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Calibration

Radar Echoes from Metal Spheres Large and Small
Pax Wei, November 2016

Wave scattering from a perfectly conducting sphere provides an important example for theoretical studies as well as RCS calibrations [1, 2].  At the Boeing 9-77 Range and the Millimeter Wave Range in Seattle, we measured spheres of large and small diameters, supported by strings or a foam tower, and through a wide range of frequencies.  In addition to co-polarized calibration, the emphasis was also on uncertainty analysis in order to verify that the experiments carried out under different conditions were mutually consistent [3].  Aside from the well-defined conditions for an indoor range, metal spheres may be dropped from the air free fall while being measured [4].  A news article on January 5, 2016, reported that three metal spheres were picked up in three provinces in northern Vietnam [5].  Though details of the experiments were obscure, from the pictures they happened to correspond to spheres of sizes from large to small.  Based on our experiences, some speculation will be discussed.  References [1]. E. F. Knott, "Radar Cross Section Measurements," (Van Nostrand Reinhold,  New York, 1993), pp. 176-180, (on spheres and the Mie series).   [2]. E. F. Knott, E. F. Shaeffer, and M. T. Tuley, "Radar Cross Section," (Artech House,      2nd ed, 1993), pp. 86 & 234-235, (on creeping waves).  [3]. P. S. P. Wei, A. W. Reed, C. N. Ericksen, and J. P. Rupp, “Uncertainty Analysis and      Inter-Range Comparison on RCS Measurements from Spheres,” Proc. 26th AMTA,      pp. 294-299 (2004).   [4]. “Mysterious silver balls fall down on town; can the black helicopters be far behind?”   By Steve Vogel, The Seattle Times, August 7, 2000, (from the Washington Post).  [5]. “3 mysterious spheres fall onto 3 Vietnam provinces,”  Tuoi Tre,  Tue, 05 Jan 2016.  http://www.sott.net/article/309800-3-mysterious-spheres-fall-onto-3-Vietnam-provinces

Compact First-Order Probe for Spherical Near-Field Antenna Measurements at P-band
Oleksiy Kim, November 2016

A number of European Space Agency's (ESA) initiatives planned for the current decade require metrology level accuracy antenna measurements at frequencies extending from L-band to as low as 400 MHz. The BIOMASS radar, the Galileo navigation and search and rescue services could be mentioned among others. To address the needs, the Technical University of Denmark (DTU), who operates ESA’s external reference laboratory “DTU-ESA Spherical Near-Field (SNF) Antenna Test Facility”, in years 2009-2011 developed a 0.4-1.2 GHz wide-band higher-order probe. Even though the probe was manufactured of light-weight materials -- aluminium and carbon-fibre-reinforced polymer (CFRP) -- it still weighs 22.5 kg and cannot be handled by a single person without proper lifting tools. Besides that, a higher-order probe correction technique necessary to process the measurement data obtained with such a probe is by far more demanding in terms of the computational complexity as well as in terms of calibration and post- processing time than the first-order probe correction. On the other hand, conventional first-order probes for SNF antenna measurements utilizing open-ended cylindrical waveguides or conical horns fed by cylindrical waveguides operating in the fundamental TE11-mode regime also become excessively bulky and heavy as frequency decreases, and already at 1 GHz an open-ended cylindrical waveguide probe is challengingly large. For example, the largest first-order probe at the DTU-ESA SNF Antenna Test Facility operates in the frequency band 1.4–1.65 GHz and weighs 12 kg. At 400 MHz, a classical first-order probe can easily exceed 1 cubic meter in size and reach 25-30 kg in weight. In this contribution, a compact P-band dual-polarized first-order probe is presented. The probe is based on a concept of a superdirective linear array of electrically small resonant magnetic dipole radiators. The height of the probe is just 365 mm over a 720-mm circular ground plane and it weighs less than 5 kg. The probe covers the bandwidth 421-444 MHz with more than 9 dBi directivity and |µ| ? 1 modes suppressed below -35 dB. The probe design, fabrication, and test results will be discussed.

Characterizing Multiple Coherent Signals Near 60 GHz Using Standard RF Hardware for MIMO and 5G Applications
Alexandra Curtin, David Novotny, Joshua Gordon, November 2016

In wireless communication technology, the growth of 5G and MIMO (multiple- input and multiple-output) systems has revealed a gap in the methods to characterize and calibrate hardware for high frequency and coherent MIMO applications. For coherent array configurations and ad hoc systems we need to measure transmission loss and phase/delay over every element. We demonstrate the use of standard RF hardware to generate and receive multiple signals in a system that is a tabletop analogy for an ad hoc system. The initial test system consists of using a single WR-15 VNA extender to detect two separate modulated signals. As our sources, we individually modulate WR-15 VNA extenders to generate continuous waveform, modulated signals around 60 GHz. On the receive side, our IF signal is first measured with a high-dynamic- range spectrum analyzer and then later collected in a digital oscilloscope. All the signal generators for the receiver LO and transmitter(s) RF IN are tied together with a common 10 MHz reference. Characterizing this initial 2x1 system is then extensible to multiple-receiver applications. We will use these coherent sources to get full complex waveform characterization element-by-element in a receiving array. We report on measurement and calibration methods to characterize the response of these systems for continuous waveforms, modulated signals, and multi-frequency applications needed for next generation coherent MIMO systems.

Uniaxial Anisotropic Material Measurement using a Single Port Waveguide Probe
Alexander Knisely, Milo Hyde, Michael Havrilla, Peter Collins, November 2016

Anisotropic material characterization requires versatile sample fixtures in order to provide sufficient measurement diversity for material parameter extraction.  However, extensive sample preparation is often required prior to making a measurement, especially for anisotropic materials.  An alternative nondestructive material measurement approach using a Single Port Waveguide Probe (SPWP) is proposed to simplify measurement of uniaxial anisotropic media.  Instead of cutting a material sample to fit into a given fixture, nondestructively interrogating a sheet of material via the SPWP greatly simplifies sample preparation and measurement.  The SPWP system measures a metal-backed sample of a known thickness.  A flange with a waveguide aperture cut in the center is placed on the metal backed sample (thus forming a parallel plate region) and a length of rectangular waveguide is connected to the flange aperture. A Vector Network Analyzer port is connected to the end of the rectangular waveguide to collect calibration and sample data.  Measurements of two different thicknesses of a sample are performed to provide sufficient data for extracting the sample permittivity tensor.  The sample permittivity tensor is computed via comparison of the measured and theoretical S-parameters using a least squares minimization algorithm.  The theoretical S-parameters are derived using a magnetic field integral equation which utilizes a uniaxial parallel plate Green’s function to constitute the fields in the parallel plate region.  Love’s Equivalence Principle is used to relate the fields in the parallel plate flange region to the fields in the waveguide (assumed to be the dominant TE10 mode only).  In this paper, the SPWP theoretical development, measurement and material parameter extraction are discussed.  Measurements and simulations of isotropic and uniaxial samples are made to assess the SPWP performance.

60 GHz Reference Chip Antenna for Gain Verification of Test Chambers
William McKinzie, Per Iverson, Edward Szpindor, Michael Smith, Bradley Thrasher, November 2016

We have developed a 60 GHz chip antenna designed for use as a gain and pattern verification tool in the calibration process of a millimeter wave antenna test chamber. The antenna is designed to interface with ground-signal-ground (GSG) micro-probes that have a probe pitch of 150 um to 250 um.  This low temperature cofired ceramic (LTCC) chip antenna is fabricated using DuPont’s 9K7 GreenTapeTM material system with gold conductors.  Features include a wafer-probe transition, a shielded stripline corporate feed network, aperture coupled patch elements, and an integrated Sievenpiper electromagnetic bandgap (EBG) structure for surface wave mode suppression.  The use of the EBG structure enables main beam gain enhancement and side lobe level suppression.  This 2x2 antenna array is directive such that it offers a nominal gain of 12 dBi at broadside over 58-62 GHz with an antenna efficiency of at least 60%.  The entire antenna package has a nominal size of only 10.9 mm x 12.2 mm x 0.71 mm.  Since this antenna package material is hermetic, it has stable performance under varying humidity and temperature which is highly desirable as a reference antenna.

Multiple Target, Dynamic RF Scene Generator
David Wayne, John McKenna, Scott McBride, November 2016

The evaluation of RF Sensors often requires a test capability where various RF scenes are presented to the Unit Under Test (UUT). These scenes may need to be dynamic, represent multiple targets and/or decoys, emulate dynamic motion, and simulate real world RF environmental conditions. An RF Scene Generator can be employed to perform these functions and is the focus of this paper. The total test system is usually called Hardware in the Loop (HITL) involving the sensor mounted on a Flight Motion Simulator (FMS), the RF Scene Generator presenting the RF Scene, and a Simulation Computer that dynamically controls everything in real time. This paper describes the system concept for an RF Scene Generator that simultaneously represents 4 targets, in highly dynamic motion, with no occlusion, over a wide range of power, frequency, and Field of View (FOV). It presents the test results from a prototype that was built and tested over a limited FOV, while being scalable to the total FOV and full system capability. The RF Scene Generator employs a wall populated with an array of emitters that enables virtually unlimited velocity and acceleration of targets and employs beam steering to provide high angular resolution and accuracy of the presented target positions across the FOV.   Key words: RF Target Simulator, RF Scene Generator, Multiple Targets, Beam Steering Wall of Emitters, Steering Array Calibration, Plane-Wave Generator, Radar Environment Simulator.

In-situ Diagnosis of Direction Finding Antenna using Optically-fed Transmitting Miniature Probes
Serge Bories, Lama Ghattas, Dominique Picard, November 2016

Direction Finding (DF) Antennas are usually designed and tested in controlled environments. However, antenna far field response may change significantly in its operational environment. In such perturbing or not -controlled close context, the antennas calibration validity becomes a major issue which can lead to DF performance degradation and to a costly re-calibration process. Even if in-situ re-calibration is still complicated; the DF antenna response can be monitored, during the mission, in order to ensure the DOA accuracy. This paper presents an innovative design and the performance of a low-disturbing solution to detect the near field antenna response deviations from a nominal case. The proposed system is based on an array of transmitting miniature dipoles deployed all around the DF antennas. These probes are optically fed through a non-biased photodiode that carries the direct conversion into a RF signal at the desired frequency. The detection re-used the DF receiving RF chains to analyze any deviation (complex values) of the antennas array manifold. Compared to the Optically Modulated Scatterer (OMS) technique, the benefits of the proposed approach are demonstrated experimentally over a frequency decade (UHF band). First a better sensitivity is shown (higher than 80 dB on the monitored link), and secondly the phase detection is made really simple compared to the OMS technique. Finally, a relation between this in-situ diagnosis mode and the DF angular direction accuracy is established. Thus the capacity to detect, on the near field response, the presence of various types of closed obstacles (open trap on the carrier, additional antenna…) which perturb significantly the far field antenna response, is evaluated.

Measurement Uncertainties in Millimeter Wave “On-Chip” Antenna Measurements
Edward Szpindor, Wenji Zhang, Per Iversen, November 2016

As a result of recent technical and regulatory developments, the millimeter wave frequency band (30GHz – 300 GHz) is being adopted for wide range of applications.  Based on array signal processing technologies used for 4G and MIMO, companies are developing small active array antennas operating throughout the millimeter-wave bands.  These arrays may include radiating elements and feed structures that are fraction of a millimeter in size and cannot be fed via a coaxial cable.  Connection to the antenna is instead performed through a micro-probe more commonly used in the chip industry.  MVG-Orbit/FR has developed a compact antenna measurement system which integrates hardware and software necessary to provide antenna gain and radiation patterns of antennas fed with such a micro-probe. To evaluate uncertainties in the measurements of the Antenna Under Test (AUT) gain, directivity, efficiency, pattern, or VSWR, reference antennas are an invaluable tool.  The authors have recently driven the development of a micro-probed chip reference antenna.  This reference antenna was designed to be mechanically and electrically stable and with reduced sensitive to its mounting fixture and feeding method.  Close agreement between measured and simulated characteristics has been achieved.  With low losses, the antenna provides good dynamic range and confidence in the measured antenna efficiency and gain. Without chip antenna gain standards, a micro-probed antenna test system requires the use of the insertion loss method for gain calibration.  This method requires correction for additional losses such as cables, attenuators, or adaptors that are included in the calibration but not in the subsequent measurement of the AUT.  In addition, the micro-probe (which is in the measurement but not in the calibration) should be calibrated and de-embedded from the measurement.   Each of these measurements and associated connections and related processing, increases uncertainty and chance of mistakes by the user.  It is therefore essential to validate the calibration using a well characterized reference antenna. This paper will outline design requirements and present test results of 60 GHz Chip Reference antennas.  Several dozen antennas have been tested.  The related uncertainties in the micro-probed antenna measurements will be evaluated with particular emphasis on the gain calibration uncertainty.  The paper will also describe the next steps towards developing a chip antenna gain standard, that should reduce gain uncertainties while also significantly simplifying the calibration process.

Dual-calibration Processing Based on Minimum Weighted Mean Squared Error (MWMSE) in RCS Measurement
Xiaojian Xu,Yongze Liu, November 2015

Dual-calibration was first proposed by Chizever et al. in 1996 [AMTA'1996] and had get wide applications in evaluation of the uncertainty in radar cross section (RCS) measurement and calibration. In 2013, LaHaie proposed a new technique based on jointly minimizing the mean squared error (MMSE) [AMTA'2013] among the calibrated RCS of multiple calibration artifacts, which estimates both the calibration function and the calibration uncertainty for each artifact. MMSE greatly improves the estimation accuracy for the radar calibration function as well as results in lower residual and RCS calibration errors. This paper presents a modified version of LaHaie's MMSE by minimizing the weighted mean squared error (MWMSE) for RCS calibration processing from  multiple calibrator measurements, which is related to the following functions and parameters: the calibration function; the theoretical and measured RCS; the number of calibration artifacts the number of frequency samples and the weight for ith calibration artifacts which may be defined in terms of the theoretical RCS of all the calibration artifacts. For example, if the weight is defined as the inverse of the total theoretical RCS of the ith calibration artifacts for all frequency samples, the error then represents the total relative calibration error instead of an absolute error as in MMSE. MWMSE then means that an optimal calibration function is found in terms of minimum total relative calibration error, which is expected for most applications. Numerical simulation results are presented to demonstrate the usefulness of the proposed technique.

Insertion Phase Calibration of Space-Fed Arrays
Jacob Houck,Brian Holman, November 2015

Calibrating a passive, space-fed, phased array antenna is more difficult and time consuming then calibrating corporate-fed arrays because individual elements cannot be activated or deactivated. We will present our method of determining element state-phase curves and insertion phase bias between elements. We will also explain this method’s theoretical basis and validate it by comparing data measured in an anechoic chamber with data measured in a planar near field range. The anechoic chamber data will be compared with the typical, proven, but more time-consuming planar near field calibration method.

A New Method for VHF/UHF Characterization of Anisotropic Dielectric Materials
John Schultz,James Maloney, November 2015

Recent interest in anisotropic metamaterials and devices made from these materials has increased the need for advanced RF material characterization. Moreover, the quest for measurement of inhomogeneous and anisotropic materials at VHF and UHF frequencies has long been one of the primary stretch goals of the RF materials measurement community. To date, the only viable method for these types of materials has been either fully filled or partially filled VHF waveguides, which are large, expensive, and slow. This paper introduces a new fixture design that greatly simplifies the process of obtaining intrinsic properties for inhomogeneous and anisotropic dielectric materials. The fixture combines low frequency capacitance and high frequency coaxial airline concepts to measure cube shaped specimens, and is termed an “RF Capacitor”. Furthermore, a significant limitation of past measurement methods is their reliance on approximate analytical models to invert material properties. These analytical models restrict the available geometries and frequency ranges that a measurement fixture can have. The present method avoids this limitation by implementing a new inversion technique based on a full-wave, finite difference time domain (FDTD) solver to exactly model the measurement geometry. In addition, this FDTD solver is applied in a novel way to enable inversion of frequency-dependent dielectric properties within seconds. This paper presents the fixture design and calibration for this new measurement method, along with example measurements of isotropic and anisotropic dielectric materials. In particular, 3” cube specimens are measured and the bulk dielectric properties in the three principal planes are determined by measuring the same specimen in three different orientations within the measurement fixture. Finally, calculations are presented to show the relative accuracy of this method against a number of probable uncertainty sources, for some characteristic materials.

Characterization of Reflectivity Losses in Space Reflector Antennas at Temperatures above 350°C
Luis Rolo,Eric van der Houwen, Elena Saenz, November 2015

In the recent years, the microwave and mm wave communities have been experiencing a strong interest in the characterisation of the RF proprieties of materials used in the manufacture of antennas and structures that, in one way or another, interact with propagating electromagnetic fields. Of particular interest are materials used for for space applications, where antennas face a harsh environment at all times making it challenging to keep antenna performances in all orbital conditions, whether in eclipse or under full sunlight exposure. A particular example is the coming Solar Orbiter mission, where the antenna reflector will be exposed to a high intensity of solar energy. This paper describes a measurement system with a custom-built setup that enables the measurement of reflectivity losses of space antenna materials and coatings at very high temperatures - up to 500 degrees Celsius. The design of the high temperature fixture will be presented in detail, together with the development of the necessary measurement and calibration techniques. The paper will conclude with a critical assessment of the obtained results and system performance and achieved accuracies.

A Calibration Method Using Interpolation to Reduce Measurement Errors in Electromagnetic Compatibility Measurements
Vince Rodriguez,Dennis Lewis, November 2015

MIL STD 461 is the Department of Defense standard that states the requirements for the control of electromagnetic interference (EMI) in subsystems and equipment used by the armed forces. The standard requires users to measure the unintentional radiated emissions from equipment by placing a measuring antenna at one meter distance from the equipment under test (EUT). The performance of the antenna at 1m distance must be known for the antenna to measure objects located at this close proximity. MIL STD 461 requires the antennas to be calibrated at 1 m distance using the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 958. This SAE ARP 958 document describes a standard calibration method where two identical antennas are used at 1m distance to obtain the gain at 1m for each antenna. In this paper the authors show using simulations that the SAE ARP 958 approach introduces errors as high at 2 dB to the measured gain and AF. To eliminate this problem the authors introduce a new method for calibrating EMC antennas for MIL STD 461. The Method is based on the well-known extrapolation range technique. The process is to obtain the polynomial curve that is used to get the far field gain in the extrapolation gain procedure, and to perform an interpolation to get the gain at 1 m. The results show that some data in the far field must be collected during the extrapolation scan. When the polynomial is calculated the antenna performance values at shorter distances will be free of near field coupling. Measured results for a typical antenna required for emissions testing per the MIL STD 461 match well with the numerical results for the computed gain at 1 m distance. Future work is required to study the use of this technique for other short test distances used in other electromagnetic compatibility standards, such as the 3 m test distance used by the CISPR 16 standard. Keywords: Antenna Calibrations, EMC Measurements, Extrapolation Range Techniques

Comparison of Payload Applications in Near Field and Compact Range Facilities
Carsten Schmidt,Josef Migl, Alexander Geise, Hans-Juergen Steiner, November 2015

For satellite applications payload measurements are a crucial part of the radio frequency validation campaign before the launch. Parameters like Equivalent Isotropic Radiated Power (EIRP), Input Power Flux Density (IPFD), Gain over Noise Temperature (G/T), Gain over Frequency (G/F), Group Delay, and Passive Intermodulation (PIM) are to be measured in suitable facilities on satellite level. State-of-the-art payload measurements are conducted in compensated compact range facilities which offer a real-time test capability which is easy to setup and use. Closed link tests are straightforward to realize with two compact range feeds employing feed scanning. The measurement techniques as well as the error budgets are well known. Near-field facilities are widely used for antenna pattern measurements. However, there is not much literature available discussing in particular measurements of G/T, G/F, and Group Delay in the near field. Measurements of the above parameters in the near field seem to be feasible, however, the processing of the measured data has to be adapted and further calibration measurements are required. In this paper methodologies for payload parameter measurements in compact range and near field facilities will be described. A comparison of payload measurement campaigns in near field and compact range facilities will be drawn. The techniques will be compared in terms of measurement timing and effort, practicability for satellite applications, and achievable accuracies.

Development of a FMCW Radar Sensor For Soil Humidity Estimation
Maria C. Gonzalez,Christian Hurd, Jose Enrique Almanza Medina, Xiaoguang Liu, November 2015

To determine the proper moisture content in the soil is critical to get maximum grow in plants and crops and its estimation it is used to regulate the amount of irrigation that it is needed. For this reason, many sensors that measure water content have been developed to give the grower some feedback of the water content.   Some methods such as the ones based in gravimetric properties are accurate but labor consuming, other such as the tension meters require periodic service, the neutron probe is also accurate but expensive. The more popular sensor is based in electrical resistance measurement that gives acceptable accuracy and it is not expensive. However, this sensor has the disadvantage that needs to be buried in the soil. Here, we are exploring the characteristics of electromagnetic propagation and its scattering properties as a tool to identify the physical soil composition. The presence of water changes drastically the dielectric properties of the soil affecting the reflected signal. In this research, we are assessing the viability of a sensor based in FMCW radar technology for water detection with the advantage of being portable and low cost. The research involves the fabrication of a directive antenna operating in a broadband regimen, transmitter/ receiver circuit and the signal processing of the return signal adjusted to the detection of moisture in soil. We present the calibration methods and graphic results of the intensity of the reflected signal of dry bare soil, wet soil, and soil covered by plants.

Monostatic RCS Calibration of Radar Target Using Extrapolation Method in Millimeter-wave Frequency Band
Michitaka Ameya,Satoru Kurokawa, Masanobu Hirose, November 2015

In this paper, we propose a calibration method for monostatic radar cross section (RCS) of simple radar targets (e.g. trihedral corner reflectors and square flat-plate reflectors) using extrapolation method. By the proposed method, we can calibrate the monostatic RCS of radar targets from 1-port S-parameter measurements. In our system, the applicable size of radar targets are 75 mm to 125 mm for corner reflectors and 40 mm to 75 mm for square flat-plate reflectors, respectively. The nominal RCS of reflector targets calculated by physical optics ranges from +3 dBsm to +15 dBsm in W-band.  The measured results are agree well with simulation results calculated by method of moment (MoM).

Experimental In-Situ Antenna Array Calibration with Signals of Opportunity
Andrew L. Kintz,Inder J. Gupta, November 2015

We present experimental results for on platform (in-situ) calibration of an antenna array with signals that are treated as signals of opportunity. In-situ calibration is required for antenna arrays installed on vehicles as platform scattering significantly perturbs the radiation patterns of the antenna elements. In-situ measurement of the array response requires: determining location of the unknown signals; determining the array’s response in the direction of the signals; and synthesizing the array pattern from the measured data. For this work, a seven element L-band antenna array was mounted on a generic aircraft platform. The platform was mounted on a dual rotator setup and emitters were placed nearby.  The platform was then rotated while the emitters transmitted, and the signals received by the antenna elements were digitized.  The collected data was then post processed to obtain the array calibration.  We found that the calibrated array manifold enables more accurate direction of arrival estimation and provides additional gain in direction constrained beamforming. The present work serves as experimental verification of earlier simulation studies on in-situ array calibration.

Research on Unwanted Reflections in an OATS for Precise Omni Antenna Measurement
Donglin Meng,Xiao Liu, Dabo Li, November 2015

Open-area test site (OATS) is a basic range for measuring omni antennas at VHF/HUF band. Reflections from the trees nearby, from the edge of the metal ground plane of an OATS are researched with the aid of ultra-broadband calculable dipole antennas (CDAs). Usually, these reflections are detrimental to precise antenna measurements from 20 MHz to 1 GHz; however they are very difficult to analyze accurately, since no rigorous theory exists on the relationship between the reflections and the configurations of an OATS. For this difficulty, a pair of very accurate and broadband CDAs are manufactured and verified with a slightly modified near-field method, whose site-insertion-loss deviation (?SIL) between measurements and simulation is less than 0.3 dB over 10 MHz to 340 MHz for a single pair of dipole elements resonated at 90 MHz. Based on the optimized CDAs, the effects of ground plane sizes, wire mesh shapes around the edge of the metal ground plane, trees nearby and especially masts are researched. The research shows that the reflections from the edge of an optimized ground plane is less than 0.1 dB at 10 m range. Finally, the performance of an OATS with these optimizations is verified: for 10 m separation, ?SIL is within 0.26 dB at horizontal polarization (HP) and within 0.34 dB at vertical polarization (VP) for the typical 24 frequencies from 30 MHz to 1 GHz; at 20 m separation, ?SIL is within 0.59 dB (HP) and 0.85 dB (VP) from 20 MHz to 500 MHz. An example for the uncertainty of calibration the free-space antenna factor of tuned dipole antennas are provided, too.

A Comparison of Antenna Range Polarization Correction Techniques
Justin Dobbins,Jason Jerauld, November 2015

Antenna range calibration is commonly performed with the goal of obtaining the gain of an antenna under test.  The most straightforward calibration procedure makes assumptions about the polarization properties of the range illumination, which can lead to both polarization and gain errors in the measured patterns.  After introducing the concept of polarization correction we describe three published range polarization correction techniques and provide an example of polarization correction applied to a compact antenna test range measurement.  We then discuss the practical aspects of incorporating polarization correction into the range calibration workflow.

Slotted Waveguide Array Beamformer Characterization Using Integrated Calibration Channel
Akin Dalkilic,Caner Bayram, Can Baris Top, Erdinc Ercil, November 2014

In military applications, where low sidelobes and high precision in beam pointing are vital, a phased array antenna beamformer requires to be calibrated regarding the cabling that connects the beamformer to the antenna and mutual coupling between antenna elements. To avoid problems associated with mismatched phase transmission lines between the beamformer and the antenna and include the coupling effects, beamforming network characterization must be done with the antenna integrated to the beamformer. In this paper, a method to characterize the beamformer of a slotted waveguide array antenna in the antenna measurement range is introduced. The antenna is a travelling wave slotted waveguide array scanning in the elevation plane. The elevation pattern of the antenna is a shaped beam realized by a phase-only beamformer. The calibration channel includes serial cross-guide couplers fed by a single waveguide line. The channel is integrated to the waveguide lines of the antenna.  In the first phase of the characterization, the far field pattern of each antenna element is obtained from the near field measurements at the “zero” states of the phase shifters. In the second stage, all states of the phase shifters are measured automatically using the calibration channel described above. The results of calibration channel measurements are used to determine the changes in phase and magnitude for different states of phase shifters. The phase and magnitude of the peak point of the far field pattern is referenced to the zero state measurement of the calibration channel. Phase only pattern synthesis is carried out using the results of both zero-state near field and calibration channel measurements and the required phase shifter states are determined accordingly. Measured patterns show good agreement with the theoretical patterns obtained in the synthesis phase.







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