AMTA Paper Archive
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Dual Polarized Near Field Probe Based on OMJ in Waveguide Technology Achieving More Than Octave Bandwidth
In classical probe-corrected spherical near-field measurements, one source of measurement errors, not often given sufficient consideration is the probe [1-3]. Standard near-field to far-field (NFFF) transformation software applies probe correction with the assumption that the probe pattern behaves with a µ=±1 azimuthal dependence. In reality, any physically-realizable probe is just an approximation to this ideal case. Probe excitation errors, finite manufacturing tolerances, and probe interaction with the mounting interface and absorbers are examples of errors that can lead to presence of higher-order spherical modes in the probe pattern [4-5]. This in turn leads to errors in the measurements. Although probe correction techniques for higher-order probes are feasible , they are highly demanding in terms of implementation complexity as well as in terms of calibration and post-processing time. Thus, probes with high azimuthal mode purity are generally preferred. Dual polarized probes for modern high-accuracy measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. As a desired feature of modern probes 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. Consequently, the probe design is a trade-off between performance requirements and usable bandwidth. High performance, dual polarized probe rely on balanced feeding in the orthomode junction (OMJ) to achieve good performance on a wide, more than octave, bandwidth [5-7]. Excitation errors of the balanced feeding must be minimized to reduce the excitation of higher order spherical modes. Balanced feeding on a wide bandwidth has been mainly realized with external feeding network and the finite accuracy of the external components constitutes the upper limits on the achievable performance. In this paper, a new OMJ designed entirely in waveguide and capable of covering more than an octave bandwidth will be presented. The excitation purity of the balanced feeding is limited only by the manufacturing accuracy of the waveguide. The paper presents the waveguide based OMJ concept including probe design covering the bandwidth from 18-40GHz using a single and dual apertures. The experimental validation is completed with measurements on the dual aperture probe in the DTU-ESA Spherical Near-Field facility in Denmark. References: Standard Test Procedures for Antennas, IEEE Std.149-1979 Recommended Practice for Near-Field Antenna Measurements, IEEE 1720-2012 J. E. Hansen (ed.), Spherical Near-Field Antenna Measurements, Peter Peregrinus Ltd., on behalf of IEE, London, UK, 1988 L. J. Foged, A. Giacomini, R. Morbidini, J. Estrada, S. Pivnenko, “Design and experimental verification of Ka-band Near Field probe based on wideband OMJ with minimum higher order spherical mode content”, 34th Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2012, Seattle, Washington, USA L. J. Foged, A. Giacomini, R. Morbidini, “Probe performance limitation due to excitation errors in external beam forming network”, 33rd Annual Symposium of the Antenna Measurement Techniques Association, AMTA, October 2011, Englewood, Colorado, USA T. Laitinen, S. Pivnenko, J. M. Nielsen, and O. Breinbjerg, “Theory and practice of the FFT/matrix inversion technique for probe-corrected spherical near- eld antenna measurements with high-order probes,” IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2623–2631, Aug. 2010. L. J. Foged, A. Giacomini, R. Morbidini, "Wideband dual polarised open-ended waveguide probe", AMTA 2010 Symposium, October, Atlanta, Georgia, USA. L. J. Foged, A. Giacomini, R. Morbidini, “ “Wideband Field Probes for Advanced Measurement Applications”, IEEE COMCAS 2011, 3rd International Conference on Microwaves, Communications, Antennas and Electronic Systems, Tel-Aviv, Israel, November 7-9, 2011.
Polarimetric Weather Radar Antenna Calibration Using Solar Scans
The US NEXRAD weather surveillance Doppler radar (WSR-88D) was recently upgraded to polarimetric capability. This upgrade permits identification of precipitation characteristics and type, thus providing the potential to significantly enhance the accuracy of radar estimated rainfall, or water equivalent in the case of frozen hydrometeors. However, optimal benefits are only achieved if errors induced by the radar hardware are properly accounted for through calibration. Hardware calibration is a critical element in delivering accurate meteorological information to the forecast and warning community. The calibration process must precisely measure the gain of the antenna, the Polarimetric bias of the antenna, and the overall gain and bias of the receive path. The absolute power measurement must be accurate to within 1 dB and the bias between the Polarimetric channels must be known to within 0.1 dB. These requirements drive a need for precise measurement of antenna characteristics. Engineers and scientists with the NEXRAD program employ solar scanning techniques to ascertain the absolute gain and bias of the 8.53 m parabolic center fed reflector antenna enclosed within a radome. They are also implementing use of daily serendipitous interference strobes from the sun to monitor system calibration. The sun is also used to adjust antenna gain and pedestal pointing accuracy. This paper reviews the methods in place and under development and identifies some of the challenges in achieving the necessary calibration accuracies.
Revising the Relationships between Phase Error and Signal-to-Noise Ratio
Within RF measurement systems, engineers commonly wish to know how much phase ripple will be present in a signal based on a given signal-to-noise ratio (SNR). In a past AMTA paper (Measurement Considerations for Antenna Pattern Accuracy, AMTA 1997), John Swanstrom presented an equation which demonstrated how the bound on the phase error could be calculated from the peak SNR value. However, it can be shown that the Swanstrom bound is broken when the signal has a peak SNR value of less than approximately 15 dB. This paper introduces a new equation that bounds the maximum phase error of a signal based on the signal’s peak SNR value. The derivation of this new bound is presented, and comparisons are made between the old Swanstrom bound and the new bound. In addition, the inverse relationship (i.e., calculating the SNR value of a signal from phase-only measurements) is investigated. In the past, analytical equations for this relationship have been presented by authors such as Robert Dybdal (Coherent RF Error Statistics in IEEE Trans. on Microwave Theory and Techniques) and Jim P.Y. Lee (I/Q Demodulation of Radar Signals with Calibration and Filtering in a Defense Research Establishment Ottawa publication). The analytical equations for calculating the SNR value using phase-only measurements are reviewed and discussed, and a brand new numerical relationship based on a polynomial curve fitting technique is proposed.
Calibration of Multi-Channel Adaptive Array Receivers
In array signal processing; i.e. direction of arrival estimation, digital beam forming/ nulling one needs to know the antenna array manifold (relative gain and phase of individual antenna elements) as well as the relative response of RF front end of the various channels. Here the RF front end is defined as LNA, filters, various down convertors and A/Ds. Since RF front end response is highly dependent on the physical environment, in general, nearly real time calibration of the RF front end is carried out. To accomplish this, a pilot signal is injected in various channels or a strong signal is received from a known direction. The received pilot is isolated, and the isolated signal is processed for relative calibration of the various front ends. One simple processing technique is to divide the signal received at a given frequency by the channel of interest with the signal received by the reference channel at that frequency. This technique works fine when the pilot signal has high SNR (20 dB or so). For low SNR, the front end calibration will have large variance and also bias, which is undesired. One can also cross correlate the signal received by the two channels at the frequency of interest, and normalize the cross correlation with the autocorrelation of the signal received by the reference channel. We will show that for low SNR of the pilot signal, this approach can lead to bias in the estimated relative magnitude. We will also present a novel approach that leads to unbiased estimate with small variance. The approach is based on signal space idea. At the given frequency, we generate a covariance matrix that contains the correlation between the signals received by various channels. Next, the principle eigenvector (corresponding to the largest eigenvalue) of the covariance matrix is calculated. The eigenvector is adjusted such that its element corresponding to the reference channel is unity. Then the other elements of the eigenvector yield the relative response of the front end of various channels. We have applied the suggested approach to real world data with very good results. Some examples are discussed in the paper.
Gain and Phase Center Calibration of Log Periodic Dipole Arrays using Complex Fit Algorithm
Abstract – This paper introduces a method for calibrating the gain and the frequency dependent phase center locations of Log Periodic Dipole Arrays (LPDAs). The method builds upon the three antenna method, but is conducted over a PEC ground plane in an Open Area Test Site (OATS). Similar to the traditional three antenna method, three pairings of transmission measurements are taken. In each measurement, one antenna is set at a fixed height above the ground plane, while the other antenna is scanned in height over 1 to 4 m heights. Magnitude and phase responses between the two antennas are taken at multiple heights. Measured results are fit to a theoretical model using a complex fit algorithm. From this process, the gain and frequency dependent phase center locations of each antenna can be solved. Measurement data show that it is effective in reducing systematic uncertainties associated with assuming fixed phase center locations. In addition, unlike other calibration methods over a conducting ground plane, no assumptions are made about the antenna patterns. This method provides an accurate, versatile and fast method for calibrating LPDAs from as low as 100 MHz.
Absolute GPS Antenna Calibration at the US National Geodetic Survey
In this paper we describe the NGS calibration facility and calibration hardware, and discuss the motivation for providing calibration services. We provide the time-difference, single-difference carrier phase observable models and estimation strategy currently used to generate NGS absolute calibrations. Calibration examples are provided in the accompanying poster presentation.
Millimeter Wave Polarization Calibration for Near-Field Measurements
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.
Benefit of a monitoring system in-situ for direction finding 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.
Low-cost GNSS Antennas Phase Center Variations Characterization for UAV Attitude Determination Application
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.
G/T Measurement in an Anechoic Chamber
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.
An Improved Capacitance Model for Permittivity Measurement
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
Common Radar Cross Section & Antenna Gain Measurement Calibration
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.
MEASURING LOW CROSS POLARIZATION USING A BROAD BAND, LOG PERIODIC PROBE
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.
Accurate Broadband Microstrip Permeameter to measure Permeability of Thin Film Samples
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.
Wideband Measurements Of The Forward Rcs And The Extinction Cross Section
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.
Electronically Controlled Tilt Angle Of A Linearly Polarized Signal At Ka-Band
As part of a target simulator , 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.
Focused Beam Measurement Of Antenna Gain Patterns
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.
On The Truncation of the Azimuthal Mode Spectrum of High-Order Probes in Probe-Corrected Spherical Near-Field Antenna Measurements
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.
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