surface were detected and reduced with noise shielding material. The overall engine noise level was reduced by 1.5dBA as the first stage in this noise control work.

There is an increasing interest in studying supersonic jet noise due to the immense noise level and poor understanding of the generation mechanisms. In many earlier studies, some nonuniform nozzle geometries were proposed to attain noise reductions in supersonic jets. However, the effectiveness and efficiency of noise reduction vary with the nozzle geometries and the fundamental mechanisms responsible for noise reduction are still unclear due to limitations of experimental measurement techniques for supersonic jet flows. Thus the present effort aims to study the noise radiated from different nonuniform nozzles to provide an experimental database for detailed characterizations. An equally important goal is to study acoustic properties in the near-field region to better understand the effect of nozzle exits on the generation of different noise components. In the present study, six lip-modified nozzles were designed and their detailed acoustic fields are compared to that of the circular baseline nozzle. Two different stepped nozzles were firstly tested at over- and perfectly- expanded conditions. The far-field noise spectrum analyses show that the stepped exits play an insignificant role in altering noise emissions at perfectly-expanded condition. At over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. To explore the physics responsible for the observed noise reduction, detailed near-field acoustic measurements were conducted to locate the sources of different noise components. The location, extent and intensity of large-scale turbulent noise are slightly modified by stepped nozzles at different azimuthal angles. More importantly, no prominent behaviour of broadband shock noise appears to exist in the near-field of the longer stepped nozzle. Subsequently, cross-correlation technique was employed to study the effects of nozzle geometry on the spatial structure of acoustic field. Baseline and stepped nozzles lead to large coherence noise sources at far downstream regions and the noise coherence region appears to be enlarged by the stepped nozzles, especially for the longer stepped nozzle. An experimental investigation for the stepped nozzles was also conducted at under-expanded conditions. When compared to the circular baseline nozzle, far-field acoustic measurements show that stepped nozzles lead to significant noise reductions at certain polar and azimuthal angles. Detailed spectral analysis shows that the noise reduction is mainly due to reduction in broadband shock associated noise and elimination of jet screech phenomenon. Subsequently, qualitative schlieren technique was employed to visualize the resulting shock structures and the results show that shock cell structures within the jets are asymmetrically modified by stepped nozzles. Quantitative schlieren measurements were performed to correlate the shock structures and density gradient fields with the resulting noise components. The results also show that the reduction in broadband shock associated noise is related to the lower shock strengths, as demonstrated by the density gradient profiles. Additionally, acoustic fields of bevelled, double-bevelled and stepped nozzles were compared using detailed far-field acoustic measurements at different jet expansion conditions. At perfectly-expanded condition, the lip-modified nozzles lead insignificant effect on noise emission, while they generate prominent noise reduction at forward quadrant for imperfectly expanded jets. Particually, the S60 nozzle produces the highest noise reduction of 5.5dB and 6dB at conditions of NPR=2.8 and 4, respectively. At under-expanded condition of NPR=5, the B60 is the optimal nozzle configuration where it generates the maximum noise reduction of 9dB at ϕ=90° and θ=180°. It should be mentioned that the studied nozzles occasionally generate noise tradeoff at shallow polar angles, i.e. ϕ≤50°, where they produce comparable or slightly higher noise level to the baseline nozzle. Finally, the effects of nozzle trailing shape on screech radiation are also investigated through acoustic measurements at an under-expanded condition of NPR=5. Bevelled nozzles eliminate jet screech by producing asymmetric shock structures which results in instability waves within the jet column, as well as mismatches in phase and amplitude. Additionally, double-bevelled nozzles produce symmetric shock structures and amplified screech intensity, even when jet-mixing effects have been significantly enhanced. To study the screech dynamics, wavelet transform was employed to analyze the near-field microphone signals. Jet screech and harmonics of different nozzles are highly unsteady and undergo non-periodic and stochastic temporal variations. Subsequently, near-field acoustic measurements were conducted to locate the screech sources. Double-bevelled nozzles have significant impact upon screech intensity levels and peak noise locations, as well as conferring different changes along different measurement planes.

Aperiodic spike noise signals generated by pulse-frequency modulation (PFM)-controlled direct-current (DC)–DC converters introduce random jitter into the switching timing, rendering it challenging to isolate individual power supply noise sources. Conventional methods based on cross-correlation analysis suffer from inaccuracies owing to the random jitter and aperiodic characteristics inherent in PFM converters. This study proposes a method for accurately separating radiated electromagnetic noise sources from multiple PFM-controlled DC–DC converters. An asynchronous near-field and far-field measurement scheme is adopted owing to its considerably lower implementation cost than that of its synchronous counterpart. The proposed method extracts individual spike noise, and then identifies and associates each with its corresponding source based on the spatial and temporal characteristics captured by the asynchronous measurements. The experimental results demonstrate improved estimation accuracy across a broad bandwidth, thereby enabling the identification of dominant noise sources. This enhanced accuracy supports more efficient mitigation of electromagnetic interference and reduces the complexity and cost of implementing noise suppression. As a result, the time-to-market can be shortened, facilitating the development of cost-effective electronic products.

So that readers may draw from their understanding of planar and cylindrical scanning, a unified theory of near-field data processing is given, treating planar, cylindrical, and spherical scanning as mere special cases.

Near-field (NF) measurement techniques and the related NF to far-field (FF) transformations are gaining interest for their ability to allow an accurate evaluation of the radiation characteristics of those antennas whose electric sizes do not make it possible to perform direct FF measurements in a controlled and reflection-free environment, such as the anechoic chamber. This book provides a comprehensive treatment of classical NF-FF transformation techniques and describes the significant improvements achieved in their performance by correctly applying the non-redundant sampling representations of antenna radiated electromagnetic (EM) fields. Non-Redundant Near-Field to Far-Field Transformation Techniques is designed to meet the needs of students of antenna NF measurements, as well as of engineers and physicists working in the area. It has been written keeping in mind the fulfilment of two main objectives. The former is to provide all the analytical details on the derivation of the classical NF-FF transformation techniques without and with probe compensation, which are not easily available elsewhere in the literature. The latter is to give a comprehensive description of effective representations of the EM fields radiated over arbitrary rotational surfaces, which make use of a non-redundant (i.e., minimum) number of samples collected on these surfaces or along proper spirals wrapping them.

The Surface Wave Scattering-Microwave Scanner (SWS-MS)

An unsymmetrical printed circuit board with a ground plane has been modeled using Genetic Algorithm by its equivalent dipole set for the Near-Field matching and Far-Field prediction. Far-Field measurements have been predicted using Near-Field measurements in which the actual radiation source has been replaced by an equivalent set of dipoles. In previous work, the Genetic Algorithm has been employed to find out the equivalent dipole set only within the surface boundary of the radiation source. Since most of the practical radiation sources are unsymmetrical in nature, the magnetic fields above and below the ground plane are significantly different. In this paper, the dipole locations are allowed to lie outside the surface boundary of the original source to match the unsymmetrical Near-Field distribution and to predict its Far-Field strength. Simulation results show that the improved Genetic Algorithm not only demonstrates its viability as a Far-Field predictor but also that Genetic Algorithm can be optimized to search for an equivalent dipole set outside the boundary of source. By taking four different cases of observation planes, it is also shown that increasing the number of observation planes as compared to increasing the number of matching points on a single observation plane gives improved results for the Near-Field matching and Far-Field approximation of unsymmetrical printed circuit board. Other important aspects like the selection of observation points, the number of equivalent dipoles required have been addressed.