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Abstract—In order to optimize accuracy of near field measurements, it is required not only to acquire data for two orthogonal polarizations, but the relative amplitude and phase balance between the two channels must also be accurately matched. This can be difficult at millimeter wave frequencies because of the transmission lines and other components involved. ORBIT/FR has explored multiple methods of achieving optimum vertical and horizontal polarization matching and found a very simple solution to achieve acceptable results. Some of the methods investigated included the use of dual-polarized feeds, dual single-polarized feeds mounted adjacently, waveguide rotary joints with a mechanically rotated feed, and a mechanically-rotated feed using a 1.0 mm coaxial-based cable. Interestingly, the mechanically-rotated feed with coaxial cable provided acceptable results on par with or better than the other methods, which moreover results in a very simple implementation in the measurement system. Measured results are presented for the chosen implementation demonstrating the near field data quality is adequate for a variety of antennas.
Abstract— Antenna arrays works at their peak performance when they are well calibrated at the factory. Once they are employed in a real environment, they might be subject to unpredictable disturbances. That’s why recalibration after operational deployment is required but is usually not done due to practical difficulties. In some applications such as Direction Finding (DF), direction of arrival estimation is susceptible to the antenna model errors. However, the evolution of Direction finding antenna, as the strong integration of an antenna array mounted on a vehicle and the use of more efficient antennas tend to increase this type of disturbances. This paper proposes to evaluate the benefit of an in-situ measurement system for detecting and compensating the disturbance of antenna radiation. The influence of permanent scatters on one hand and variables (open door…) on the other hand in the vicinity of antenna array is investigated. We present a quantitative study of a biased calibration using a model combining 3D electromagnetic simulation, a complete receiver model and a MUSIC direction of arrival algorithm characterization. Two antennas arrays with same height are compared: a standard dipole array and an electrically small UWB antenna array.
In the present paper, a non-dedicated mass market GNSS antenna calibration method is discussed, with a special focus on the significant error component due to phase variations of receiving antennas in precise GNSS applications. Different calibration methods are compared from the literature; the indoor (anechoic chamber) calibration has been selected. The algorithm used to compute the mean Phase Center (PC) and its associated Phase Center Variation (PCV) for all angular directions is also described and has been validated on simulated canonical antennas. PC and PCV are then computed when four antennas are placed near the command unit of an unmanned aerial vehicle (UAV), which emulates the final application scenario. The impact of this structure is evaluated thanks to PCV cartographies. Two low-cost COTS antennas have been selected and their PCV maps are compared with regards to their geometry. Lastly, a reproducibility study based on the PCV characterization of ten copies of one of the selected COTS antennas concludes on the robustness of the PCV calibration.
The improved calibration model proposed in this paper is based on the traditional capacitance model which suffers from errors caused by the assumption that the capacitance is independent of frequency and the permittivity of the ambient medium under test. By analyzing the near-zone field of the coaxial opening, we introduce the new near-field capacitance to account for the dependency on the external permittivity. Simulation results show that the calibration error is significant reduced for low and moderate loss medium. And the calibration of the unknown coefficients simply requires the premeasurement of three known material including air, which provides convenience for the real field measurement. Measurement results obtained by a novel wideband in-situ coaxial probe are included to prove the accuracy improvement improved calibration model. by using this
Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.
Many modern antennas are incorporating LNAs into the aperture to maximize system receive performance. G/T (Gain over Temperature) quantifies the performance of these antenna systems. Historically, G/T measurements needed knowledge of absolute effective temperature of multiple noise sources, which is not practical in an anechoic chamber. A Y-factor method is presented which uses a reference antenna system with a known G/T to determine the G/T of the Antenna Under Test (AUT). This paper will review G/T, describe the measurement process, cover calibration of the reference antenna system and discuss error sources and their mitigation.
There are a number of near-field measurement situations where it is desirable to use a broad band probe to avoid the need to change the probe a number of times during a measurement. But most of the broad band probes do not have low cross polarization patterns over their full operating frequency range and this can cause large uncertainties in the AUT results. Calibration of the probe and the use of probe pattern data to perform probe correction can in principle reduce the uncertainties. This paper reports on a series of measurements that have been performed to demonstrate and quantify the cross polarization levels and associated uncertainties that can be measured with typical log periodic (LP) probes. Two different log periodic antennas were calibrated on a spherical near-field range using open ended waveguides (OEWG) as probes. Since the OEWG has an on-axis cross polarization that is typically at least 50 dB below the main component, and efforts were made to reduce measurement errors, the LP calibration should be very accurate. After the calibration, a series of standard gain horns (SGH) that covered the operating band of the LP probe were then installed on the spherical near-field range in the AUT position and measurements were made using both the LP probes and the OEWG in the probe position. The cross polarization results from measurements using the OEWG probes where then used as the standard to evaluate the results using the LP probes. Principal plane patterns, axial ratio and tilt angles across the full frequency range were compared to establish estimates of uncertainties. Examples of these results over frequency ranges from 300 MHz to 12 GHz will be presented.
This paper addresses the difficulties in measuring the broadband complex permeability of thin films using conventional stripline or microstrip permeameters and outlines a novel methodology to solve them. It is shown using full-wave simulations that several of the conventional assumptions made for extracting permeability from a permeameter are not justified. In particular, the proportionality factor used to relate the measured effective permeability to the actual film permeability is shown not to be a constant. Another typical drawback is the need for a known reference sample for calibration. By exploiting the analyticity of the function relating effective to true permeability we have come up with a general methodology to derive this proportionality function for permeameters free of the problems mentioned before. The validity of the method is confirmed with fullwave simulations. Moreover, this general approach can be applied to other similar test devices. A key issue in measuring a thin film’s permeability over a broadband frequency range is assuming that its permittivity is known. More often than not, this data is not available. We show a method to extract a film’s permeability without the need to assume or know its permittivity value. This is done measuring two identical films of equal widths.
This paper describes the development of a method based on measurements of the radar cross section (RCS) in the forward direction to determine the extinction cross section for the 2.5-38GHz frequency range using the optical theorem. Forward RCS measurements are technically complicated due to that the direct signal has to be subtracted from the total signal at the receiving antenna in order to extract the forward RCS. The efficiency of this subtraction as a function of time is evaluated. A traditional calibration method using a calibration target and a second method that does not require a calibration target are investigated and compared. The accuracy of the forward RCS measurements is determined using small spheres of different sizes. The spheres have a forward RCS that is straightforward to calculate with good accuracy. The method is also extended to polarimetric measurements on a small helix that are compared to theoretical calculations.
As part of a target simulator [1], a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance. A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in this band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive an orthomode transducer. A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a crosspolarization of -25 dB, exceeding the required performance. This paper provides an overview of the concept, a block diagram of the design, discussion of the calibration and measurement procedure, and a summary of the results achieved.
The focused beam measurement technique has proven to be a solid technique for free space measurement of electromagnetic material properties. This paper presents the use of the focused beam method to measure swept frequency antenna gain as well as antenna patterns. A calibration and signal processing procedure has been developed to properly handle the range of incident waves inherent in the Gaussian beam illumination. One disadvantage of this technique is that the size of the antenna under test is limited by the spot size of the focused beam. However, the GTRI focused beam system uses lenses that are easily reconfigured to realize various spot sizes. The advantage of the focused beam illumination is that the number of measurements and thus measurement time is reduced by roughly an order of magnitude when compared to spherical near-field scanning techniques. More importantly, focused beam systems can be used in a lab environment and do not require large dedicated chambers. We present both model/theory predictions and measured data of how a too-small spot size of the focused beam leads to systematically lower peak gain measurements and wider beam widths.
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.