AMTA Paper Archive
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AMTA Paper Archive
Unifying G/T and Noise Figure Metrics for Receiver Systems
The conventional method for comparing the performance of antenna-receiver systems is the classical G/T metric. The G/T metric is the ratio of antenna-circuitgainrelative to the thermal noise temperature evaluated at the input of the low noise amplifier; the thermal noise at the input to the LNA consists of the received sky noise, the LNA's effective input noise temperature, and post LNA noise referenced to the LNA's input. While this has been a standard for many years, it will be shown that G/T does an incomplete job of describing the performance under all conditions. The noise figure metric was developed as a characteristic describing signal-to-noise degradation to be applied to circuit based input/output topologies, and cannot easily be applied to hybrid systems such as an antenna-receiver system in which the input power is described by spatial field density levels, and the output power is stated in terms of a circuit-based voltage-current environment. This paper presents a noise figure metric which has been expanded to include systems that are a hybrid of wave and circuit characteristics such as the marriage of an antenna and receiver. It will also be shown that whereas a system's noise figure is dependent upon a chosen noise reference temperature, the intrinsic Effective Input Noise Temperature of the system is an invariant that does not change when a different reference temperature is selected. It will also be shown that, in contrast to G/T, the effective input noise temperature of an antenna/receiver system will accurately predict the system's output SNR for all values of system input SNR. It will be shown in detail, how to measure the antenna/receiver system's Effective Input Noise Temperature (TE), resulting in the following equation: TE = (TD1 - Y£ TD2 )/(Y - 1) Where: TD1 , and TD2 are measured noise power densities at the face of the antenna, TE is the Effective Input Noise Temperature of the system, and "Y" is the classical "Y factor" metric.
Loadbox Design for EMC Testing in Automotive GNSS/SDARS Application
In this paper, a loadbox was developed to perform theconductive and radiative Electromagnetic Compatibility (EMC) emission and immunity testing of the Global Navigation Satellite System (GNSS) and Satellite Digital Audio Radio Service (SDARS). To perform these tests, the supplier must purchase and build bias-tee, lowpass filter, choke, Diplexer and coupling circuitry to develop a loadbox. This means that same product made by different suppliers have different test set-up in place and therefore variability in the testing which create uncertainties in the test results and product approval or rejection. This is not reliable and can cause large amount of money wasted or bad product pass through. In this work, we propose an integrated loadbox design that can be built in-house with low cost and provide unique solution across the board. A loadbox consisting distributed Printed Circuit Board (PCB) made diplexer was developed that is easy to fabricate with low cost high durability and reliability during EMC testing and keep all the EMC testing consistent across the board which is enabling factor for proper decision making. A dual band diplexer was realized to separate the combined signal coming from the LNA's output port to two separate GNSS and SDARS ports. At the GNSS and SDARS frequencies due to the short wavelength of the RF signals, inductors and capacitors can be implemented using different transmission lines widths and lengths in short, open, parallel and straight-line formation. Distributed diplexer was designed using ADS RF Momentum simulator tool from Keysight and fabricated on a 2-layers PCB, FR2 of thickness 0.787 mm and a copper thickness of 35 um with overall size of 5.8x3.1 cm. Simulated and measured s-parameter for all of diplexer ports are in good agreement with measured insertion loss of better than 1.9 dB, return loss of 11.4 dB and GNSS-SDARS isolation of 16.8 dB at the GNSS frequency band and measured insertion loss of better than 2.1 dB, return loss of 14.1 dB and SDARS-GNSS isolation of 10.9 dB at the SDARS frequency band.
Three Antenna Polarization Measurement Revisited
Three-antenna methods  are fundamental to modernantenna metrology. They enable the simultaneous determination of the on-axis polarizations and gains of three unknown antennas. For example, on-axis characterization of a probe antenna is necessary for the accurate far-field measurement of test antenna transmitting and receiving functions. Recently after renovation of antenna ranges, NIST has beeninvolved in an internal program to re-certify its polarizationcharacterization services. While reviewing the theory , werealized that a small modification to the standard algorithmcould improve the accuracy of the polarization determinationin many cases. Three-antenna techniques measure the antennas in pairswith one antenna of each pair rotating about its axis (Figure1). The ideal form of the measured signal is very simple (6). Previous methods , take an economical approach in which a minimal number of measurements are used to extractthe polarization parameters from the model. Some allow forthe averaging of multiple determinations to improve results. We propose, on the other hand, to use the discrete Fourier transform (DFT) to isolate the exp (¤i?) behavior in the data, . The pair-polarization ratios (8) are easily computedfrom this transform. References  and  only came tolight after our analysis was completed. Rather the drop theproject, we have decided to offer this note as a tutorial andto call attention to what appears to be an under-appreciatedapproach to polarization measurement. All of the above methods work well when the error signalis small. Otherwise, the global nature of Fourier interpolationis expected to yield advantages over any local analysis. This hypothesis is supported by the simulations discussed below. Data were simulated for a number of combinations of axialratio, tilt angle, and sense of polarization. Noise was added atvarious levels. NOTE: The abstract refers to a figure, equations, and references not included in the abstract for brevity but which are available upon request
Bi-static Reflectivity Measurements of Vulnerable Road Users using Scaled Radar Objects
The future of cooperative automated and connected driving lies in the fusion of multiple mobile wireless sensor and data transmission nodes, covering technologies like radar, cellular and ad-hoc communications, and alike. Current developments indicate enormous potential to increase the environmental awareness through joint communication and radar sensing. In this respect, future wireless channel models aim at including bi-static reflectivities of road users, depending on different illumination and observation angles, in the nearfield as well as in the far-field. The limitations of the measurement distances within anechoic chambers unavoidably induce nearfield effects, especially for electrically large radar objects like realistic road users, and conventional bi-static RCS calibration techniques would eventually fail. In order to model the transition from the nearfield to the far-field reflectivity of road users, this paper uses the object scaling approach, with combined measurements and electromagnetic simulations. Bi-static reflectivity measurements of selected vulnerable road users are described, from the chamber setup all the way up to data post-processing. The approach of electromagnetic object scaling is applied to such bi-static reflectivity measurements, and the results are evaluated and discussed in comparison with numerical simulations. Initial proof-of-concept measurements of differently sized metal spheres confirmed the applicability of the scaling approach under far-field conditions very convincingly. Based on this, scaled models of radar objects, namely a bicycle and a pedestrian, were 3D printed and then metallized with copper paint. Compared to corresponding electromagnetic simulations of the original bi-static reflectivity of the radar objects, the results measured for the scaled models show very promising agreement with the numerical expectation. This study contributes to the further development of future wireless channel models considering bi-static multipath components of different road users, being an indispensable prerequisite to enhance the safety in future road traffic.
Correction of the Measured Phase of the Radiation Pattern of Millimeter-Wave Antennas
To characterize the radiation characteristics of an antenna, determining the power pattern of the antenna is often sufficient. In some cases, however, both the amplitude and phase response are important. For instance, for accurate channel modeling, the antenna has to be de-embedded, requiring knowledge of the complex radiation pattern of the antenna. A vector network analyzer typically measures complex S-parameters, hence, determining the complex radiation pattern seems like a straightforward task. When measuring at higher frequencies, as the wavelength becomes shorter, antenna phase measurements are very sensitive to positioning and alignment errors. Using sophisticated measurement tools, the position and orientation of the antennas can be determined, and this information can be used to correct the measurement data. The stringent requirements on positioning and alignment at millimeter-wave frequencies, however, makes correcting the data based on physical insight, in some cases, a more practical solution. The results of a radiation pattern measurement of a WR-28 rectangular open-ended waveguide will be shown in the full paper. The magnitude of the radiation pattern is symmetric in its two principal planes, which is to be expected, but the phase of the radiation pattern is not symmetric. To explain this lack of symmetry, a two-parameter misalignment model will be presented. It will be shown that the measured phase is much more sensitive to the misalignment than the measured magnitude, explaining why the symmetry is only lacking in the measured phase. Based on the 1,708 available planar cuts, the two parameters in the misalignment model are determined with great confidence. Subsequently, the parameters are used to correct the phase of the measured radiation pattern, restoring the expected symmetry in the phase measurement.
A Genetic Algorithm Approach for Deriving Direction Finding Antenna Placement on Platforms
Placing a Direction Finding (DF) array onto an existing aircraft is typically a difficult endeavor due to the limitations placed by existing antennas or structures which mandate keep-out areas, the additional infrastructure required for potentially dispersed DF antennas, and getting all of the modifications for the DF antennas flight certified. Because of these challenges, along with the basic expense of modifying an aircraft for external antennas, the ability to optimize the antenna lay down for peak DF performance is absolutely essential. This paper will describe the use of a Genetic Algorithm (GA) in the application of defining antenna locations on a platform to form an optimized broadband Direction Finding (DF) array. For this optimization study, the Correlation Interferometry Direction Finding (CIDF) algorithm, will be used to assess candidate array solutions generated by the genetic algorithm. CIDF Correlation domain statistics such as main correlation beamwidth (proportional to DF accuracy), and correlation sidelobe levels (average relates to array robustness, maximum relates to potential for wild bearings) were used to assess each candidate array over the entire frequency band of interest. This paper will show how that the use of a genetic algorithm, with an optimization function based on CIDF correlation statistics, and a fitness function adjusted in population size and mutation rate, will yield the derivation of a robust DF antenna array configuration. This paper will derive the critical optimization and fitness functions, and then use examples of a large jet aircraft, a medium size business jet, and a small Unmanned Aerial Vehicle (UAV) to demonstrate the genetic algorithm capability to solve the DF antenna placement problem. As the reader may not be familiar with the theory of interferometric direction finding, or genetic algorithms, a brief tutorial will be provided in Sections I and III respectively.
CATR Reflector Measurement System with Multiple Reflectors for Multiple Angles of Arrival in Millimeter Wave Frequency Bands
This paper presents a novel method using multiple compact antenna test range (CATR) reflectors to simulate the Radio Resource Management (RRM) measurements required for 5G devices capable of beam-forming in the millimeter wave frequency range (i.e. FR2). Four CATR reflectors are arranged on a semi-circle with the device under test (DUT) on a dual axis positioner in the center of the intersection of four planar waves in order to generate five sets of two Angles of Arrival (AoA), thereby capable of simulating multiple basestations from different directions for the 5G device to monitor and perform handovers. The reflectors create far-field conditions at the device under test (DUT) such that quiet zones of up to 20-30cm in size can be achieved. Absorber baffles are strategically placed as to reduce scattering from adjacent reflectors. In addition to RRM measurements, one reflector can be used to also perform in-band RF beam characterization[JMFL2] while additional reflectors can measure out of band emissions at the same time, thereby decreasing total measurement times by a factor of 2-3 times.
Nearfield Antenna Measurements over Seawater - Challenges and Prospects
The question of how to perform a nearfield antenna measurement in the presence of the air-sea interface is one that has been raised previously by the author. When discussing spherical near field measurements various approaches have been proposed for addressing this problem, that are also applicable to measurements taken over a conducting ground plane. In this paper we shall discuss some of the practical challenges involved in data collection and measurement methods when performing this type of measurement. Examples shall be taken from both spherical nearfield measurements of simple sources along with single-point at-horizon measurements to examine the challenges associated with these approaches. A notional approach for measuring realized power gain at the horizon will also be discussed.
Open Source Antenna Pattern Measurement System
An open-source antenna pattern measurement system comprised of software-defined radios (SDRs), standard PVC tubing, and 3-D printer hardware will measure the radiation patterns of student-built prototype antennas. The antenna pattern measurement system developed at Weber State University (WSU) was inspired by the published work of Picco and Martin . Their low-cost and practical system utilized commercially-available 2.4 GHz wireless routers. Open-source firmware was loaded on the routers to access the received signal strength indicator (RSSI) data. The RSSI was recorded versus antenna-under-test orientation using National Instruments LabVIEW. The WSU antenna pattern measurement prototype utilizes wideband software-defined-radios to generate, transmit, and receive the test signal. Synchronous belts, gears, and 3-D printer parts were chosen and designed to address mechanical problems described by Picco and Martin. Position control is achieved using an Arduino microcontroller with open-source software developed for 3-D printer systems. Measured principal plane gain patterns for three antenna prototypes are compared to simulated results. Models were constructed using commercial Method-of-Moments (FEKO) for comparison. Measured Radiation pattern data was scaled to the simulated Gain values for a quarter-wave monopole over a finite ground plane, a Yagi-Uda directional antenna, and an air-backed circular microstrip patch antenna. The low-cost, open-source nature of the measurement system is ideal for undergraduate-level investigation of antenna theory and measurement. It is anticipated the SDRs will permit future research of modulation methods and encoding to improve measurements in non-anechoic environments.
Aircraft Antenna Placement Investigation Utilizing Measuered Sources in Simulation Model
Antenna placement or antenna in-situ performance analysis on large and complex platforms such as ships, airplanes, satellites, space shuttles, or cars has become even more and more important over the years. We present a systematic investigation of different antenna types for space applications in G- and S-band on an experimental aircraft. In this process, the individual antennas are measured with the help of a dual reflector compact antenna test range (CATR) under far-field conditions in various configurations. These results are validated and compared utilizing a finite element method (FEM) based solver simulation model. At first, the antennas are simulated and measured alone without any supporting or mounting structure. Subsequently, the effect of mounting structures on the overall radiation performance is added by analyzing the antennas over a large conducting ground plane, on top and the side of winglets, and on top of a cylinder body with dimensions on the order of the actual aircraft. For the detailed in-situ investigations, a second method of moments (MoM) based simulation tool is employed which works on measured sources. These measured sources are obtained from the CATR measurements of the isolated antennas. By means of a spherical wave expansion (SWE), they are transformed into a near-field source for the simulation model. These measured data based results are again compared and validated with the full FEM simulation for the complete aircraft setup and the simplified cylinder body. By this means, the expensive design and measurement of a full-scale electromagnetically equivalent mock-up of the aircraft could be saved. Furthermore, the pure simulation of the installed antenna performance often suffers from the limited availability of exact antenna design parameters. In some cases, the antenna design data remains undisclosed deliberately due to IP reasons. The presented results reveal the influence of physical structure on the radiation characteristics and demonstrate the benefits of working with measured data in simulation tools.
Reducing phase-measurement errors due to RF-source band breaks
A signal source can introduce phase-measurement errors when its output crosses through internal frequency-band breaks. The source phaselock circuits in this band-break region sometimes report approximate phaselock before complete phaselock occurs. The result of this approximate phaselock is a minor error in the output frequency, which can lead to phase-measurement errors at the system level. The magnitude of the phase errors depends on the amount of frequency offset and the difference in electrical lengths between the measurement system's signal and phase-reference paths. If this behavior were deterministic, then the resulting phase errors might be tolerable. Unfortunately, it was found that the final settling time (measured in many hundreds of milliseconds) was not consistent, depended in part on the two specific frequencies surrounding the band break, became more confused if a second sweep encountered the band break before the first break had settled, and of course changed behavior if the frequencies were sequenced in reverse order or measured one at a time. The design approach described herein reduced to negligible the phase-measurement errors due to frequency errors in two large multioctave test systems. The approach relies on managing range transmission line lengths so that propagation time is sufficiently equal among the various signal and reference paths. Measured data are presented that show the advantage of the optimized system design.
Polyhedral Sampling Structures for Phaseless Spherical Near-Field Antenna Measurements
In conventional Spherical Near-Field (SNF) antenna measurements, both amplitude and phase are necessary to obtain the Far Field (FF) of the Antenna Under Test (AUT) from the Near-Field (NF) measurements. However, phase measurements imply the use of expensive equipment, e.g., network analyzer, and rely on the assumption of having access to the reference phase, which is, for example, not the case in Over The Air (OTA) measurement scenarios. For these reasons, phaseless approaches gain attention and different methods have been investigated such as two-sphere techniques, indirect holography, or the use of different probes. Recent research on two-sphere techniques introduces algorithms originally developed for solving the so-called phase retrieval problem like PhaseLift or Wirtinger-Flow. Applied to SNF, the phase retrieval problem corresponds to obtaining the phase of the Spherical Mode Coefficients (SMCs) from amplitude NF measurements only. It has been shown that Wirtinger-Flow benefits from taking measurements over different structures, decreasing the redundancy. First investigations examined the combination of two spheres resp. a sphere and a plane and showed better reconstruction of the FF with the second combination. Furthermore, it has been shown that increasing the distance between both structures improves the reconstruction of the FF. Note that so far investigations have been based on the plane wave expansion. We currently deepen the knowledge presented above in a framework solely based on the spherical wave expansion. From a mathematical point of view, planes can be seen as spheres of infinite radius, i.e., a plane combined with a sphere may be interpreted as a special case of combining two spheres. This interpretation goes hand in hand with the observation that an increased radius difference between both spheres leads to better reconstruction performance. Consequently, we analyze different polyhedral sampling structures composed of planes (such as tetrahedrons or cubes), mimicking several spheres of infinite radius in different spatial directions. For the mathematical analysis of non-spherical structures in the basis of spherical waves, pointwise probe correction is used. First experiments show a better reconstruction of the FF compared to the standard two-spheres/sphere-plane sampling.
Utilization of Microwave Imaging for Chipless RFID Tag Reading and Verification
Chipless RFID is a subset of the RFID field where the tags possess no power source and no electronics. Information is instead stored in the structure of the tag and extracted by examining how the tag responds to an illuminating electromagnetic wave. These responses are most commonly viewed in the frequency-domain as a radar cross-section (RCS) vs. frequency response or as a complex reflection coefficient (S11) response. Binary codes are then assigned to the responses through a variety of procedures depending on the application and user preference. By manipulating the structure of the tag or the environment the tag is in, the response and therefore the binary code consequently experience changes. This mechanism is used to perform identification and sensing. While in simulation it is straightforward to extract the tag response, measurement poses additional challenges. These challenges include limited read range, extreme sensitivity to slight rotation or tilts of tags relative to the reader antenna, and noise in the response, all of which make it difficult to extract the response of tags and to verify proper tag performance. One sensing application of interest, is embedded materials characterization where the tag's response changes as a function of the dielectric properties of the material the tag is in. This work examines how microwave imaging with synthetic aperture radar (SAR) processing can be used to extract tag responses, verify tag performance (e.g., determine if tag manufacturing inaccuracies are present), and better understand tag environments in sensing applications. Through gaining a deeper understanding of the environment a tag is in (e.g., voids or material differences around a tag in an embedded application) during use in sensing applications, better models can be created. These models can then be used to help validate chipless RFID sensing approaches. Multiple tag designs - those with separated resonators and those with interlaced resonators - are utilized for this work to also understand the role and impact image resolution plays in the proposed techniques. This investigation is performed through a collection of simulations and measurements with a focus on using embedded chipless RFID tags for materials characterization applications.
Wideband Double-Ridged TEM Horn for Nondestructive Evaluation and Imaging Applications
Antenna performance plays a significant role in synthetic aperture radar (SAR) image quality, particularly for nondestructive evaluation (NDE) applications. To obtain high image quality and target detectability, SAR imaging systems should possess good resolution (cross- and along-range), and a relatively large penetration depth. Consequently, the antenna used must be wideband with a relatively wide beamwidth for high resolution and operate at low starting frequency for sufficient penetration depth. Meanwhile, antenna aperture size should be small rendering it sufficiently portable for scanning purposes or when employed within imaging arrays. However, increasing frequency bandwidth, reducing minimum frequency of operation while maintaining small aperture size (resulting in wide beamwidth), all at the same time is difficult. To this end, double-ridged horn (DRH) antenna, with flared aperture for improved radiation efficiency and performance is found to provide a good compromise among these parameters. Therefore, an improved modified design of DRH is proposed. The dimensions of its geometry are optimized to provide low unwanted reflections. Curved surfaces are attached at the end of the two ridged walls for better aperture matching. The final aperture size of the antenna is 230 ? 140 mm2, operating in the 0.5-4.0 GHz frequency range, and with a relatively wide beamwidth in its near-field region where most NDE imaging measurements are conducted. Measured reflection coefficient by using the fabricated antenna is used to verify the simulation results. Comparisons are also made with similar designs of DRH found in the literature showing that the proposed antenna has smaller electrical length with respect to the lowest operating frequency for designs without using absorbing material. Moreover, to conduct wideband SAR imaging, a new phase calibration method, using a small electric field monopole probe, to measure the phase change between the antenna aperture center and the input feed port for each frequency component is developed. Imaging results over a large concrete slab with delamination and voids simulated by foam and plastic sheets show that the proposed calibration approach works well, and the proposed antenna can effectively detect all of these defects with different scattering properties.
Single Antenna Dual Circularly-Polarized Chipless RFID Tag Reading Methodology
RFID technology can be classified as active, passive, or chipless based on tag design. While active and passive tags rely on electronics to modulate and return the irradiating signal, chipless tags rely on geometry to produce a distinct signal which is viewed in the time-, frequency-, or spatial-domain. Within the field of chipless RFID, frequency-domain tags are the most popular and different design approaches with different polarization schemes have emerged. Primarily, these tag design approaches can be categorized as linearly polarized (LP), orientation insensitive (OI), and cross-polarizing. This diversity in tag designs leads to a variety of requirements for reader antennas and also leads to current reader antennas being non-universal (i.e., reader antennas can only be used for specific types of tags rather than all tags). LP and cross-polarizing tags require that the reader antennas have their polarization be perfectly aligned with that of the tag, as a small tag rotation with respect to the reader can greatly affect the response. Cross-polarizing tags additionally require either a dual-polarized reader antenna or a bistatic measurement setup. While specialized chipless RFID reader antennas and bistatic reading schemes have been developed, there are still limitations with these approaches, such as requiring tag/reader polarization alignment, hardware complexity, mutual coupling, and other related issues in bistatic setups. Tag interrogation with circular polarization (CP), however, accommodates the polarization diversity of different tag designs, while also relaxing the tag/reader relative alignment requirements. This work proposes a novel chipless RFID tag reading methodology that utilizes a single existing dual CP X-band (8.2-12.4 GHz) septum polarizer antenna as a universal (i.e., all types of tags) frequency-domain reader antenna that can generate and receive both right-hand and left-hand CP, as well as LP (through mathematical manipulation). This antenna has been optimized for this application and its specifications are provided. Additionally, through post processing the rotation offset of LP tags can be determined, a capability which can then be used for rotation sensing. To demonstrate the tag reading methodology and the rotation determination capabilities of the method, simulation and measurement results are presented for LP and OI tags.
A Validated Model for Non-Line-of-Sight V2X Communications
As vehicle autonomy and active safety features become more advanced and ubiquitous, it becomes clear that vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication, together under the umbrella of vehicle-to everything (V2X), can help to enable these autonomous and active safety features by providing useful sensing and localized network capabilities. One problem facing the development of such communication networks is the difficulty involved in accurately predicting the key performance indicators (KPIs) associated with it - e.g., packet error rate (PER) and received signal strength indicator (RSSI) - for an arbitrary setup of antennas and occluders. In this paper we will present a model for V2X communications using a commercial simulation software, Altair's FEKO and WinProp suites, for predicting electromagnetic field intensity at microwave frequencies under different scenarios of antenna placement and occluder setup. We will also validate this model against field data collected using Cellular V2X (C-V2X) radios. We considered three distinct non-line-of-sight (NLOS) scenarios involving a pair of vehicles operating a CV2X inter-vehicle communication channel: 1. A stationary vehicle located directly behind a shipping container and a moving vehicle performing loops around it. 2. So-called urban canyon with tall metal walls on either side of the moving vehicle for a portion of its loop around the stationary vehicle. 3. Fully-covered tunnel, where both vehicles are moving, one leading the other around a loop and through a tunnel. We will discuss the simulations of these three scenarios in light of real-world data taken from field tests of identical scenarios, for the dual purposes of validating the simulation model against real-word data, and developing a model to predict the PER and RSSI at any theoretical receiver location. Together, these will allow us to perform a simulation for any arbitrary potential (NLOS) scenario and get an estimate of the channel PER and RSSI between any two spatial points, thereby help in developing standards for V2X communications, optimizing antenna placement for vehicles and infrastructure, and better understanding of these V2X systems overall.
Measuring G/T with a Spherical Near-Field Antenna Measurement System via the CW-Ambient Technique
In modern systems where RF front ends are tightly integrated, the parameters of passive aperture gain and active electronics noise figure become difficult to obtain, and, in many cases, impossible to measure directly. Instead, the parameter referred to as Gain over Noise Temperature, or G/T, becomes the performance metric of interest. Recently, the antenna measurement community has seen an increased demand to use near-field measurement systems for determining G/T values. Papers presented at AMTA over the past few years have shown that it is possible to determine G/T values using measurements taken in planar near-field antenna ranges. The CW-Ambient technique was one of the techniques proposed for computing G/T values by utilizing planar near-field measurements [1,2]. In this paper, we show how the CW-Ambient technique can also be applied to calculate G/T values in spherical near-field antenna measurement systems. This paper provides a brief summary of the CW-Ambient technique, and then presents the procedure and equations required for computing G/T using a spherical near-field system. To validate the recommended procedure, we compare predicted and measured G/T values for a separable unit under test (UUT). Since the passive aperture for this UUT is separable from the back-end active electronics, we measure the aperture gain of the UUT and the noise figure of the back-end electronics individually, and then compute the composite G/T value for this assembly. We then compare these composite values against G/T measurements from a spherical near-field antenna measurement system. We summarize these comparisons and provide conclusions regarding the validity of using a spherical near-field system to measure G/T.
Reduced Azimuthal Sampling for Spherical Near-Field Measurements
This paper investigates on the use of under-sampling over the azimuthal dimension to reduce measurement time on spherical near-field scanning. This means that the number of angular phi samples is reduced, which allows to reduce the number of positioner steps, obtaining measurement time savings virtually proportional to the number of samples reduction. Of course this under-sampling introduces an error, which can be interpreted as an aliasing term over the retrieved Spherical Wave Expansion of the Antenna Under Test (AUT). The axial symmetry of the vast majority of antennas allows the application of significant under-sampling ratios with little aliasing errors. However, this information is not a priori known due to the lack of a reference AUT radiation pattern, or in the case of malfunctioning antennas with degraded symmetry. Here we proposed a measurement procedure for the exploitation of the AUT axial symmetry. The procedure consists on an iterative AUT measurement with increasing number of azimuthal cuts. As the number of cuts increases, the aliasing error decreases, thus obtaining the final radiation pattern with a lower uncertainty. We will introduce an aliasing error estimator, which estimates the error caused by the under-sampling without any a priori knowledge of the AUT. This estimator can be used as a stopping criterion of the iterative measurement procedure when the desired accuracy is achieved. The proposed technique will be demonstrated using different antennas, showing considerable reductions in measurement time with low errors in the transformed far-field pattern, and with the guarantee that the error is below a given threshold thanks to the derived estimator.
Balun Design for CISPR 16-1-5 Calibration and Reference Test Site Verification
The CISPR 16-1-5 standard requires site attenuation (SA) measurements for the validation of Calibration Test Sites (CALTS) and Reference Test Sites (REFTS). CALTS validation requires horizontally-polarized SA measurements, while REFTS validation requires both horizontally- and vertically-polarized measurements. These measurements are made with tuned linear dipole antennas driven from coaxial transmission lines via balancing networks (baluns). According to the CISPR standard, the effects of the baluns are removed with a substitution measurement. Specifically, the baluns are connected back-to-back (balanced to balanced) with the elements removed and the port-to-port insertion loss then measured. This insertion loss is then subtracted from the port-to-port insertion loss with the antennas assembled and in place on the OATS. Thus, the measurement is a true RF substitution measurement. The baluns must be perfectly symmetric for this measurement to be sound. It is then accurate only if the baluns are very well matched simultaneously to both to the coaxial transmission lines and the dipole antennas. Essentially, the dipole-to-dipole transmission, the 2-port network which is substituted, would have to behave as a matched attenuator. In the CISPR standard SA measurements are made a a minimum of 24 specific frequencies between 30 and 1000 MHz. The height of the transmitting antenna above the ground plane in all cases is 2 m, but the height of the receive antenna varies in order to avoid a transmission null. For each one of these measurements it is possible to obtain a perfect match for each dipole antenna. However, the matching network would be different for each frequency and also for the different heights involved. Thus, there is impetus to use broadband baluns and resistive matching pads. If this approach is selected, neither dipole can be perfectly matched. Moreover, if the balun is required to operate over a broad bandwidth, it is difficult for itsperformance to be made so good that it could be considered ideal. By employing a full 4-port model for antenna-to-antenna transmission on an OATS between linear dipoles with imperfect baluns and thus unbalanced antennas, we assess measurement error for topologies of balun/attenuator combinations for the CISPR 16-1-5 SA measurements.
Examining and Optimizing Compact Antenna Test Ranges for 5GNR OTA Massive MIMO Multi-User Test Applications
Direct far-field (DFF) testing has become the standard test methodology for sub-6 GHz over the air (OTA) testing of the physical layer of radio access networks with the far-field multi-probe anechoic chamber (FF-MPAC) being widely utilized for the test and verification of massive multiple input multiple output (Massive MIMO) antennas when operating in the presence of several users. The utilization of mm-wave bands within the 5th generation new radio (5G NR) specification has necessitated that since the user equipment should, preferably, be placed in the far-field of the base transceiver station (BTS) antenna, excessively large FF-MPAC test ranges are required or, the user equipment is paced at range-lengths shorter than that suggested by the classical Rayleigh criteria or, a modified compact antenna test range geometry must be developed and utilized. This paper presents a novel design for a new compact antenna test range (CATR) design that uses a parabolic toroid as the main reflector. The folded optics utilized within this design possesses superior pseudo-plane wave scanning capabilities than those available from equivalent, classical, point source offset parabolic reflector CATR designs. This wide-angle scanning capability is a crucial feature for successful over-the-air testing and measurement of mm-wave 5G NR Massive Multiple Input Multiple Output (MIMO) antenna systems within multi-user applications providing 60 degrees of azimuth scan and 15 degrees of elevation scan to the incoming plane wave at the AUT. CATR quiet-zone results are presented, compared and contrasted with more classical designs before results of the effects of the CATR test channel on a number of commonly encountered communication system figures of merit in wide scanning cases are presented.
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