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This paper describes test methods and challenges for performing radio frequency (RF) characterization of a phased array antenna with element-level digital beamforming using planar near-field (PNF) and compact range technologies. The characterization of a digital array requires the synchronization of measurement equipment including positioner controllers, transmitters, and receivers. All hardware and software must remain synchronized with the array clock to achieve accurate amplitude and phase samples and ensure a coherent phase front. This synchronization is achieved through handshake triggers and communication protocols that are managed through external software. The acquisition of element-level data over large PNF scans presents unique challenges in data and post-processing that precipitate the need for optimization of array architecture as well as design of processing software. Advantages of the digital array architecture include being able to generate multiple receive beams from a single near-field scan for each frequency and the ability to compare multiple calibration methods efficiently using off-array processing.
A novel probe design for measuring complex permittivity of soils in-situ from 10 to 1000 MHz without taking soil samples is presented. The dielectric constant and conductivity of soil is derived from step-frequency reflection taken inside a small freshly bored hole. As a result, permittivity at various depths with in-situ moisture content and soil texture can be obtained in the fields. A novel calibration method was developed to account for the frequency- and material-dependent geometrical factor which causes bias errors in conventional calibration methods. Experimental measurement results and simulation results are used to prove the efficiency and accuracy of this method.
A method is presented to accurately characterize the backscatter and attenuation properties of vegetation using high resolution measurements with the vegetation placed on a turntable. By this method we obtain a controlled scenario of realistic vegetation. To obtain high cross range resolution, 2D-ISAR technique was used. The full obtainable resolution is then defined by the registered bandwidth (2 GHz) and aspect angle width. 2D-ISAR images were produced from which areas of interest were gated out where the vegetation backscatter coefficient was calculated. This, along with antenna tapering compensation and distance compensation allowed us to accurately normalize the vegetation backscatter coefficient. The received signal power was made independent of range and system parameters by calibration. Hence the received power signal can be written to be only dependent on the backscattering radar cross sections. The resulting values of the volume backscattering and extinction parameters are presented for reeds and birch vegetation at HH and VV polarization.
Azimuthal mode (µ mode) truncation of a high-order probe pattern in probe-corrected spherical near-field antenna measurements is studied in this paper. The results of this paper provide rules for appropriate and sufficient µ-mode truncation for non-ideal first-order probes and odd-order probes with approximately 10dBi directivity. The presented azimuthal mode truncation rules allow minimizing the measurement burden of the probe pattern calibration and reducing the computational burden of the probe pattern correction.
Calibration of probes for planer near field range measurements is generally required to obtain accurate cross-polarization (xpol) data; however, probe calibration is costly and time consuming. Using analytical models in place of calibration is generally much more cost effective, but may result in larger measurement errors. In a previous paper [1], we showed that simple models of copol probe patterns with zero xpol can give accurate measured results, provided that the probe xpol is much better, generally 10-15 dB better, than the Antenna Under Test (AUT). The next question is “Can a lower performing (and cheaper) probe be used if both the copol and xpol probe patterns are modeled?” In this paper, we compute AUT xpol measurement errors that result from probe xpol errors, and we compare far field AUT patterns processed using probe models with patterns processed with calibrated probe files.
A spherical near-field measurement range at Nearfield Systems Inc. has recently been used to measure gain, pattern and polarization of a multi-element helix array operating in the UHF band. Verification of gain performance over the operating band was of primary importance and so major efforts were made to obtain the best possible gain results and to quantify the gain uncertainty through a complete error analysis. A single element helix gain standard was first calibrated and the estimated uncertainty in this calibration was 0.35 dB. A double ridged horn was to be used as the probe for the spherical near-field measurements and so the patterns of the horn at all test frequencies were measured on the spherical range using identical horns as the AUT and the probe. From these measurements, probe pattern files were generated that could be used to perform the probe correction in the measurements of the helix gain standard and the multi-element array. The helix gain standard was then installed in a new spherical near-field range at NSI with the double ridged horn as the probe. The range used a specially designed phi-over theta rotator that could support and rotate the array and maintain the required position accuracy. The chamber was lined with 36 inch absorber. Spherical measurements were then performed and the data processed to provide the far-field peak amplitudes at each frequency that were necessary for gain measurements. The far-field peak values are equivalent to the far electric field for the gain standard and are compared to the same parameter for the multi-element array to produce the final gain results. The helix array was then installed in the spherical range and a series of measurements were performed to produce the far-field gain, pattern and polarization results and also to provide the data for the complete 18 term uncertainty analysis. The uncertainty in the gain measurements was 0.45 dB and the axial ratio uncertainty was 0.11 dB.
A fully-polarimetric compact radar range operating at 240 GHz has been developed for obtaining Ku-band RCS measurements on 1:16th scale model targets. The transceiver consists of dual fast-switching, stepped, CW, X-band synthesizers driving dual X24 transmit multiplier chains and dual X24 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 45 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 27” FWHM quiet zone. In this paper a description of this 240 GHz compact range is provided along with an ISAR measurement example.
Wideband dual polarized probes are often used for modern high precision measurement systems. A desired feature of a good probe is that the useable bandwidth should exceed that of the antenna under test so that probe mounting and alignment is performed only once during a measurement campaign [1]. This paper describes a new field probe taking full advantage of the 1: 4 bandwidth of the Ortho Mode Junction (OMJ) overcoming the aperture size problem by applying different apertures on the same field probe. The apertures are circularly symmetric so the exchange of apertures can be performed rapidly without the need to repeat calibration and alignment procedures for the full probe.
Standard Gain Horns (SGH) are utilized frequently either as measurements antenna or as reference antenna in antenna gain measurements by comparison or substitution method [1]. They also find use as source antennas in anechoic test chambers and for many other purposes such as fixed site antennas. The most widespread SGH geometry has a rectangular cross-section and is pyramidal with optimized geometry to achieve maximum gain [2, 3, 4]. When used as a precision gain reference in antenna measurements the SGH is often calibrated by a reference facility or another third party. When external or internal calibration means are not available the SGH peak gain is often determined directly from the reference tables of the NRL report [2]. The quality of the original work is such that even today the associated uncertainty on these peak gain values are generally accepted to be within +/-0.3dB [1]. In this paper the accuracy of the NRL gain tables are investigated by comparison with a full wave numerical method based on FDTD [7] and measurements in different antenna test ranges. Performance variation of the SATIMO Standard Gain Horns due to the manufacturing and measurement accuracy has been also investigated with conducted and radiated experiments.
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
The calibration and measurement of bistatic reflectivity at short range (3.3 m) presents challenges that are significantly different from the usual test range measurements (typically monostatic at 100 to 150 m). In order to overcome this an object-free calibration procedure has been applied, eliminating crosstalk, reducing other interferences and removing errors associated with the RCS and alignment of calibration objects. It is based on calibrating the transmitter and receiver antennas as a pair by directing the antennas toward each other. The method thus requires that the antennas can be separated. Furthermore the signal level needs to be handled e.g. by the separation distance or attenuators. The bistatic reflectivity measurements were performed by placing a clutter sample on a turntable which is located at the centre of a bistatic arc. This configuration enables us to do ISAR images. Background contributions were discriminated using a combination of synthetic resolution and zero-doppler filtration. The sensitivity variation across the antenna footprint was handled by calculating an equivalent area from measured off-axis antenna sensitivities. Reflectivities have been measured for a metallic test surface and for grass. The metallic test surface had been manufactured to correspond to typical theoretical bistatic clutter models.
There are only a handful of commercially available antenna calibration laboratories in the US that are accredited to ISO-17025. Satimo has been operating an accredited laboratory in Atlanta since 2005 and an accurate and documented evaluation of measurement uncertainty has been a key element of the accreditation process. In order to develop the budgets, the parameters affecting the accuracy of the antenna measurement must be well understood. There are several references [2-9] that outline the method for preparing a measurement uncertainty budget, but few encompass the unique attributes associated with antenna measurements. Many of the papers published on the subject of the measurement uncertainty for antenna measurements address the characterization of a specific error term associated with an uncertainty budget, but few describe all of the terms contributing to the error budget nor how to practically determine their values. The intent of this paper is to outline and briefly describe the derivation of the uncertainty terms that contribute to the overall error budget for antenna measurements. Topics that will be discussed include: the uncertainty types, how to obtain or derive the error term for each uncertainty type, the distributions associated with each uncertainty type, the determination of the confidence level and coverage factor, how to combine the error terms as the references listed above are not in agreement on the method for combining the terms.
We report on the initial phase of our study to assess the electromagnetic interference and electromagnetic compatibility measurement and calibration procedures at the National Institute of Standards and Technology. We are developing a measurement-based uncertainty analysis of calibrations and measurements in the anechoic chamber. We intend to characterize all sources of uncertainty, which include power and probe-response measurements, noise, nonlinearity, polarization e.ects, multiple re.ections in the chamber, drift, and probe-position and probe-orientation errors. We present simple and repeatable measurement procedures that can be used to determine each individual source of uncertainty, which then are combined by means of root-sum-squares to state the overall measurement or calibration uncertainty in the anechoic chamber. We report on work in progress and future plans to characterize other EMI/EMC facilities at NIST.
Over-the-air (OTA) performance testing of wireless devices has become a major component of product qualification for today's wireless networks. The methodologies used currently expand on traditional passive antenna pattern measurement techniques and operate under the fundamental assumption that the radiation pattern of the DUT does not change for the duration of the test. Emerging wireless technologies such as MIMO that use multiple antennas and adaptive algorithms for communication violate that assumption. In addition, the enhanced feature set and higher complexity of these newer technologies means that there are other performance criteria to be evaluated beyond just radiated power and sensitivity. A new OTA test system has been developed that can simulate complex multipath systems in a fully anechoic environment. This technology allows evaluating the performance of the DUT in a host of simulated real-world environments. Existing spatial channel models used for conducted testing of these radios can be easily adapted to OTA testing. This paper describes the details of such a system, including requirements for calibration and validation, as well as showing typical test results and available metrics.
Polarimetric radar cross section systems are characterized by polarimetric system parameters Eh and Ev. These parameters can be measured with the use of rotating dihedrals. The full polarimetric dataset as a function of the angle of rotation can be analyzed with a nonlinear set of calibration equations to yield the system-parameter complex constants and the four polarimetric calibration amplitudes. These amplitudes appropriately reproduce the system drift and satisfy a drift-free system con.guration criterion very accurately. The results indicate that the nonlinear approach is better than the previously studied linear approach, which yielded system parameters that are seriously distorted by drift.
Accurate calibration of near-field measurements requires the probe used for the measurement be well characterized. The determination of the absolute gain of rectangular open-ended waveguide probes is difficult due to the broad beamwidth in both the E-plane and H-plane which increase the likelihood of multi-path affecting the accuracy of the measurement. Multi-path may be minimized by reducing the separation distance, but at the price that far-field conditions may no longer apply. A variation of the two matched antenna method is to use a large reflecting plate to form an image of the probe. Use of the entire bandwidth of the probe, and time-gating the results to isolate the signal reflected from the plate allows the gain to be determined. The procedure also allows the determination of the aperture reflection coefficient used by theoretical probe models used for pattern compensation in the near-to-far-field transformation.
The gain-transfer technique is the most commonly used antenna gain measurement method and involves the comparison of the AUT gain to that of another antenna with known gain. At microwave frequencies and above, special pyramidal horn antennas known as standard-gain horns are universally accepted as the gain standard of choice. A design method and gain curves for these horns were developed by the US Naval Research Laboratory in 1954. This paper examines the ability of modern numerical electromagnetic modeling to predict the gain of these horns and possibly achieve greater accuracy than with the NRL approach. Similar computational electromagnetic modeling is applied to predict the gain and pattern of open-ended waveguide probes which are used in near-field antenna measurements. This approach provides data for probes that are not available in the literature.
A broad band interferometer antenna was designed and manufactured by Saab Avitronics. Saab Aerotech has installed a test facility for calibration of the interferometer antenna. The main purpose of the facility is to measure the interferometric function of the antenna. The interferometric function of the antenna can be measured directly but this method puts very high demands on the test range performance. An alternative method where each element is centered on a short far-field range is evaluated and compared by measurement with a large compact range at Saab Microwave Systems. The paper also describes the design aspects when measuring broad band, broad beam interferometer elements together with the actual design of critical components such as positioners, RF-system and absorber treatment.
A new simple approach is presented to calibrate the gain of standard gain horn antennas operating in the millimeter-wave frequency band. In terms of calibration, it is difficult to accurately measure the gain of standard gain horn antennas in the far-field region due to the space limitation. Therefore, near-field measurement methods are generally used to calibrate the gain of standard horn antennas. The extrapolation range method is one of the most accurate measurement methods in the near-field region. In the conventional extrapolation range method, a moving average process is applied to remove multiple reflections between antennas. Moving average can only remove multiple reflections between antennas. Therefore, electromagnetic absorbers are required to remove other reflections increasing measurement uncertainties. The time-domain gating method in extrapolation range allows us to remove all reflection waves, and achieve accurate antenna gain calibration without absorbers. The time-domain gating also reduces the number of measurement positions in the extrapolation ranges and obtains the gain of antennas in wide frequency ranges. In this paper, we compare the theoretical value with the time-domain gating method without absorbers by measuring three W-band standard gain horn antennas.
Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) are the two metrics most commonly used to characterize the over the air (OTA) performance of a wireless device. Measurement of these quantities requires a reference measurement of the loss from the origin of the spherical coordinate system to the power measurement device. Calibrated dipoles are typically used as gain standards for the reference measurement. These narrow bandwidth dipoles can provide low uncertainty reference measurements, but numerous dipoles are required to cover all of the wireless frequency bands. Since typical wireless measurement systems must be calibrated from 700MHz to 6GHz, calibration of the measurement system with narrow bandwidth dipoles becomes a tedious and time-consuming exercise. Broadband gain standards can be used, but due to the uncertainty in their absolute gain and their interaction with the measurement system, these add uncertainty to the reference measurement. This paper reports on a broadband gain standard and a measurement procedure that allow an extremely fast reference measurement while at the same time does not appreciably increase the uncertainty of the reference measurement.