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Rock core specimens collected during surveys for oil drilling have, in a standard form, a 4" diameter. Cores are cut in half or in 1/3-2/3 sections to provide core slab. We developed a measurement procedure based on spot probe illumination to characterize geological and/or geochemical properties of core slab specimens via their complex permittivity for frequencies between 2.5 GHz and 20 GHz. Conventional reflectometer methods are based on illumination of a thin slab of air-or metal-backed material. However, in this case only the front surface is flat and the back surface is semicircular. A measurement method was developed based on time-domain gating to separate the back-surface reflection from that of the front. Material inversion is then based on the amplitude and phase of the reflection just from the front surface. This paper presents details of the calibration for this reflectometer measurement method, along with example measurements of core slab materials. Two different inversion methods are applied to these measured data. The first is a more conventional frequency-by-frequency method for inverting complex permittivity from the amplitude and phase of the reflection. The second method applies a physical model, the Debye relaxation model, to the data. This model-based approach minimizes the errors from edge diffraction from the small sample size.
We use a simple program to compute fields radiated by a collection of elementary electromagnetic dipoles located at arbitrary points within the measurement sphere. The simulated measurement data have been used to provide a direct and convincing demonstration of the accuracy and robustness of both the standard and position compensated NIST SNF code.
Shipboard phased array radar antennas typically have high gain, low sidelobe specifications, and testing after initial production, overhaul or repair often reveals sidelobes that fail specifications, requiring rework. Further, some systems only allow phase adjustments as a means to fine tune the pattern. To correct sidelobe failures in these systems, the phase distribution of the array is first mapped using near-field scanning techniques, then specific element phases are adjusted, such as by using phase shifters. The standard method of determining phase changes has been based on trying to achieve a nominal phase profile; however, this method does not allow targeting specific problematic sidelobes. The authors have developed a novel method, dubbed "Whack-a-Lobe", which targets suppression of specific sidelobes while minimizing other impacts to the pattern. Recognizing that far-field sidelobes are a summation of complex vectors of the individual elements in the direction of the sidelobe, the authors have developed a cross product technique that identifies elemental vectors orthogonal to a far-field sidelobe vector such that only a minimal phase change to these elemental vectors is needed to reduce the sidelobe level. This technique is targeted, deterministic, and reduces tuning cycles, labor hours and antenna test chamber time.
Antenna gain is the product of directivity and antenna loss. Antenna gain is typically measured by comparing the antenna under test (AUT) to a standard gain horn (SGH) or direct gain measurement using a calibrated probe. This requires an accurate account of power into the AUT and SGH, the loss of all test cables and switches must be measured to obtain an accurate AUT gain. Additionally, SGH calibration uncertainty reduces the quality of the measurement. The gain measurement technique describe here exploits the near-field range capability of accurately producing the pattern of high gain antennas. The near-field range allows the full wave capture of antenna aperture fields and transformation to the far-field with high resolution. The new technique uses the directivity obtained by integrating the far-field pattern, accounts for the spill-over energy not measured by the near-field range, and uses measured network losses of the AUT. It does not require measured losses of test cables and switches. Since AUT losses are typically measured as part of antenna integration the technique reduces overall measurement burden. Accurate calculation of spill-over energy is the key to success. The technique has been shown to yield better accuracy than the typical gain calibration method for multi-beam high gain antennas.
Testing of automotive antennas are commonly performed in large Spherical Near Field (SNF) ranges [1-3] able to host the entire vehicle to test the effect of the antenna coupling with the structure [3]. The impact of a realistic ground, such as asphalts or soil, on the radiation performance of the vehicle mounted antennas is often a desired information. As long as the free-space response of the vehicle is available, such information can be obtained with fairly good accuracy considering post-processing techniques based on the Image Theory (IT). Automotive systems with absorber material on the floor [3] are thus ideal for estimating such effects because the free-space signature of the vehicle is directly measured and because the radiation pattern is usually available on more than just a hemisphere. In this paper an IT-based technique which allows for the estimation of a realistic ground is proposed and validated with simulations where the measurement setup of a typical multi-probe free-space automotive system is emulated. The impact of the truncation of the scanning area is analyzed in detail showing how advanced post-processing techniques [4-6] can be involved to mitigate the truncation errors and thus obtain a better estimation of the realistic ground effect.
Time domain gating is a well-known technique to remove or isolate responses in a multiple reflective environment. It is well known that time domain gating algorithm can cause unreliable data near band edges, known as band edge effects. The edge effects are caused by discontinuities at the frequency band edges. Even with mitigation techniques, such as windowing and post-gate renormalization (commonly used in commercial Vector Network Analyzers), edge effects can still be significant. In this paper, we propose a method based on spectral extension, referred to as Spectrum Extension Edgeless Gating (SEEG). Frequency domain data is first extended beyond the edges in a smooth and physically meaningful manner before applying time domain gating. In a typical antenna measurement application, it is shown that the SEEG method reduces gating edge errors significantly.
Planar near-field ranges are popular facilities to evaluate far-field antenna patterns. These ranges typically have the scanner plane parallel to the Antenna Under Test (AUT). Having the scanner plane parallel to the AUT can limit the maximum far-field angles that can be properly measured due to the mechanical extents over which the range can accommodate. This paper summarizes a test approach where the AUT is rotated in the near-field such that sufficient energy is concentrated within the range extents, ultimately resulting in an accurate far-field pattern. Measured results will be shown which demonstrate the limitations of the current testing approach, as well as the benefits of the near-field rotation approach.
This paper documents the various elements of the 2017/18 upgrade and presents results from the performance validation measurements with the DTU-ESA 12 GHz Validation Standard antenna conducted before and after the upgrade. The upgrade concerned several major improvements to the building infrastructure, the ventilation system, the antenna positioner, and the probe positioner. The validation measurements involved the averaging of measurements at different distances between the antenna under test and the probe to compensate the multiple reflections between these. This in turn necessitated the investigation of the compensation of the system drift between the measurements and of the sensitivity of the probe calibration to the position of the probe on the probe positioner.
NB-IoT (Narrowband Internet of Things) is a narrowband radio technology showing very different characteristics compared with traditional wireless protocols. For the first time based on authors' best knowledge, this paper compares the Over-the-Air (OTA) performance of NB-IoT in the Reverberation Chamber (RC) and Anechoic Chamber (AC), which involves two major RF test environment variations in the OTA test arena. In this paper, the Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS), related to the transmitter and receiver performance of NB-IoT, respectively, are investigated. For TIS test, an early exit algorithm with 95% confidence level based on Chi-Square distribution has been developed to improve the test speed. The test results show a good match (Within CTIA allowed measurement uncertainty) between AC and RC. Our analysis also includes several key parameters, such as test repeatability, measurement uncertainty, and test time, which gives a comprehensive comparison of different aspects between RC and AC for NB-IoT OTA test. It could be noticed as well that the early exit algorithm based on Chi-Square distribution improves the test time performance significantly without compromising the test accuracy.
This paper presents the second phase of the development of a new measurement technique to determine antenna system noise temperature using data acquired from a planar near-field measurement. In the first phase, it was shown that the noise temperature can be obtained using the plane-wave spectrum of the planar near-field data and focusing on the portion of the spectrum in the evanescent region or "imaginary space". Actual evanescent modes are highly attenuated in the latter region and therefore the spectrum in this region must be produced by "errors" in the measured data. Some error sources such as multiple reflections will produce distinct localized lobes in the evanescent region and these are recognized and correctly identified by using a data point spacing of less than /2 to avoid aliasing errors in the far-field pattern. It has been observed that the plane wave spectrum beyond these localized lobes becomes random with a uniform average power. This region of the spectrum must be produced by random noise in the near-field data that is produced by all sources of thermal noise in the electronics and radiated noise sources received by the antenna. By analysing and calibrating this portion of the spectrum in the evanescent region the near-field noise power can be deduced and the corresponding noise temperature determined. In the current phase of tests, planar near-field data has been acquired on a measurement system and the analysis applied to determine the system noise parameters. Measurements have been performed with terminations inserted at three different locations in the RF receiving path: the IF input to the receiver, the input to the mixer and the input to the probe that is transmitting to a centre-fed reflector antenna. The terminations consist of either a load that serves as the "cold" noise source or a noise source with a known noise output for the "hot" noise source.
In this paper, we present a chip antenna in the 27GHz band, targeting 5G measurements. This antenna can be used as reference in mm-wave measurement systems, such as the MVG µ-Lab, feeding the antenna under test through a micro-probe station. The reference antenna is employed to calibrate in gain through the substitution method. The antenna shown in this paper is an array of four patches, fed through a strip-line beam forming network. A transition strip-line to coplanar waveguide allows the antenna be fed by the micro-probe.
Indoor antenna measurement facilities are usually dedicated to characterize all the parameters of an antenna. In order to perform phase center position measurements, the CEA has designed a specific experimental layout to characterize this parameter with a very high accuracy. This paper describes this measurement facility and deals with technical decisions made during its design phase. Finally, we will talk about possibilities offered by this specific layout and the advantages of this layout compared to a classical antenna test-bench.
This paper presents a time domain antenna measurement technique by using a low cost software defined radio receiver. The technique aims to resolve measurement challenges derived from antennas where the reference signal is not accessible. The phase reconstruction implemented in this work is based on calculating the Fast Fourier Transform of the time domain signal to estimate the power spectrum and the relative phase between measurement points. In order to do that a reference antenna is used to retrieve the phase, providing a full characterization in amplitude and phase of the electric field and allowing source reconstruction. The results demonstrate the potential of this technique for new antenna measurement systems and reveal some of the limitations of the technique to be optimized, like the undesired reflections due to the interactions between the probe and the reference antenna.
The Low frequency Coaxial Reflectometer is the recommended procedure to measure the absorbers' reflectivity as per the IEEE 1128-1998 standard. The standard recommends the operable frequency range up to 500 MHz with a permissible error of 2 dB and higher error beyond 600 MHz. This paper studies and discusses the error on different types of absorber. Each of the absorber type is simulated in the square section of the reflectometer setup to compute the absorber's reflectivity using Ansys HFSS. An effective time gating technique is applied to reduce the effect of edge effects. These results are compared to the unit cell simulation results with a plane wave excitation and periodic boundary conditions. The absorbers are then simulated in the complete reflectometer setup to include the mismatch associated with the transition and compared to the unit cell model results. The errors associated with the comparison of the absorbers' simulation results for these different models are analyzed. The combination of these different absorbers is simulated in unit cell model. The absorbers are placed in different regions and orientations inside the reflectometer. The comparison between the unit cell results of the combination of the absorbers and the results of the absorbers inside the reflectometer in different orientations give the effect of the non-uniform field distribution inside the reflectometer.
The need for thermal stability in a test chamber is a well-established requirement to maintain the accuracy and repeatability sought for high frequency planar near-field (PNF) scanner measurements. When whole chamber thermal control is impractical or unreliable, there are few established methods for maintaining necessary precision over a wide temperature range. Often the antenna under test (AUT) itself will require a closed-loop thermal control system for maintaining stable performance due to combined effects from transmission heat dissipation and the environment. In this paper, we propose a new approach for near-field system design that leverages this AUT stability, while relaxing the requirement of strict whole chamber thermal control. Fixed reference monuments strategically placed around the AUT aperture perimeter, when measured periodically with a sensing probe on the scanner, allow for the modeling and correction of the scanner positioning errors. This process takes advantage of the assumed stability of the reference monuments and attributes all apparent monument position changes to distortions in the scanner structure. When this monument measurement process is coupled with a scanner structure that can tolerate wide thermal variations, using expansion joints and kinematic connections, a robust structural error correction model can be generated using a bilinear mapping function. Application of such a structure correction technique can achieve probe positioning performance similar to scanners that require tightly controlled environments. Preliminary results as well as a discussion on potential design variations are presented.
Prior literature in the subject area of far-field antenna measurements has demonstrated an extrapolation technique to isolate and correct the errors due to near-zone proximity effects as well as multi-path range reflections, thus allowing data to be collected at distances much less than the conventionally defined far-field criteria. This paper describes a modern, indoor, far-field antenna measurement range specifically designed to support this extrapolation technique. A multi-axis positioning system featuring a mobile horn tower capable of motion along the chamber Z-axis is emphasized. High-speed RF instrumentation and advanced software control support the full automation of the extrapolation method. This contemporary approach is demonstrated, and measurement examples are provided for an X-band slotted waveguide array. The resultant far-field gain calculations are also compared to similar data extracted using near-field scanning techniques.
An ambitious resurfacing campaign was launched in late 2017 to correct for large reflector surface distortions present at the NASA LaRC Experiment Test Range (ETR) in support of performing Europa Clipper flight High Gain Antenna (HGA) measurements at X-and Ka-band frequencies. The effort was successful as the worst case peak-to-peak amplitude ripple was reduced from 4.0-dB to 1.5-dB across the 4.1-meter quiet zone.
The current method of measuring system G/T performance is with the use of celestial noise sources (sun and cold sky). This paper details a method using man-made noise sources to measure system performance within an anechoic chamber, followed by an outdoor measurement to obtain G/T performance in a real world operational environment. A simple method is presented and equations derived that relate system performance in unknown environments to performance with known noise sources.
The performance of a compact antenna test range is evaluated by field-probing measurements of the quiet zone. The comparison between the simulated and measured data, however, is misleading due to the finite measurement accuracy and the limited nature of the numerical model. In order to allow a comparison, the uncertainty terms of the field-probing measurements and the numerical model are identified based on the National Institute of Standards and Technology 18-term uncertainty analysis technique. The individual terms are evaluated with simulations or measurements using the equivalent-stray-signal model. Bearing the differences between the model and the actual measurements in mind, the electrical field can be estimated precisely within the overlapping region of both uncertainty budgets.
This paper presents a new measurement method based on the parallel plate capacitor concept, which determines complex permittivity of dielectric sheets and films with thicknesses up to about 3.5 mm. Unlike the conventional devices, this new method uses a greatly simplified calibration procedure and is capable of measuring at frequencies from 10 MHz to 2 GHz, and in some cases up to 6 GHz. It solves the parasitic impedance limitations in conventional capacitor methods by explicitly modeling the fixture with a full-wave computational electromagnetic code. Specifically, a finite difference time domain (FDTD) code was used to not only design the fixture, but to create a database-based inversion algorithm. The inversion algorithm converts measured fixture reflection (S11) into dielectric properties of the specimen under test. This paper provides details of the fixture design and inversion method. Finally, example measurements are shown to demonstrate the utility of the method on typical microwave substrates.