Direct Far-field Measurements Research Articles

Convolutional Neural Network Combined With Stochastic Parallel Gradient Descent to Decompose Fiber Modes Based on Far-Field Measurements

Modal decomposition (MD) of fiber modes based on direct far-field measurement combining the convolutional neural network (CNN) with a stochastic parallel gradient descent (SPGD) algorithm is investigated both numerically and experimentally. For obtaining the modal coefficients of fiber modes guided in a large-mode-area fiber, the fiber modes are decomposed into a finite number of Hermite gaussian modes, the initial conditions of the modal coefficients are obtained through the CNN, and further optimization of them are carried out through the SPGD. The ambiguity problem that may happen in the CNN owing to the existence of the pair-beam field is resolved by properly labelling the phase differences with a single-valued parameter set in consideration of the mode-order indices. The feasibility and effectiveness of the proposed MD method is verified both numerical simulations and experimental demonstrations with both recorded image data and online real-time image data. The correlation error incurred by the proposed method is below 6.6×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and 8.7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in the numerical simulations and the experimental demonstrations, respectively. The online real-time operation of the proposed method is also experimentally demonstrated at a decomposing rate of ∼2 Hz.

Integrated-Antenna Over-the-Air Testing for Millimeter-Wave Applications: An Overview of Systems and Uncertainty [Measurements Corner

Recent wireless communication systems, including the 5G communication New Radio (NR) operating in the millimeter-wave (mm-wave) bands, demand performance measurements using an over-the-air (OTA) test instead of using the traditional conducted methods. This article provides an overview of various OTA measurement systems that measure the integrated antenna inside a 5G-and-beyond mm-wave device, including the direct far-field (DFF), indirect FF (IFF), near-field (NF), and midfield measurements. By considering several significant parameters, including minimum measurement distance required, metrics that can be measured, and complexity of the electrical and mechanical components of the system, suitable OTA measurement methods for integrated antennas can be selected. Furthermore, factors contributing to the measurement uncertainty of OTA systems based on the technical report (TR) of the 3rd Generation Partnership Project (3GPP) are summarized and discussed. By analyzing the significant parameters of each OTA system and contributors of the measurement uncertainty, the reader can determine the OTA method that is most suitable to measure the integrated antennas for mm-wave communications.

Photonics-Based Near-Field Measurement and Far-Field Characterization for 300-GHz Band Antenna Testing

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.

Numerical Green’s Function-Based Method for Modeling Radiated Emission From PCBs in Shielding Enclosures

Due to stringent electromagnetic compatibility (EMC) regulations, all electronic devices need to go through rigorous radiated emission tests before launching to market. Thus, efficient, accurate, and cheap characterization approaches are quite of interest. Instead of utilizing the time-consuming and expensive direct far-field measurement, in this work, a novel methodology on the basis of numerical Green’s function (NGF) is proposed to acquire the parasitic EM emissions from PCBs placed in a shielding enclosure with ventilation slots. Referring to Schelkunoff’s equivalence principle, the whole modeling procedure can be decomposed into two steps in order to obtain the emissions in the entire space. For the first step, the tangential electric fields over the surface of ventilation slots are sampled first; then, the NGF over a spherical surface tightly enclosing the perfectly electric conductor (PEC) filled shielding box is numerically evaluated; finally, the tangential component of the magnetic field over the spherical surface enclosing the shielding box is calculated in terms of the NGF and the sampled near electric field. As for the second step, by filling the sphere with a perfectly magnetic conductor (PMC) first, the pertinent Green’s function is further expanded by the spherical modal functions; later, the radiated emission outside this sphere can be analytically evaluated in terms of the previously obtained tangential magnetic field and the modal function expanded Green’s function. Therefore, the novelty of the proposed method has two folds: 1) only the tangential electric field over the ventilation slots is required, thus saving a lot of time in near-field measurement and 2) the NGF is independent of the PCBs in the shielding enclosure and, thus, only needs to be calculated once even though the PCBs inside the shielding enclosure is changed. To validate and verify the effectiveness of this proposed modality, several representative examples are presented.

Modal decomposition of fiber modes based on direct far-field measurements at two different distances with a multi-variable optimization algorithm.

We present a novel method for modal decomposition of a composite beam guided by a large-mode-area fiber by means of direct far-field pattern measurements with a multi-variable optimization algorithm. For reconstructing far-field patterns, we use finite-number bases of Hermite Gaussian modes that can be converted from all the guided modes in the given fiber and exploit a stochastic parallel gradient descent (SPGD)-based multi-variable optimization algorithm equipped with the D4σ technique in order for completing the modal decomposition with compensating the centroid mismatch between the measured and reconstructed beams. We measure the beam intensity profiles at two different distances, which justifies the uniqueness of the solution obtained by the SPGD algorithm. We verify the feasibility and effectiveness of the proposed method both numerically and experimentally. We have found that the fractional error tolerance in terms of the beam intensity overlap could be maintained below 1 × 10-7 and 3.5 × 10-3 in the numerical and experimental demonstrations, respectively. As the modal decomposition is made uniquely and reliably, such a level of the error tolerance could be maintained even for a beam intensity profile measured at a farther distance.

A DGTD Scheme for Modeling the Radiated Emission From DUTs in Shielding Enclosures Using Near Electric Field Only

To meet the electromagnetic interference regulation, the radiated emission from device under test such as electronic devices must be carefully manipulated and accurately characterized. Instead of resorting to the direct far-field measurement, in this paper, a novel approach is proposed to model the radiated emission from electronic devices placed in shielding enclosures by using the near electric field only. Based on the Schelkkunoff's equivalence principle and Raleigh–Carson reciprocity theorem, only the tangential components of the electric field over the ventilation slots and apertures of the shielding enclosure are sufficient to obtain the radiated emissions outside the shielding box if the inside of the shielding enclosure was filled with perfectly electric conductor (PEC). In order to efficiently model wideband emission, the time-domain sampling scheme is employed. Due to the lack of analytical Green's function for arbitrary PEC boxes, the radiated emission must be obtained via the full-wave numerical methods by considering the total radiated emission as the superposition between the direct radiation from the equivalent magnetic currents in free space and the scattered field generated by the PEC shielding box. In this study, the state-of-the-art discontinuous Galerkin time-domain (DGTD) method is utilized, which has the flexibility to model irregular geometries, keep high-order accuracy, and more importantly involves only local operations. For open-region problems, a hybridized DGTD and time-domain boundary integration method applied to rigorously truncate the computational domain. To validate the proposed approach, several representative examples are presented and compared with both analytical and numerical results.

Millimeter-Wave Near-Field Measurements Using Coordinated Robotics

The National Institute of Standards and Technology (NIST) recently developed a new robotic scanning system for performing near-field measurements at millimeter-wave (mm-wave) frequencies above 100 GHz, the configurable robotic millimeter-wave antenna (CROMMA) facility. This cost-effective system is designed for high-frequency applications, is capable of scanning in multiple configurations, and is able to track measurement geometry at every point in a scan. The CROMMA combines real-time six-degree-of-freedom optical spatial metrology and robotic motion to achieve antenna positioning to within $25\;\upmu\text{m}$ rms. A unified coordinated metrology approach is used to track all positional aspects of scanning. A vector network analyzer is used to capture amplitude and phase. We present spherical near-field measurements of the forward hemisphere of a 24-dBi standard gain horn at 183 GHz. Using the configurable scanning ability, two different scanning radii were used. Near-field data were taken at a 100-mm radius. Direct far-field measurements were also taken at 1000-mm radius. The E- and H-plane patterns are determined from the measurements and compared to theoretical patterns. We describe the system components of the CROMMA and the coordinated metrology approach used. An analysis of the positional repeatability and accuracy achievable is also presented.

Far-field measurements of short-wavelength surface plasmons

We present direct far-field measurements of short-wavelength surface plasmon polaritons (SPP) by conventional optics means. Plasmonic wavelength as short as 231 nm was observed for 532 nm illumination on a Ag−Si3N4 platform, demonstrating the capability to characterize SPPs well below the optical diffraction limit. This is done by scaling a sub-wavelength interferometric pattern to a far-field resolvable periodicity. These subwavelength patterns are obtained by coupling light into counter-propagating SPP waves to create a standing-wave pattern of half the SPP wavelength periodicity. Such patterns are mapped by a scattering slit, tilted at an angle so as to increase the periodicity of the intensity pattern along it to more than the free-space wavelength, making it resolvable by diffraction limited optics. The simplicity of the method as well as its large dynamic range of measurable wavelengths make it an optimal technique to characterize the properties of plasmonic devices and high-index dielectric waveguides, to improve their design accuracy and enhance their functionality.

A quasi-optical vector near-field measurement system at terahertz band.

This paper describes a vector near-field measurement system at terahertz band based on a high sensitivity superconducting receiver equipped with a quasi-optical probe for high resolution near-field sensing. A novel single-receiver rather than commonly used dual-receiver configuration is adopted for vector measurement. Performances of the measurement system including stability and dynamic range are studied. Vector near-field measurement of a diagonal feedhorn at 850 GHz is presented and shows good agreement with simulation and direct far-field measurement.

Accurate target-plane focal-spot characterization in high-energy laser systems using phase retrieval

Target-plane intensities on the short-pulse beamlines of OMEGA EP, a petawatt-class laser, are characterized on-shot using the focal-spot diagnostic (FSD), an indirect wavefront-based measurement. Phase-retrieval methods are employed using on-shot and offline camera-based far-field measurements to improve the wavefront measurements and yield more-accurate, repeatable focal-spot predictions. Incorporation of these techniques has improved the mean cross-correlation between the FSD predictions and direct far-field fluence measurements in the target chamber from 0.78 to 0.94.

Coordinate-transformation-based ultra-directive emission

The modelling, practical implementation and characterisation of an ultra-directive antenna around 10 GHz are presented. The design of the antenna is based on the transformation optics concept by transforming a radiating cylindrical space into a rectangular one. Metamaterials presenting electric and magnetic resonances are used to achieve the transformation. Field intensity mappings and direct far-field measurements are performed to experimentally demonstrate the narrow beam profile.

Antenna Measurement on Cylindrical Surface in Fresnel Region Using Direct Far-Field Measurement System

The small anechoic chambers built by many small-/medium-sized companies and universities present difficulties in testing electrically large antennas because the chamber size cannot satisfy the far-field criterion of large antennas. In this paper, a method for Fresnel-region measurement on a cylindrical surface with variation of the measurement height is proposed and verified by both calculations and experiments. We implement the proposed method using a direct far-field measurement system by adding a few supporting structures. The results show good accuracy.

Asymptotic expansion for spherical mode phase with application to near-field measurements

The exact-circuit representation of spherical modes allows the derivation of a convenient asymptotic relationship for the phase difference between a plane wave and a spherical wave as a function of the radial position and spherical mode index. One application of this asymptotic form is to allow the effects of range uncertainties on patterns measured in a near-field antenna range to be related simply to the more familiar finite range effects in a direct far-field measurement.

Nearfield calibration array utilizing piezoelectric polymer

The Trott nearfield calibration array (NFCA) concept allows the farfield acoustic properties of underwater transducers to be determined in measurement facilities that are too small for direct farfield measurements. As a projector, the NFCA produces a nearly uniform plane wave throughout a large adjacent volume. The farfield sensitivity of a receiver can then be measured directly by placing it in the “plane‐wave” volume. As a receiver, the NFCA measures only a single plane‐wave component of the sound radiated by a projector located in the “plane‐wave” volume. Existing NFCA's are primarily used as receivers and utilize small piezoceramic hydrophone elements arranged in planar or cylindrical configurations. The spacing between elements limits the upper frequency of operation of the NFCA; e.g., a 1‐in. spacing allows operation up to about 50 kHz. This paper describes the design and construction of a planar NFCA utilizing a mosaic of nonelectroded sheets of the piezoelectric polymer PVDF attached to full area electrodes that subdivide the array into independently controlled annular elements. The PVDF NFCA was tested successfully as both a projector and a receiver by calibration of a high‐frequency transducer. Although the available electronics limited the upper frequency of the test to 150 kHz, the PVDF NFCA should be useable, with suitable electronics, up to megahertz frequencies. [Work partially supported by ONR Code 220.]

A 5- to 50-kHz synthetic cylindrical nearfield calibration array

The direct measurement of farfield acoustic radiation from underwater transducers in a shore-based facility is often impossible because of the limited size of measurement tanks. The Trott nearfield calibration array (NFCA) concept solves this problem by allowing the farfield radiation to be determined from measurements made close to the transducer. We describe here the development of a 5- to 50-kHz synthetic cylindrical NFCA that is suitable for measurement tanks as small as about 2.5 m in each dimension and that can be operated at hydrostatic pressures up to 6.8 MPa. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54-cm apart center-to-center and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. We then obtain the azimuthal farfield pattern of the transducer by computer processing the angular responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer in a cylindrical measurement tank less than 2.5 m in diameter. The resulting computed farfield patterns are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake.

5–50 kHz cylindrical nearfield calibration array

In acoustic measurement tanks it is often impossible to directly determine the farfield radiation properties of underwater transducers because of limited space. The Trott nearfield calibration array (NFCA) concept solves the problem by allowing the farfield properties to be obtained by measurements made close to the transducer. We describe here the development of a 5–50 kHz synthetic cylindrical NFCA that is suitable for medium size measurement tanks such as the pressure tank at the Underwater Sound Reference Detachment of the Naval Research Laboratory. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54 cm apart and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. The azimuthal farfield pattern of the transducer is obtained by computer processing the responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer. The resulting computed farfield patterus are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake. [Work supported by NAVSEA 63R.]

Recent experimental results in near-field antenna measurements

Recent experimental results on determination of antenna pattern and power gain from near-field measurements are described. Two new antenna measurement theorems were applied. Measurements were made on an electrically large horn lens, a standard-gain horn and a nominal duplicate of the measuring antenna. Some comparisons with direct far-field measurement results were made.