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Polarization
Topology for Maintaining Symmetry in Hybrid LPDABroadbandDipole Antennas
Topologies for hybrid LPDAbroadbanddipole antennas (hybrid antennas) are systematically presented and evaluated regarding their ability to provide symmetric response as defined and required in recent standards. The symmetry property of the hybrid antenna is fundamentally related to the intrinsic infinite balun, the choke structure, and the matching transformer for the broadband dipole, if one is employed. In general, hybrid antennas incorporating matching transformers are more symmetric if the transformer is effectively centertapped. More specifically, in a hybrid antenna employing an impedance matching transformer derived from an equaldelay hybrid, the sum port can be advantageously connected via a lowimpedance load to the center of a symmetric choke arrangement. A specific topology for a hybrid LPDAbroadbanddipole antenna is given here which employs a 1:4 impedance transforming balun between the LPDA and broadband dipole but at the same time provides symmetry such that the antenna satisfies the requirements given in recent standards. Thus, the advantages of the impedance transforming balun are realized, but the symmetry of the antenna is maintained. Finally, it is shown that a hybrid antenna satisfies the symmetry requirements if a 180 • rotation about the bore sight axis is equivalent to a 180 • electrical phase shift in the source and that this behavior is obtained with a combination of 2fold rotational symmetry in the radiating structure and electrical symmetry in the intrinsic balun structure.
Deembedding Radome Depolarization from Measurements with a NonIdeal Circularly Polarized Antenna Source
An approximation method is developed to remove the source antenna's crosspolarization discrimination (XPD) contribution from the total measured XPD. This modeling is shown to correlate very well on a flatpanel test with a radome's predicted (idealsource) XPD. Additionally, a mathematical formulation of the theoretical crosspolarization discrimination (XPD) bounds is presented to validate the proposed method. The measured axial ratio should not exceed these bounds. The measured result is within these bounds and thus this model serves as an additional validation step to both the proposed method and the measured results.
Indoor 3D Spherical Near Field RCS Measurement Facility: A new high resolution method for 3D RCS Imaging
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor Near Field monostatic RCS assessment [3]. This experimental layout is composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. 3D imaging is a suitable tool to accurately locate and characterize in 3D the main contributors to the RCS. However, this is a noninvertible Fourier synthesis problem because the number of unknowns is larger than the number of data. Conventional methods such as the Polar Format Algorithm (PFA), which consists of data reformatting including zeropadding followed by an inverse fast Fourier transform, provide results of limited quality. We propose a new high resolution method, named SPRITE (for SParse Radar Imaging TEchnique), which considerably increases the quality of the estimated RCS maps. This specific 3D radar imaging method was developed and applied to the fast 3D spherical near field scans. In this paper, this algorithm is tested on measured data from a metallic target, called Mx14. It is a fully metallic shape of a 2m long missilelike target. This object, composed of several elements is completely versatile, allowing any change in its size, the presence or not of the front and / or rear fins, and the presence or not of mechanical defects, … Results are analyzed and compared in order to study the 3D radar imaging technique performances.
Experimental Validation of Minimum Redundancy Scanning Schemes in PNF Measurements at V band
The planar widemesh scanning (PWMS) methodology is based on a nonredundant sampling scheme [1], [2] and is thus without loss of accuracy. It has the potential to enable much faster measurements than standard Planar Near Field (PNF) scanning that is based on denser, regular, equally spaced NF sampling fulfilling Nyquist criteria. In [3], the nonredundant methodology has been validated numerically by simulated measurements on a highly shaped reflector antenna and with actual measurements on a pencil beam antenna in Kuband and on a navigation antenna in Lband. In this paper, we present the experimental verification of the PWMS methodology, at V band using dedicated PNF measurements of a Standard Gain Horn antenna MVG SGH4000. The results accuracy of the nonredundant methodology has been investigated against FarField patterns, implemented by standard scanning methods, by visual comparison, and by computation of the Equivalent Noise Level (ENL). The achieved undersampling factor is equal to 12, corresponding to similar time reduction in the stepped measurement system employed for the presented validation.
Recent Developments in International Facility Comparison Campaigns
The EurAAP (the European Association on Antennas and Propagation) [1] Measurements working group (WG5), constitutes a framework for cooperation to advance research and development of antenna measurements. An important ongoing task of this group is to sustain the Antenna Measurement Intercomparisons. The comparison of each facility measurement of the same reference antenna in a standard configuration results in important documentation and validation of laboratory expertise and competence, allowing to validate and document the achieved measurement accuracy and to obtain and maintain accreditations like ISO 17025. An additional outcome is the improvement in antenna measurement procedures and protocols in facilities and contributions to standards, which is one of the longterm objectives of the EurAAP WG5. Several participants among Europe but also USA and ASIA have joined the activity. These campaigns will also serve for a new task, recently approved within the WG5, of selfevaluation from comparison of the measurement results. An important ongoing campaign involves a X/Ku/Kaband high gain reflector antenna MVISR40 fed by SH4000 Dual Ridge Horn. In this paper we report the results here for the first time. The medium gain ridge horn, MVISH800, equipped with an absorber plate to enhance the correlation in different facilities has been the reference antenna of another campaign. In [2] the preliminary results were shown. In this paper we present the final validation. The comparison is reported plotting the gain/directivity patterns and computing the equivalent noise level and the Birge ratio with respect to the reference pattern obtained taking into account the uncertainty declared by each facility.
A Review of the CWAmbient Technique for Measuring G/T in a Planar NearField Antenna Range
Techniques for measuring G/T have been previously presented at AMTA; however, there are very few papers that discuss how to measure G/T in a nearfield antenna range. One recent paper discussed such a method and gave a brief description within the larger context of satellite payload testing [1]. The paper's treatment of G/T was necessarily brief and gives rise to several questions in relation to the proposed method. Other papers that treated this topic required the antenna aperture to be separable from the backend electronics, which may not be possible in all cases [23]. In this paper, we discuss in great detail a slightly modified version of the G/T measurement method presented in [1]. A signal and noise power diagram is presented that can be useful for understanding how system signaltonoise ratio (SNR) relates to G/T, and a few common misconceptions concerning the topic of G/T are addressed. The CWAmbient technique for computing G/T of a Unit Under Test (UUT) from measurements in a planar nearfield system is described in detail, and a list of assumptions inherent to the CWAmbient technique is presented. Finally, the validity of the CWAmbient technique is assessed by analyzing measured data collected from a separable UUT.
Experimental Verification of 3D Metal Printed Dual CircularPolarized Horn Antenna at VBand
In this paper, a 3D metal printed dual circularpolarized horn antenna operating in the Vband is proposed, built and tested. This antenna consists of a horn and a circular waveguide, a single groove polarizer and is sidefed by orthogonally placed rectangular waveguide ports. The groove is placed at 45° with respect to the input ports and provides a phase delay of 90° to generate rightor lefthand circular polarization (RHCP or LHCP). The proposed antenna provides symmetric patterns for all planes and exhibits polarization isolation of more than 30 dB at broadside. This antenna is analyzed to realize wide impedance matching bandwidth and wide 3dB axial ratio (AR) bandwidth. A prototype of the horn antenna has been fabricated using 3D metal printing technology. Metal material with finite surface roughness is considered when modeling this antenna.
A Methodology for Instantaneous Polarization Measurements Using a Calibrated DualPolarized Probe
Accurately measuring the polarization of an antenna is a topic that has garnered much interest over many years. Methods abound including phasereferenced measurements using two orthogonal polarizations, phaseless measurements using two or three pairs of orthogonal polarizations, spinning linear probe measurements, and the rigorous threeantenna polarization method. In spite of the many publications on the topic, polarization measurements are very challenging and can easily lead to confusion, particularly in accurately determining the sense of polarization. In this paper, we describe a method of accurately and rapidly measuring the polarization of an antenna with the aid of a multichannel measurement receiver and a dualpolarized probe. The method acquires phasereferenced measurements of two orthogonal polarizations. To enable such measurements, we describe a methodology for calibrating the probe. We also describe a tool for automating the polarization measurement and display of the polarization state. By automating the process, it is hoped that the common challenges and confusions associated with polarization measurements may be largely obviated.
Fast Spherical NearField Measurements on Arbitrary Surfaces by Application of Pointwise Probe Correction to Compressed Sampling Schemes
The major disadvantage of Spherical NearField (SNF) measurements is their long acquisition time. To calculate the Antenna Under Test's (AUT) farfield radiation characteristics , a sphere containing the AUT must be sampled. Classically, equiangular sampling is chosen, being the resulting sphere heavily oversampled. Since the Spherical Mode Coefficients (SMCs) are usually sparse, an approach to reduce the measurement time of SNF measurements is to undersample the sphere and to reconstruct the SMCs using compressedsensing techniques. Using a sampling matrix with a minimum mutual coherence for the given bases of the SMCs increases the probability of recovery. The SMCs are defined in the basis of the spherical harmonics or Wigner Dfunctions, which limits the geometries in which this technique can be applied. In this work, the application of pointwise probe correction for the description of nonspherical surfaces in the WignerD basis expansion is suggested. The chosen sampling points are radially projected onto the measurement surface and the new distance to each point is calculated. New equivalent probe response coefficients are calculated per measurement point according to their distance to the AUT. To compensate for different orientations other than the probe pointing to the AUT's minimum sphere's center, the probe's SMCs are rotated to reflect the real orientation of the probe at each point prior to the calculation of the probe response coefficients. Although more computationally demanding than classical probe correction, this technique allows measurements with different, potentially faster geometries and enables the application of compressed sensing to other, nonspherical conventional scanning systems.
EMC Measurement System Based on Software Defined Radio and Diagnostic Techniques
In a previous paper a new referenceless measurement setup based on a reference antenna was used for characterizing the radiation of antennas in the planar scanner [1]. The method is based on using a lowcost receiver to retrieve the amplitude and phase of the signal. This paper explores the limitations of the method for different geometries and implements a multiprobe electromagnetic compatibility measurement system. Once the amplitude and phase are recovered, diagnostic techniques can be applied and also nearfield to near or farfield transformations to calculate the field at distances defined by standards. The results demonstrate the good accuracy of the method in comparison with traditional electromagnetic compatibility laboratories.
Measurements on extended objects for radar field probes
An extended long object usually gives rise to a bright reflection (a glint) when viewed near its surface normal. To take advantage of this phenomenon and as a new concept, a discrete Fourier transform (DFT) on the RCS measurements, taken within a small angular range of broadside, would yield a spectrum of incident wave distribution along that object; provided that the scattering is uniform per unit length, such as from a long cylinder [1, 2]. In this report, we examine the DFT spectra obtained from three horizontal long objects of different lengths (each of 60, 20, and 8 feet). Aside from the end effects, the DFT spectra looked similar and promising as an alternative to the conventional field probes by translating a sphere across a horizontal path. Keywords: RCS measurements, compact range, field probes, extended long objects 1. The Boeing 977 compact range The Boeing 977 indoor compact range was constructed in 1988 based on the largest Harris model 1640. Figure 1 is a schematic view of the chamber, which is of the Cassigranian configuration with dualreflectors. The relative position of the main reflector and the upper turntable (UTT) are as shown. The inside dimensions of the chamber are 216ft long, by 80ft high, and 110ft wide. For convenience, we define a set of Cartesian coordinates (x: pointing out of the paper, y: pointing up, z: pointing downrange), with the origin at the center of the quiet zone (QZ). The QZ was designed as an ellipsoidal volume of length 50ft along z, height 28ft along y, and width 40ft along x. The back wall is located at z = 75 ft, whereas the center of the rolledged main reflector (tilted at 25 o from vertical) is at z =110 ft. It is estimated that the design approach controls the energy by focusing 98% of it inside the QZ for target measurements. The residual field spreading out from the main reflector was attenuated by various absorbers arranged in arrays and covering the chamber walls., Tel. (425) 3920175 2. Anechoic chamber In order to provide a quiet environment for RCS measurements, the inside surfaces of an anechoic chamber are typically shielded by various pyramidal and wedgedshaped absorbers, which afford good attenuation at nearnormal incidence for frequencies higher than ~2 GHz. At low frequencies and oblique angles [3], however, Figure 1. A schematic view of the Boeing 977 compact range with dimensions as noted. insufficient attenuation of the radar waves by the absorbers may give rise to appreciable backgrounds. Figure 2 shows a panorama view inside the compact range, as viewed from the lower rear toward the main reflector and the UTT. With the exception of the UTT, all other absorbers are nonmoving or stationary. A ring of lights on the floor shows the rim around the lower turntable (LTT), prior to the installation of absorbers. In order to minimize the targetwall interactions, the surfaces facing the QZ from the ceiling, floor, and two sidewalls are covered with the Rantec EHP26 type of special pyramidal absorbers.
NearField Techniques for MillimeterWave Antenna Array Calibration
A reliable technique for antenna array characterization and calibration is demonstrated for two stateoftheart antenna measurement systems, a nearfield system and a compact antenna test range system. Both systems are known to reduce the measurement distance between device under test and the probe antenna in comparison to classical farfield systems, which need to provide at least the Fraunhofer distance as minimum range length. Equivalent magnetic surface currents are derived from measurements, which represent the electric field on the applied Huygens surface. The calculated equivalent magnetic currents are utilized for characterizing two completely different antenna arrays in the millimeterwave region. Magnitude and phase calibration opportunities of antenna arrays are discussed, as well as the accuracy provided by the proposed calibration technique.
Extraction of MagnetoDielectric Properties from MetalBacked FreeSpace Reflectivity
Intrinsic magnetic and dielectric properties of magnetodielectric composites are typically determined at microwave frequencies with both transmission and reflection data. An iterative method, such as rootfinding, is often used to extract the properties in a frequencybyfrequency basis. In some situations, materials may be manufactured on a metal substrate that prevents transmission data from being obtained. This happens when the materials are too fragile or too strongly bonded to the substrate for removal and must be characterized with the metal substrate in place. This paper compares two different freespace extraction algorithms, developed for the simultaneous extraction of complex permittivity and permeability from metalbacked reflection. One of the algorithms relies on reflection measurements of the same material with two known thicknesses. The second method takes advantage of wide bandwidth measurements to fit the reflection to analytical models (e.g. Debye). The accuracy of these methods are evaluated and the stability criteria for the techniques will be discussed, as well as the tolerance of the techniques to various measurement errors.
Toploaded 60:1 Wideband DirectionFinding Sensor
A wideband, four element array is designed to create suitable radiation patterns for angle of arrival estimation over a field of view of 0 • to 80 • in elevation and 360 • in azimuth, using the CramerRao Lower Bound (CRLB) as the figure of merit. The antenna elements are truncated monocones over a circular ground plane and operate over 100 MHz to 6 GHz. A study of the antenna geometry was performed to meet size constraints while minimizing the reflection losses at the input for frequencies up to 1 GHz. A method is presented to find the ideal loading impedance required for each frequency using multiport Sparameters derived from field simulations. The loading improves the maximum return loss from 0.4 dB to 6 dB. The study reveals a tradeoff between minimal reflection losses and direction finding (DF) performance evaluated using the CRLB over the operating frequency. For the best investigated geometry using top loading the maximum root mean square error of the azimuth DF estimate remains below 13 • .
Waveguideintegrated Rydberg Atombased RF Field Detector for Nearfield Antenna Measurements
We demonstrate simultaneous amplitude and phase measurements of a radiofrequency (RF) field through the use of a Rydberg atombased sensor embedded inside a waveguiding structure. This measurement uses the Rydberg atombased sensor in a mixer configuration, which requires the presence of a local oscillator (LO) RF field. The waveguiding structure supplies the LO field. The combined waveguide and Rydberg atom system is used to measure phase and amplitude in the nearfield of a horn antenna to extract the farfield pattern.
Update of IEEE Std 17202012 Recommended Practice for NearField Antenna Measurements
The IEEE Standards Association Standards Board (IEEESASB) approved the IEEE Std 1720™ "Recommended Practice for NearField Antenna Measurements" in 2012 [1]. More than forty dedicated people from industry, academia and other institutions contributed to the creation of this new document. The main motivation for a new standard dedicated to nearfield measurements was to complement the existing IEEE Std 1491979™ "Test Procedures for Antennas" [2]. According to the IEEESA policies, the existing standard IEEE Std 17202012™ is approaching expiration in 2022. A working group of the APS Standard Committee has been formed to review the current document. Most of the current standard is still relevant and useful for individuals designing and evaluating nearfield antenna measurement facilities and other people involved in antenna measurements. However, the standard needs update and renewal in areas in which new developments and technologies have matured. This paper gives an overview of the current standards and discusses the suggested potential changes.
A Review of the Changes and Additions to the Antenna Measurement Standard IEEE Std 149
The IEEE Standard 149, Standard Test Procedures for Antennas, has not been revised since 1979. Over the years the Standard was reaffirmed, that is, its validity was reestablished by the IEEE APS Standards Committee, without any changes. Recently however, the IEEE Standards Association stopped the practice of reaffirming standards. This change in policy by the IEEE has been the "medicine" that this Standard needed. A working group was organized and a project authorization request (PAR) was approved by IEEE for the document to be updated. In this paper, the expected changes to the document are described and commented. The main change is to convert the Standard to a recommended practice document. Additionally, some new techniques to measure antennas, such as the use of reverberation chambers for efficiency measurements and more information on compact ranges, is discussed. Other topics inserted are more guidance on indoor ranges and an updated section on instrumentation. Most importantly, a discussion on uncertainty is included. The result will be a very useful document for those designing and evaluating antenna test facilities, and those performing the antenna measurements.
Measurement procedures and postprocessing for fast antenna characterization
Three measurement procedures and associated postprocessing for the fast characterization of antennas are presented. First, an approach for the fast diagnosis of antenna under test (AUT), ie. the identification of potential defaults with respect to an ideal antenna, is described. The technique leverages the knowledge of the ideal (expected) radiation pattern and uses a sparse recovery algorithm to locate the few potential defaults. Second, a scheme is proposed to interpolate the near field radiated by the AUT. It exploits the low complexity of the electromagnetic field and does not resort to any knowledge on the AUT. Third, an approach to speed up the measurement of the AUT far field radiation pattern is detailed. The only input is the maximum dimension of the AUT. The technique relies on the sparse expansion of antenna radiation patterns on spherical harmonic basis. For each of the three examples, experimental results will be shown for various complex radiating structures in different frequency bands.
Small Antenna Testing in a Compact Antenna Test Range
The Compact Antenna Test Range (CATR) was initially conceived as an efficient way of testing electrically large antennas at very much reduced, fixed, range lengths than would otherwise be the case. However, when testing lower gain, physically smaller antennas, the measurements can become susceptible to inhomogeneities within the CATR QZ including phenomena associated with edge diffraction effects, feed spillover, chamber multipath etc. Whilst it has been demonstrated experimentally that many of these measurement artefacts may be effectively mitigated using standard and modern more sophisticated postprocessing techniques. This paper supports those findings through simulation of the direct and indirect far field ranges and by careful examination of the data processing chain. Results are presented, the relative success of the various techniques examined and the utility of this is set, and expounded, in the context of modern, i.e. 5G, communications systems.
On Wacker's Essential Equation in the Extrapolation Measurement Technique
The generalized threeantenna method is a standard method for measuring onaxis gain and polarization of an antenna without a priori knowledge. The cornerstone of the method is the use of the extrapolation technique and the key relationship in the extrapolation technique is Wacker's equation. This equation expresses the received signal as a function of the separation distance between any two antennas. The derivation of Wacker's equation is not readily available in the literature. In this paper, we provide a streamlined derivation of Wacker's equation and address some of the common misconceptions associated with it.

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