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Near-field (NF) measurements is considered as a very accurate and versatile antenna testing technique. It became widely used as a preferred measurement technology in antenna measurement systems about four decades ago. Today, hundreds of near-field antenna test facilities are installed worldwide. The IEEE Std 1720™ “Recommended Practice for Near Field Antenna Measurements” is specifically dedicated to near-field antenna measurements. It therefore complements the IEEE Std 149-1979™ “Standard Test Procedures for Antennas” which describes general antenna measurement procedures. The Std 1720™ was originally approved in 2012 as a completely new standard by the IEEE Standards Association Standards Board. It is highly relevant for users performing NF antenna measurements but also the design and evaluation of NF antenna measurement facilities. After 10 successful years, the standard expires this year and will no longer be an active standard under the IEEE. A revision is required to update the document with new developments and technologies that have matured since the first edition. This is the scope of project P1720 that was approved by IEEE-SA in 2019 to undertake “minor revision” of the current standard. A Working Group (WG) of the Antennas and Propagation Society Standards Committee (APS/SC) has been formed for this task. The WG is transversal, with users and experts of the near field measurement field and consist of approximately fifty dedicated volunteers from industry, academia, and government. This paper gives an update on the running activities and discusses the suggested changes to the standard.
With the growing applications of wireless systems in different aspects of everyday life, from the consumer-electronic devices to internet-of-thing applications to wireless health-monitoring systems, there is an expanding need for reliable measurement of the radiating performance of these devices. The electromagnetic compatibility (EMC) tests, including immunity and interference tests, are normally done in shielded rooms the walls of which are covered by the so-called hybrid absorbers. These absorbers are made of a magnetic lossy layer giving absorption at low frequencies and a dielectric lossy geometrical absorber which is mainly responsible for the electromagnetic absorption at higher frequency range. The heart of hybrid-absorber design, which can be reduced to a wide-band matching problem, is to match the dielectric lossy part to the magnetic lossy layer. The magnetic layer which is mostly made up of a ferrite material, is a relatively thin layer, i. e. less than 1 cm thick, which is supposedly homogeneous. The dielectric geometrical absorber part has a relatively large thickness, e.g. 30 inches, and incorporates geometries like a pyramid to make a tapering against the wave which is going to be absorbed by the absorber. Because of the relatively large thickness of the dielectric lossy part and the production-process techniques used to make these parts, there are inhomogeneities in this part. In the current paper, the impact of the inhomogeneity of the dielectric part on the matching performance of the hybrid absorber is investigated in details. In the 1st stage, a 30-ich absorber is chosen and sliced in 15 different layers the permittivity is of each measured separately. Using these permittivity values, a complex model is made and simulated to get an expected reflectivity value on the ferrite layer. In the 3rd step, the absorbers of the same production batch are chosen to be measure in real-scale measurement setup and the reflectivity values are measured. Finally, the measurement and simulation results are compared and the impact of inhomogeneity of the dielectric absorber on the hybrid-absorber performance in concluded.
For millimeter-wave wireless devices used in the close proximity to the human head or body, the compliance evaluation to regulatory exposure limits is determined from near-field free-space power density measurements. For mobile phones and other portable equipment, the standard assessment technique involves the characterization of the power density on planes, as close as 2 mm from each facet of the device under test (DUT), as well as on anthropomorphic surfaces. These measurements are typically realized by means of a pseudo-vector diode-detected probe, acquiring the electric field magnitude and polarization ellipse at multiple locations over the scanning area. Phaseless techniques have been employed to deduce the required phase and magnetic field information for the calculation the Poynting vector. Although accurate, this technique presents some limitations: prohibitive test times; the inability to distinguish between various frequency contributions of the electric field due to the detection process; or the necessity to implement specific test modes in the DUT to fix the radiated beam in a given state. A recent paper proposed an alternative method which overcomes these listed limitations. The approach relies on the use of spherical antenna / over-the-air (OTA) phasor electric field acquisitions in an anechoic chamber environment, performed in the radiative field region and combined with near-field processing through equivalent currents reconstruction. This paper proposes an extensive validation of this method based on simulations and measurements of reference antennas, as defined in the IEC 63195. The reference measurements are realized with a standard-compliant 6-axis robot assessment system. The uncertainty contribution coming from probing at distances where reactive field components cannot be captured is investigated, demonstrating a negligible influence on reconstructed field distributions down to a third of the wavelength from the reference antenna, for which a theoretical interpretation is provided. It is also shown that characterizing the detailed changes in the reactive field is not necessary to obtain an accurate peak spatial-average power density value, which is the relevant metric for compliance assessment. A systematic analysis of the error affecting this specific quantity is also provided.
The characterization of antennas is a time-consuming task. Its acceleration leads often to large and sensitive numerical problems. Therefore, special care must be taken of the choice of the parameters, the optimization, and the stability of the employed resolution methods. Based on Huygens’ principle, the radiation operator can be defined from an equivalent surface enclosing the Antenna Under Test (AUT). The discretization of this operator leads to the so-called radiation matrix. An expansion basis of the fields radiated from the equivalent to the measurement surface is constructed by the Singular Value Decomposition (SVD) of that matrix. The Reduced-Order Model (ROM) is the compressed representation of this basis obtained by truncating the SVD. The truncation order, T, is computed by inspection of the singular value distribution and is strongly linked to the number of degrees of freedom of the radiated fields. Several practical and technical aspects are studied in this article to provide a systematic, efficient and reliable procedure for the characterization of the radiated fields using the ROM. Analytical criteria are used to define the dimensions of the radiation matrix enabling a stable determination of the compressed basis. The truncation order, T, is the key-point of this method as it determines the size of this basis. Therefore, its variation is studied with respect to the discretization step and the geometry of both equivalent and measurement surface. Finally, the Randomized SVD (RSVD) is used in order to significantly reduce the computation time with negligible impact on the accuracy. To illustrate our procedure, it is applied to various scenarios and experimental results of spherical measurements. Estimations of the time savings by using the RSVD are also provided.
With the growing of vehicular communication technologies, the need for performing radiated measurements accounting for the full vehicle is becoming increasingly important. In modern cars, antennas are an integral part of the vehicle which is too complex to be represented by a simple ground plane during the tests. 5GAA test report [1] unifies measurement procedures for vehicle mounted antennas for both passive (at the antenna level) and Over-the-Air (OTA - at the modem level) measurements. Both Far-Field (FF) and Near-Field (NF) measurement systems are considered for testing of the full vehicle. In NF systems the radiated signals are measured on a closed surface at reduced distance, and a NF-to-FF transformation is applied. Since the transformation requires phase coherence between the transmitter and the receiver, NF systems are suited for passive tests. On the other hand, FF ranges are more suited for OTA tests, but requires large measurements distances, and hence expensive testing environments. OTA testing at reduced distances offers several advantage including the possibility to consider smaller and cost-effective anechoic chambers and the reduction of the system path losses (improved dynamic range). Moreover, the use of multi-probe systems dramatically reduces the testing time. The possibility of performing automotive OTA tests in spherical multiprobe systems at a reduced distance will be investigated in this paper. Simulations of a realistic vehicle with antennas installed in different locations will be considered to assess the uncertainty introduced by the reduced measurement distance. Figure of merits like different partial radiated powers [1] will be considered. Experimental automotive OTA measurements of a monopole-like antenna, installed on the roof of a vehicle will also be presented. These measurements have been performed at LTE bands in a spherical multiprobe system with a 4m radius. The measurements will be analysed comparing direct OTA measurement with a two-stage method, where passive antenna measurements and a few sampled OTA measurement points are combined. The outcome of the simulated analysis and experimental tests will be used to preliminary assess the uncertainty of automotive OTA measurements at reduced distance considering metrics relevant to automotive technologies [1].
NASA mission requirements have driven an increased interest in active phased arrays antennas for space-based user communication terminals. Recent advancements in 5G technology have driven down the cost of phased array development and manufacturing all while providing a technology solution that covers many existing Ka-Band satellite communication spectrum bands. While these developments have provided ample opportunities to leverage new chips and arrays for use in space, there is also a need to evaluate these antennas in a relevant environment. Active arrays, as designed for use in 5G, require thermal management to avoid damage to the array as well as to maintain performance. Measured performance under various thermal conditions is essential both for understanding array performance and ensuring operation is within required tolerances. To address these measurement needs, the SmallSat Ka-band Operations User Terminal (SKOUT) at the NASA Glenn Research Center (GRC) developed a test environment that combines traditional antenna and communication system metrology with a conduction cooling/heating thermal control system approximating the space thermal environment. This paper will address the metrology system design and performance specifications as well as test article setup and operation. Tests that are typically performed at a single operating temperature can be performed over the typical temperature range of a Low Earth Orbit (LEO) mission. These tests include error vector magnitude (EVM), gain to noise temperature (G/T), antenna patterns, and non-linear characterization (e.g. P_in/P_out, intermodulation distortion products). The paper will cover the specific configuration for each test and provide results from recent test campaigns. The results illustrate the importance of higher fidelity environmental testing when evaluating the performance of active antennas.
Using a Higher-Order Basis Function based Method of Moments Analysis for Designing Compact Antenna Test Ranges Abstract:- Full wave electromagnetic simulation of a Compact Antenna Test Range (CATR) is not trivial given its electrical size. Typically, the reflector geometry is simulated using asymptotic methods using an assumed feed pattern, while RF absorber and its effects are ignored. A boundary element method of moments (MoM) implementation, using higher-order basis functions is a good numerical technique for analyzing these ranges since the equations are only solved at the interfaces between different homogeneous regions. There is therefore no need to discretize and solve equations for the fields in the large empty volume portion of the CATR, unlike when using Finite-Difference Time-Domain (FDTD) or Finite Element Methods (FEM). Using higher-order basis functions allows for the mesh size of the discretized CATR geometry to be as large as two wavelengths, reducing the number of unknowns while enabling fast, efficient solutions. In this paper, a commercial software package that uses MoM with Higher-Order Basis Functions is used to model a CATR that incorporates a blended rolled edge reflector. The results for the reflector and feed model are compared with asymptotic analysis results to show agreement. A realistic feed horn, support structure and RF absorber is then introduced to the model and its performance is also included to predict field distribution in the CATR test zone. Using this field solution the Poynting vector is calculated to visualize the flow of energy in the range and from these results proper RF absorber layout can be designed to ensure optimum test zone performance. It is also shown how feed structure absorber treatment impacts CATR test zone performance.
Wide-angle beam steerable antennas are critical devices for 5G and next-generation Internet of Things (IoT). In general, beam steerable antennas are realized electronically by controlling the phase of an array of elements, or mechanically through moving parts. While electronic approaches typically offer fast beam switching and low system profile, the use of a substantial number of active components can considerably impact efficiency in the millimeter-wave range and increase cost. Meanwhile, novel mechanical steering concepts such as the Risley prism antenna (RPA) have become attractive because of their low electronic complexity, low cost, and better efficiency. An RPA typically contains three co-axially placed panels, including a stationary feed source generating planar illumination and two independently rotatable beam-deflective surfaces. Beam scan is realized by in-plane rotations of the components. Therefore, RPA avoids any distributed active components, and the profile of the antenna stays unchanged while scanning. These features make RPA a competitive candidate in scenarios that have moderate requirements on steering speed, e.g., for satellite-tracking ground terminals. In this work, we propose a novel RPA configuration that realizes wide-angle 2D beam steering with merely two flat panels coaxially placed in parallel, including a rotatable feed and a rotatable transmitarray. The feed radiates a pre-defined gradient-phase and the transmitarray provides another gradient phase shift. The combination of the two gradients becomes a new gradient at the exiting aperture. In-plane rotations of the feed and the transmitarray changes the value and direction of the aperture phase gradient, and eventually scans the beam. This configuration uses fewer components than a conventional RPA, and can significantly reduce system complexity, weight, profile, and loss. Based on this concept, we present the principle, design, and verification of a K-band circularly polarized (CP) RPA at 19 GHz. A sequentially rotated truncated-corner patch array feed generates the first CP phase gradient. A CP transmitarray using S-ring element unit cells is placed on top of the feed to provide the second gradient phase shift. The total thickness of the RPA is less than a wavelength at 19 GHz. Simulated and measured results showing beam scans beyond 50°in elevation will be presented.
Recent papers have addressed making far field measurements at much less than the traditional far field distance, particularly for 5G MIMO test articles. These papers have focused on main beam measurements only, such as Total Radiated Power (TRP) and have stated that other normal antenna pattern metrics, such side lobe level measurements are not appropriate for this shortened distance. These papers have addressed fixed error levels acceptable for this quasi far field technique. This paper will present a sliding scale of main beam error versus measurement distance that can provide a more precise evaluation for the practitioner on selecting this technique. In addition, the paper will present a trade study in terms of chamber size, measurement durations and measurement methods between the quasi far field, compact range, and spherical near-field approaches. This trade study will cover three representative test articles in the C, Ka, and V frequency bands for 5G applications. A case study for a particular test article will provide an evaluation example.
Novel metamaterial and metasurface realizations provide unique control of the wave dispersion but present many challenges for accurate constitutive parameter extraction. Practical material measurement approaches for characterizing complex materials, such as biaxial anisotropic and gyrotropic media, rely on either waveguide or focus beam techniques with trade-offs in sample size and bandwidth. Multi-static focus beam setups offer many advantages for complex material measurements by enabling wider sampling bandwidths, measurement degrees of freedom and larger sample sizes. However, rotational misalignment of the principal crystal and measurement coordinates of the biaxial anisotropic media sample results in cross-polarized scattered field contributions. Likewise, interrogating gyrotropic or general bianisotropic media with a dual-polarized focus beam measurement setup produces cross-polarized scattering matrix components. These non-diagonalized S-parameters for the complex sample under test must be measured to successfully extract the constitutive parameters. A time-domain gated response isolation calibration scheme is one common approach to establish the measurement reference plane and minimize fixture uncertainties for samples with limited cross-polarization scattering. This paper extends the time-domain gated response isolation methodology for free-space focus beam systems to account for cross-polarization terms present in both the sample under test as well as the measurement setup. The featured analysis leverages a 4x4 S-parameter matrix notation to capture the polarimetric scattering at each cascaded stage. An equivalent line standard procedure is developed where four unique, linearly independent calibration standard measurements are shown to account for all unknown terms. Finally, a sensitivity analysis of the calibration standards is performed via numerical simulations to show the potential trade-off and limitations of the cross-polarization extended time-domain response isolation calibration scheme performance.
Holographic back projection to a plane from spherical pattern data offers more details than from planar data because the image is derived from data over an entire hemisphere. A previous study introduced a back projection method where the far field pattern can be projected to a plane which is orthogonal to the radial direction, with thetwo transverse axes parallel to the local θ and ∅ vectors. This limits the hologram to only certain specific planes which may not match the desired surfaces. To overcome the shortcomings, we apply a more general back projection method in this paper which allows projection to an arbitrary plane. This is achieved through a translation and a series of rotation operations of the far field pattern. Specifically, as a first step, the phase center of the far field pattern is moved to coincide with the center of the projection plane through a translation matrix. Next the far field pattern is rotated such that its x, y and z directions are aligned with the desired projection area. Then the plane wave spectrum is computed based onthe new far field pattern. The hologram can be obtained by applying inverse Fourier transform to the plane wave spectrum. The phase shiftterm exp(jγd) in the conventional back projection technique is no longer needed after the pre-process of the far field pattern because it is included in the translation operation. This is a much more general back projection algorithm which provides the holographic projection onto an arbitrary plane. The method can be especially useful for cases when the desired projection areahas an arbitrary orientation with an offset from the origin.
Modern design of phased array apertures typically begins with unit-cell design relying on an extensive use of full-wave simulations. The waveguide simulator is an equivalent, experimental simulator that allows for measurement of the active impedance of radiating elements at a prescribed pair {frequency, scan angle} in an infinite array environment. Therefore, the waveguide simulator offers a means of real-world verification of a unit-cell design before proceeding with finite array design or realization. In its simplest form, the waveguide simulator is constructed through placing an element within the walls of a rectangular waveguide. The natural imaging of the waveguide walls acts to emulates an infinite array environment. The multielement waveguide simulator described first by Gustincic (J. J. Gustincic, IEEE Trans. Antennas Propagat., vol. 20, no. 5, pp. 589–595, 1972), allows for experimental determination of the active reflection coefficient of an embedded array element for a discrete set of scan conditions corresponding to a given waveguide mode excitation. Though the theory behind the waveguide simulator is well documented in the literature, there is scant discussion of the effect of configuration on the number of and distribution of valid scan pairs. The number of valid scan pairs is proportional to the number of modes that are excited within a waveguide, which is set by the element spacing and size of the waveguide. It is desirable, for a given number of elements constituting a waveguide simulator, to maximize the number of valid scan pairs and simultaneously the information content that can be obtained from a single experimental setup. The number of valid modes for a square waveguide with half-wavelength element spacing is found to reduce to the well known Gauss circle problem. For the general case in which the element spacings are arbitrary and the waveguide is rectangular the problem is reduced to Hardy’s generalization of the Gauss circle problem (G. H. Hardy, Proceedings of the Royal Society of London. vol. 107, (744), pp. 623-635, 1925). The herto unrecognized connection to an existing and widely developed mathematical theory gives insight into the fundamental sampling limitations of the waveguide simulator.
Diagnostic and verification testing of Low Observable (LO) platforms and components requires an Ultra-Wideband (UWB) Inverse Synthetic Aperture Radar (ISAR) imaging capability. A Compact Range (CR) is a test instrument that, when fitted with an instrumentation radar and target positioner, can efficiently produce ISAR images and other Radar Cross Section (RCS) data products required for LO research, design and production programs. Key limiting factors for the instantaneous radar imaging bandwidth of a CR is the feed antenna, where the criteria of a good feed is frequency bandwidth and illumination pattern shape. Maintaining a relatively constant reflector illumination characteristic typically requires several feeds with constant patterns functioning over smaller operating bandwidths, to be mechanically sequenced in the measurements. These feed limitations increase operational costs and complexity for LO measurements, driving a need for improved illumination sources providing constant reflector illumination for UWB collections. Focal Plane Arrays (FPAs) can be utilized to resolve these issues while increasing instantaneous bandwidth and measurement quality while reducing operational costs. This paper presents a procedure for defining complex weights of an FPA aperture to optimize radiation pattern matching to the reflector. A simulated plane wave arriving from the CR quiet-zone impinges on a model of the reflector. The FPA is placed in a region near the focal point and contained within the beam waist envelope, and the FPA weights are computed using a Computed Electromagnetic (CEM) techniques. The computational complexity of CEM simulations of electrically large CRs are usually prohibitive, however this method exploits the large focal lengths of CRs to sparsely model reflectors, and produces a tractable solution even at millimeter wavelengths. Practical aspects of FPA designs are presented and discussed as applied to the large outdoor CR at the US Army, Electronic Proving Ground (EPG), Fort Huachuca, Arizona.
The Boeing 9-77 Compact Radar Range has utilized low-frequency solutions since the 1990s. However, compact radar ranges have innate challenges when it comes to low-frequency measurements, typically due to facility size limitations. Due to increasing demand for more reliable data across a broad set of frequencies, an upgrade to the existing Ultra High Frequency (UHF) antenna feeds was designed and implemented in July 2020. This antenna was developed with field quality improvements, reliability, repeatability, and maintainability in mind. Unlike the previous design, this antenna was designed as an array with weighted feeds to complement the characteristics of the pre-existing range Gregorian reflector system. This new UHF antenna array leveraged the Weighted Element Method (WEM) along with extensive electromagnetic modeling and trade studies to achieve an efficient design at a minimum size. As a result of these design choices, the new antenna has doubled the efficiency in the band of interest. In addition, the frequency bandwidth of the antenna was improved while also reducing calibration and background drift. Lastly, this array has significantly improved the field quality of the quiet zone compared to the previous antenna system and improved the signal-to-noise ratio. This paper describes the UHF Antenna Array design process and the compact range measurements results to demonstrate the benefit of the WEM for feed arrays in a compact range. Additionally, the authors present an evaluation of methods used to create a digital twin of the UHF Antenna Array and a summary of best practices for future development of weighted antenna arrays for compact radar ranges.
The imaging of the reflectivity of a target from Near-Field (NF) scattered data is nowadays well established. Generally speaking, these techniques require complex data, i.e., amplitude and phase acquisitions. In this paper, the use of only-amplitude acquisitions is investigated. We propose an approach to image the reflectivity profile of a target using only amplitude NF data under a monostatic measurement configuration. To cope with phaseless data, a phase retrieval problem is settled and dealt with as a quadratic inverse one. The phaseless procedure requires two sets of independent squared amplitude measurements of the scattered field, collected on two different scanning surfaces. A solution of the problem is reached by the search for the global minimum of an appropriate quartic functional. A proper representation of unknowns and data, exploiting the available information on the target and on the scanning geometry, allows to improve the reliability and the accuracy of the optimization process. First, a representation of the unknown target reflectivity using Prolate Spheroidal Wave Functions is used. Furthermore, properly acquiring the phaselesss NF requires a significantly high sampling rate when performed with a standard approach. From this point of view, a non-redundant sampling can be employed to drastically reduce the amount of requested NF data. Following the use of the non-redundant sampling, a two-dimensional optimal sampling interpolation expansion can be employed to accurately recover the NF scattered amplitude data on the classical Cartesian grid. Numerical results to assess the effectiveness of the proposed approach will be presented. The approach has proved to be capable, besides imaging the unknown reflectivity, to accurately reconstruct the amplitude and phase NF on a third NF test plane. In the shown example, a reduction of 95% NF amplitude-only samples is achieved.
Phased array antennas are often built from sub-arrays with identical or symmetrical layout. At an early project stage, performance verification measurements of the sub-array are valuable to proof the single module design. However, the characteristics of the final antenna are questionable without further processing. This work presents a concept that is based on far-field measurements of a sub-array in a Compact Antenna Test Range (CATR) in conjunction with planar near-field (PNF) processing to synthesize the entire phased array antenna characteristics. The procedure is explained with an example of a dual linear polarized L-band planar phased array antenna for an airborne synthetic aperture radar application. It is shown that the measured sub-array can be complemented by the synthesized twin to evaluate the characteristics of a final antenna that is not yet available in this form. The resulting performance of the synthesized entire phased array is presented and compared with simulations. The presented post-processing method would be beneficial to characterizing radiation patterns of large phased arrays by measuring only sub-arrays in a limited test-zone with any measurement principle.
The radiation and scattering pattern characteristics of open-ended rectangular waveguide with a chamfered tip are examined. Despite common and widespread use as a probe antenna for planar near-field antenna measurements, a methodical investigation of the chamfered-tip design and resultant performance has not been published. A computational electromagnetics (CEM) model for an open-ended rectangular waveguide probe with a parameterized chamfered tip has been constructed and results for both radiation and scattering patterns are presented. A comparison of results includes a probe without a chamfer and a probe typical of that available from commercial suppliers. It is shown that, for a series of standard waveguide size probes sharing a common thickness for the waveguide wall and chamfered tip, the radiation pattern is relatively insensitive to the chamfer tip designs studied until frequency increases into W-band (WR-10). The scattering pattern characteristics for the same series of standard waveguide size probes show a reduction in on-axis (boresight) monostatic radar cross section (RCS) for chamfered tip waveguides compared to blunt-ended waveguides and that this reduction increases for increasing frequency.
Countless degrees of freedom in the design of antenna test ranges are enabled if the measurement errors caused by the environment can be precisely compensated. Measurements in highly reflective measurement chambers and with broadband feeds or probes are possible since the quality of the test zone is not essential anymore. To create a reflective environment, a metal plate is installed in an anechoic chamber and a base transceiver station antenna is characterized with and without the additional scattering source at a frequency of 2:11GHz. To compensate for the undesired signals, the field of the test zone is measured on a spherical surface using a scanning arm. Via a wave expansion of the field and the antenna under test into spherical mode coefficients, the undesired signals are compensated. It is shown that the error of the compensated pattern compared with the undistorted measurements is mainly below 30dB and that the directivity is retrieved with a difference of only 0:011dB>
This communication provides the experimental validation of an effective probe-compensated near-field to far-field (NFFF) transformation with a nonconventional plane-rectangular scan suitable for flat antennas under test (AUTs). It is based on the nonredundant sampling representations of the electromagnetic fields, on the use of optimal sampling interpolation expansions, and assumes a flat AUT as enclosed in a dish having diameter equal to its maximum dimension. This source modeling results to be very effective from the NF data reduction viewpoint, since, by fitting very well the geometry of such a kind of AUT, it is able to reduce as much as possible the residual volumetric redundancy related to the use of the other modelings suitable for quasi-planar AUTs (an oblate spheroid or a double bowl). Experimental results, assessing the practical feasibility of the proposed NF-FF transformation technique, are shown.
This paper investigates on the postprocessing of spherical near-field measurements in phaseless scenarios. Traditionally, iterative algorithms have been used to propagate between two measurement surfaces to retrieve the near-field phase. In the last years, advanced phase retrieval techniques have been developed formulating the phaseless problem in matrix form. Both approaches are introduced and investigated, comparing its performance with numerical and measurement data. Preliminary results indicate that iterative propagation techniques offer superior performance, yet with a more irregular and nonlinear behavior. The matrix approach, however, offers much more flexibility on its formulation leaving room for more potential improvements.