Far-field Range Research Articles

Millimeter-Wave Far-Field Range Antenna Measurement System for a W-Band Monopulse Antenna

In this study, a millimeter-wave far-field range antenna measurement system is proposed for W-band monopulse antenna measurements. Since the W-band monopulse antenna installed in radar systems has many input/output ports, multiple calculations are required to measure all the ports. We propose a system structure capable of the simultaneous measurement of multiple channels to quickly and accurately measure the performance of a monopulse antenna. The proposed measurement system includes a multi-channel rotary joint, a pair of diplexers, and multiple vector network analyzer extenders. In addition, an RF sub-system is implemented to reliably measure the phase of the W-band radiation pattern. It utilizes a shared local oscillator source and external power amplifiers for the transmit and receive paths. Employing the proposed multi-channel antenna measurement system, the sum/difference channels of a monopulse antenna were simultaneously measured according to the azimuth angle. In addition, a comparison of the far-field measurements provided by the proposed system and the near-field measurements was conducted.

On the Assessment of Reinforced Concrete (RC) Walls under Contact/Near-Contact Explosive Charges: A Deep Neural Network Approach

In recent years, the use of machine learning has been expanded to several fields, with promising advances in structural engineering applications. Deep neural network models have been implemented to predict the structural response of systems under conventional loading. Some of those neural network models are based on datasets containing images, test data, and/or data produced by using finite element models developed for a specific environment. While the accuracy of these models relies on the size and quality of the dataset, their use for blast analysis is rather limited, as publicly available data are scarce or restricted. Reinforced concrete (RC) walls or slabs under blast loading are commonly evaluated for flexural and shear behaviour, for which performance guidelines are widely available. While such response mechanisms are typically associated with the far-field range, the target response is controlled by local failure modes when blast loads are generated by contact or near-contact detonations. This paper introduces the implementation of a neural network model for the response prediction of RC walls subjected to contact and near-contact explosions. The model predicts the damage category (i.e., no damage, spall, and breach) associated with a given explosion scenario. The model is trained using experimental data from multiple test programmes available in open-source literature. It considers several parameters associated with the explosive charge (e.g., type, geometry, charge weight, and standoff) and RC target (e.g., material properties, geometry, and reinforcement). The model is able to accurately predict 81% of the total breached specimens, 66% of the total spalled specimens, and 71% of the full set of non-damaged specimens, with an overall accuracy of 72%, with precision and recall ranging from 60 to 76% and 66 to 81%, respectively. The current model is shown to be a significantly better predictor of the damage category than the semi-empirical approach outlined in UFC 3-340-02, making it a promising tool that can be improved with the inclusion of more experimental data.

Dynamic behaviors of unreinforced and spray polyurea retrofitted brick masonry infill walls under blast loads: Shock tube test and analyses

Unreinforced masonry infill walls (MIWs) are prone to crack, fragment, and even collapse under blast loads, threatening the safety of building structures, as well as the inside occupants and equipment. Spray polyurea (SPUA) has been proven to be an excellent retrofitted material for MIWs to improve their blast resistance. Aiming to study the dynamic behaviors of brick MIWs under far-field range blast loads, the successive blast tests on seven unreinforced and SPUA-retrofitted brick MIWs with different thicknesses and boundary constraints were first carried out based on the developed compressed air-driven large cross-section (3m×3 m) shock tube. It was indicated that the blast resistance of MIWs significantly improved by increasing wall thickness and strengthening boundary constraint; SPUA coating can improve the blast resistance of MIWs and effectively suppress the debris generation and splash. Then, the equivalent single degree of freedom model based theoretical calculation and the simplified micro-model based numerical simulation methods previously established by authors were briefly introduced. Additionally, the dynamic behaviors of seven examined MIWs were further predicted and compared with test results, which verified the applicability of theoretical calculation and numerical simulation methods. Finally, the improved damage and failure evaluation criteria of unreinforced and SPUA-retrofitted MIWs were given, and the charge weight and standoff distance diagrams of typical MIWs were proposed based on the validated theoretical and numerical methods as well as the improved criteria, which would provide a helpful reference for blast-resistant evaluation and retrofitted design of brick MIWs.

Wireless Powering Efficiency of Deep-Body Implantable Devices

The wireless power transfer efficiency to implanted bioelectronic devices is constrained by several frequency-dependent physical mechanisms. Recent works have developed several mathematical formulations to understand these mechanisms and predict the optimal operating conditions. However, existing approaches rely on simplified body models, which are unable to capture important aspects of wireless power transfer. Therefore, this paper proposes the efficiency analysis approach in anatomical models that can provide insightful information on achieving the optimum operation conditions. First, this approach is validated with a theoretical spherical wave expansion analysis, and the results for a simplified spherical model and a human pectoral model are compared. The results show that although a magnetic receiver outperforms an electric one for near-field operation and both sources could be equally employed in far-field range, it is in mid-field that the maximum efficiency is achieved with an optimum frequency between 1-5 GHz depending on the implantation depth. The receiver orientation is another factor that affects the efficiency, with a maximum difference between the best and worst-case scenarios around five times for the electric source and over 13 times for the magnetic one. This approach is used to analyze the case of a deep-implanted pacemaker wirelessly powered by an on-body transmitter and subjected to stochastic misalignments. We evaluate the efficiency and exposure, and we demonstrate how a buffered transmitter can be tailored to achieve maximum powering efficiency. Finally, design guidelines that lead to optimal implantable wireless power transfer systems are established from the results obtained with the proposed approach.

Wideband Near-Field RCS Measurement Techniques With Improved Far-Field RCS Prediction Accuracies

Wideband near-field (NF) radar cross section (RCS) measurement techniques for improving the far-field RCS prediction accuracies are addressed in this article. Both the data preprocessing and NF-to-far-field transformation (NF2FFT) techniques are used to accurately measure wideband far-field RCSs in a wide azimuthal angle range. By utilizing the calibration set creation and some wideband data preprocessing techniques, including the vector background subtraction, the time-domain calibration and compensation, the time-gating steps, as well as the cylindrical mode expansion or the imaging-based NF2FFT techniques, the accurate wideband far-field RCSs in a wide azimuthal angle range can be obtained with the limited NF measurement range. Both the poor measurement accuracy and dynamic range as well as the main lobe distortion and angular undersampling effects existing in the NF measured raw data can be improved significantly. The proposed techniques are validated with an 8–12-GHz measurement of a 0.5-m-length cone target (in a 3-m measurement range, compared with the at least 30-m far-field range requirement), and the obviously improved RCS patterns and less than 0.5-dB average wideband measurement errors are achieved.

Simulation of an asymmetric hexagonal microcavity with high-ratio fluorescence and high-efficiency directional emission.

Ratiometric fluorescent sensors are widely used in biological sensing and immunoassays due to their high sensitivity detection of analytes. The high-ratio value of fluorescence can increase the sensitivity of the fluorescence sensor; in addition, the directional emission can improve the efficiency of light collection and improve the effective use of radiation power. In previous studies, low fluorescence ratios and low directional emission efficiency have restricted the application of ratio fluorescence sensors. Based on the above constraints, this paper proposes an asymmetric hexagonal microcavity structure. By destroying the complete rotational symmetry of the hexagon structure, it achieves high fluorescence ratios and high-efficiency directional emission in the far-field range in the near-infrared wavelength range, which is of significance for the development of high sensitivity fluorescence sensors.

Design and field testing of a non-linear single-beam echosounder for multi-frequency seabed characterization

Seabed mapping and characterization are best performed using several frequencies and several angles of incidence. This is often an issue because of the need to employ different sonars, with distinct frequencies but co-located as much as possible to image the same patch of seafloor. This article presents the design, calibration and field testing of a multiple-frequency single-beam echosounder (SBES), mounted on a mechanical pan-and-tilt head. It uses very high transmitting levels to produce non-linear effects and generate harmonics of a 100 kHz fundamental frequency. PZT transducers are used to transmit high acoustic powers and PDVF transducers enable the reception of scattering levels over a very broad frequency band (for the different harmonics). Tank experiments are used to verify effective harmonic generation. The shock distance (at which harmonics are at their maximum level) is measured as 2 m from the transmitter and recommended as the minimum far-field range. Non-linear transmission losses (distinct from linear losses) are calibrated using a full metal sphere 38.1 mm in diameter and of known frequency response, up to ranges commensurate with the depths expected in the field (⩽30 m). The −3 dB beamwidth varies from 5.8° at 100 kHz to 2.8° at 300 kHz. Harmonics are used to resolve phase ambiguities in detecting seabed depths. Backscattering strengths BS are matched to the Generic Seafloor Acoustic Backscatter (GSAB) model to derive the best-fitting parameters. Field validation took place in the Bay of Brest (France) in May 2016, over three different types of seafloor (namely: sandy mud; gravel; gravelly coarse sand with maerl). Additional in situ calibration was used. The echosounder was pointed at angles from 0° (nadir) to 60° by 5° steps. One of the areas surveyed (“Carré Renard”), commonly used for instrument calibration and comparison with other measurements, showed differences <1 dB at 200 kHz. Videos and photographs of the seafloor were used to ground truth interpretations of the BS curves. The results show that these BS curves measured with the echosounder are relevant for seabed classification and characterization. The different shapes and levels of BS when compared to ground truth are coherent with the Jackson model. The main limit of this prototype of echosounder is the signal to noise ratio, in particular for high frequency harmonics (⩾400 kHz). The in situ calibration is unavoidable because of the non-linear parameter variations with water characteristics (temperature, salinity…). Calibrated BS curves from 100 kHz to 300 kHz can be directly compared to other measurements, for example to calibrate other instruments.

Broadband Lens-Integrated CMOS Camera-Type THz Compact Antenna Test Range

This article presents the concept of a lens-integrated CMOS camera-type terahertz (THz) compact antenna test range (CATR) to determine the far-field characteristics of transmitters (TX), including the radiation pattern, directivity, half-power beam-width (HPBW), and radiation power. For concept validation, the frequency-dependent far-field radiation pattern of a 0.6-1.1 THz TX equipped with a 23.7-28.2 dBi standard gain horn antenna was measured using THz CATR. Directivity, HPBW, and radiation power have been extracted from THz CATR measurements. The first two parameters are compared with values obtained from calculations, full-wave simulations, and reference measurements performed at a conventional far-field range. In contrast, the third parameter is compared with a reference measurement performed with an absolute power meter. The following performance was achieved employing super-resolution imaging. Frequency-dependent directivity and HPBW were determined within a root mean square (RMS) accuracy of 0.85 dB and 1.16 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , respectively, related to reference measurements performed at the conventional far-field range. The frequency-dependent radiation power was determined within an RMS accuracy of 0.7 μW related to reference measurements with the absolute power meter. Camera-related limitations concerning the THz CATR application are discussed in detail, such as field-of-view limitations, angular resolution limits, scan loss, pixel-to-pixel variation, signal-to-noise ratio limitations, and the interaction between TX and camera lens.

Fast Determination of Single-Cut Far-Field Pattern of Base Station Antenna at a Quasi-Far-Field Distance

In this article, we present a new approach for fast determination of the single-cut far-field (FF) pattern and antenna gain of a base station antenna with remarkably unequal aperture length and width (such as linear antenna array) at a quasi- FF (QFF) range, by taking advantage of the cylindrical wave expansion (CWE). The proposed method is free of probe compensation since the probe is stationary and the antenna under test (AUT) is in the QFF range, which makes the measurements equivalent to FF measurements. The measurement distance required by the proposed approach only needs to satisfy the FF condition and QFF condition for the relatively smaller dimension and larger dimension of the AUT, respectively. To validate the proposed method, we simulated four types of antenna array, that is, an array with a uniform distribution, a low sidelobe array, a phased array, and a shaped-beam array. It is found that the results are consistent with our theoretical analysis.

金属-电介质复合结构实现荧光远场增强

金属-电介质复合结构实现荧光远场增强

Accuracy of the moment-tensor inversion of far-field P waves

SUMMARY The far-field assumption is widely used and suitable for the moment-tensor inversion in which the source–receiver distance is quite long. However, the description of far field is uncertain and an explicit far-field range is missing. In this study, the explicit far-field range is determined and the errors of moment-tensor solutions produced by the far-field approximation are analysed. The distance, for which the far-field assumption is satisfied and the effect of the near-field term can be ignored, is directionally dependent. For the shear dislocation, in the directions near the nodal lines of the far-field P waves, the far-field distance is heavily dependent on the displacement component used to invert moment tensors. The radial component of displacement, which is parallel to the wave propagation direction, is recommended for the inversion and the corresponding far-field distance is quite short. In the directions far from the nodal lines, the selection of displacement components has little influence on the far-field distance. The maximum far-field distance appears in the directions of the pressure and tensile axes of the source and the value is about 30 wavelengths of radiated waves. Using more receivers (&gt;6) in the moment-tensor inversion can shorten the far-field distance. The effect of the near-field term on the moment-tensor inversion for tensile dislocations and isotropic sources (explosion or implosion) can be ignored. The conclusions obtained in this study are helpful for determining the positions of receivers and evaluating the accuracy of moment-tensor solutions, with far-field assumption being applied in the inversion.

Enhanced Far-Field Directional Luminescence Emission by a Hybrid Structure Consisting of Silver Channel and Microlens

In microfluidic detection, the distance between the fluorescent nanoparticles (FNPs) and the structure is usually in far-field range. In previous work, the enhancement of emission will decay exponentially with the increase of the distance between FNPs and the structures, which brings a challenge to the development of high-sensitivity biosensors. In this paper, a hybrid structure consisting of a silver channel and a microlens is proposed to enhance far-field directional emission when the FNP is in far-field range. It is shown that the far-field total power enhancement of our structure is obvious compared to the polymethyl methacrylate channel. This is due to scattering, constructive interference, and resonance modes created by the silver channel and microlens. This structure will contribute to high-sensitivity biosensors.

AMPLITUDE ATTENUATION LAWS OF ACOUSTIC EMISSION WAVES IN PLATE STRUCTURES

Although acoustic emission technology has been used in the nondestructive testing of various plate structures, its further application and development remain limited because the laws governing the amplitude attenuation of acoustic emission waves in plate structures remain unclear. This study aimed to reveal the amplitude attenuation laws of acoustic emission waves in plate structures. Thus, in this study, the traditional model of amplitude attenuation, which is in the form of an exponential function, was improved in reference to cylindrical waves, and an amplitude attenuation model in the form of a power function suitable for plate structures was proposed. Moreover, an experimental system for the attenuation of acoustic emission waves in aluminum alloy plates was established to experimentally verify the proposed model. Finally, the amplitude attenuation laws of acoustic emission waves within the full field, near field, and far field of plate structures were discussed. Results show that the amplitude attenuation laws of acoustic emission waves within the full field, near field, and far field of plate structures all conform to the power function model and gradually reduce as propagation distance increases. In addition, the improved amplitude attenuation model better corresponds with the actual amplitude attenuation trend within the near-field range of the acoustic emission source than other forms of amplitude attenuation models. Specifically, the traditional model of amplitude attenuation model agrees with the actual attenuation laws of acoustic emission waves only within the far-field range of the acoustic emission source. The results of this study provide valuable references for the profound analysis of the propagation characteristics of acoustic emission waves in plate structures and will help promote the development and application of acoustic emission technology. Keywords: Acoustic emission waves, Amplitude, Attenuation, Laws, Plate

Quantifying TOLNet Ozone Lidar Accuracy during the 2014 DISCOVER-AQ and FRAPPÉ Campaigns.

The Tropospheric Ozone Lidar Network (TOLNet) is a unique network of lidar systems that measure high-resolution atmospheric profiles of ozone. The accurate characterization of these lidars is necessary to determine the uniformity of cross-instrument calibration. From July to August 2014, three lidars, the TROPospheric OZone (TROPOZ) lidar, the Tunable Optical Profiler for Aerosol and oZone (TOPAZ) lidar, and the Langley Mobile Ozone Lidar (LMOL), of TOLNet participated in the "Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality" (DISCOVER-AQ)mission and the "Front Range Air Pollution and Photochemistry Éxperiment" (FRAPPÉ)to measure ozone variations from the boundary layer to the top of the troposphere. This study presents the analysis of the intercomparison between the TROPOZ, TOPAZ, and LMOL lidars, along with comparisons between the lidars and other in situ ozone instruments including ozonesondes and a P-3B airborne chemiluminescence sensor. In terms of the range-resolving capability, the TOLNet lidars measured vertical ozone structures with an accuracy generally better than ±15% within the troposphere. Larger differences occur at some individual altitudes in both the near-field and far-field range of the lidar systems, largely as expected. In terms of column average, the TOLNet lidars measured ozone with an accuracy better than ±5% for both the intercomparison between the lidars and between the lidars and other instruments. These results indicate very good measurement accuracy for these three TOLNet lidars, making them suitable for use in air quality, satellite validation, and ozone modeling efforts.

Monitoring the Absolute Calibration of a Polarimetric Weather Radar

AbstractThe absolute calibration of a dual-polarization radar of the German Weather Service is continuously monitored using the operational birdbath scan and collocated disdrometer measurements at the Hohenpeissenberg observatory. The goal is to measure the radar reflectivity constant Z better than ±1 dB. The assumption is that a disdrometer measurement close to the surface can be related to the radar measurement at the first far-field range bin. This is verified using a Micro Rain Radar (MRR). The MRR data fill the gap between the measurement near the surface and the far-field range bin at 650 m. Using data from the first half of the warm season in 2014, a bias in radar calibration of 1.8 dB is found. Data from only stratiform precipitation events are considered. After adjusting the radar calibration and using an independent data sample, very good agreement is found between the radar, the MRR, and the disdrometer with a bias in smaller than 1 dB. The bias in is not captured with the classic one-point calibration, which is performed twice a day using a built-in test signal generator. This is attributed to the fact that the characterization of the transmit and receive path is not accurate enough. Solar interferences during the operational scanning are used to characterize the receiver. There, the bias found is small, about 0.2 dB, so that bias based on the comparison of the radar with external sensors is attributed to the transmit path. The representativeness of the disdrometer measurements are assessed using two additional disdrometers located within 200-m distance.

Extraction of Acoustic Sources Through the Processing of Sound Field Maps in the Ray Space

Our goal is to develop a model-based approach to acoustic source extraction from microphone array data, which is suitable for both near-field and far-field sources. A signal representation based on plane-wave (PW) decomposition is suitable for acoustic sources in the far field as the resulting spectrum turns out to be impulsive. When the source approaches the array, however, the curvature of the wavefront causes the spectrum of the PW components to depart from impulsive behavior, thus making source extraction harder to attain. In this paper, we adopt a sound field representation based on the local estimation of the plenacoustic function along the array line. This approach consists of dividing the array into subarrays, and applying the PW analysis on individual subarrays. This has the immediate result of extending the range of validity of the far-field hypothesis, as a source that enters the near-field range of the extended array is still in the far-field range of the subarrays. PW analysis on subarrays allows us to construct the so-called sound field map in a domain of acoustic visibility called ray space. The extraction of the desired source is accomplished through spatial filtering of the sound field map. The design of the spatial filter relies on a linear minimum mean square error criterion defined on the sound field map. The effectiveness of the proposed methodology is proven through an extensive simulation campaign as well as real experiments.

A Dual-Field Sensing Scheme for a Guidance System for the Blind.

An electronic guidance system is very helpful in improving blind people’s perceptions in a local environment. In our previous work “Lin, Q.; Han, Y. A Context-Aware-Based Audio Guidance System for Blind People Using a Multimodal Profile Model. Sensors 2014, 14, 18670–18700”, a context-aware guidance system using a combination of a laser scanner and a camera was proposed. By using a near-field graphical model, the proposed system could interpret a near-field scene in very high resolution. In this paper, our work is extended by adding a far-field graphical model. The integration of the near-field and the far-field models constitutes a dual-field sensing scheme. In the near-field range, reliable inference of the ground and object status is obtained by fusing range data and image data using the near-field graphical model. In the far-field range, which only the camera can cover, the far-field graphical model is proposed to interpret far-field image data based on appearance and spatial prototypes built using the near-field interpreted data. The dual-field sensing scheme provides a solution for the guidance systems to optimise their scene interpretation capability using simple sensor configurations. Experiments under various local conditions were conducted to show the efficiency of the proposed scheme in improving blind people’s perceptions in urban environments.

Spatial remote luminescence enhancement by a half-cylindrical Au groove

Enhancing the luminescence emission from upconversion nanoparticles (UCNPs) is crucial to reduce the limits of biosensing based on UCNPs. Photonic crystal and surface plasmon structures have been used to enhance the emission of phosphor, while the enhancement distance is only limited to the near-field range. For in vitro biodetection, the distance between the UCNP and the substrate is usually in the far-field range. This work proposes the use of a half-cylindrical gold groove to enhance the emission of UCNPs even when the nanoparticles are at a far-field distance from the substrate. Due to scattering, constructive interference and coupling of optical modes in the Au groove, large-area/far-field range resonance modes can be created. Efficient resonance coupling between the optical mode and the excitation/emission of UCNPs can occur in the far-field range, which is able to enhance the absorption, quantum yield, extraction and collection of emission of UCNPs. The enhancement region is not only limited to the near-field range of the structure but also expands to the far-field range of the structure. By comparing different shapes and materials of the grooves, the proposed structure can improve the emission by 8.25 times when the UCNP is 4.4 μm away from the bottom of the groove compared with the bare glass. Besides, our structure is experimentally feasible and will greatly contribute to the development of high sensitivity and high SNR fluorescence biosensors.

Changes in antioxidant capacity of blood due to mutual action of electromagnetic field (1800 MHz) and opioid drug (tramadol) in animal model of persistent inflammatory state

BackgroundThe biological effects and health implications of electromagnetic field (EMF) associated with cellular mobile telephones and related wireless systems and devices have become a focus of international scientific interest and world-wide public concern. It has also been proved that EMF influences the production of reactive oxygen species (ROS) in different tissues. MethodsExperiments were performed in healthy rats and in rats with persistent inflammatory state induced by Complete Freund's Adjuvant (CFA) injection, which was given 24h before EMF exposure and drug application. Rats were injected with CFA or the same volume of paraffin oil into the plantar surface of the left hind paw. Animals were exposed to the far-field range of an antenna at 1800MHz with the additional modulation which was identical to that generated by mobile phone GSM 1800. Rats were given 15min exposure, or were sham-exposed with no voltage applied to the field generator in control groups. Immediately before EMF exposure, rats were injected intraperitoneally with tramadol in the 20mg/kg dose or vehicle in the 1ml/kg volume. ResultsOur study revealed that single EMF exposure in 1800MHz frequency significantly reduced antioxidant capacity both in healthy animals and those with paw inflammation. A certain synergic mode of action between applied electromagnetic fields and administered tramadol in rats treated with CFA was observed. ConclusionsThe aim of the study was to examine the possible, parallel/combined effects of electromagnetic radiation, artificially induced inflammation and a centrally-acting synthetic opioid analgesic drug, tramadol, (used in the treatment of severe pain) on the antioxidant capacity of blood of rats. The antioxidant capacity of blood of healthy rats was higher than that of rats which received only tramadol and were exposed to electromagnetic fields.

Performance improvement of super-resolving pupil filters via combination with nonlinear saturable absorption films

With the continuous development of the field of information technology, there has been a demand for recording mark size of optical data storage, optical imaging resolving power, and characteristic linewidth of photolithography to reach nanoscale. However, it is very difficult to realize the goal due to the optical diffraction limit restrictions. Much interest has focused on the study of optical far-field super-resolution spot by using pupil filters. However, common concerns have continued to plague super-resolving pupil filters based on either scalar diffraction theory or vector diffraction theory. These concerns include the fact that the side lobe becomes non-negligible when the central lobe is squeezed to a certain extent. Moreover, it is difficult to reduce the super-resolving spot to nanoscale. In this work, we proposed a novel method to combine the super-resolving pupil filters with nonlinear saturable absorption thin films to reduce the central spot size to nanoscale, lower the intensity ratio of side lobe to central lobe, and elongate the depth of focus or tunable tolerance distance between the super-resolving spot and sample. The simulated results indicate that by using the three-zone annular binary phase filter as the super-resolving pupil filter and Sb2Te3 as the nonlinear saturable absorption thin films, the central spot size can be reduced to nanoscale, the side lobe intensity is squeezed to about 10% of the central lobe intensity, and the tunable tolerance distance between the super-resolving spot and the sample is about two times that of the depth of focus of the diffraction limited spot at the incident laser wavelength of 405nm and the numerical aperture of focusing lens of 0.95. The combination of the super-resolving pupil filters with the nonlinear saturable absorption thin films is very useful for nano-optical data storage, maskless nanolithography, and nano-optical imaging. It is also easy to use in actual applications because of the operation in the far-field range.