AMTA Paper Archive
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Three-Antenna Method for Group Delay Calibration
Antenna systems are increasing in complexity at a rapid pace as advances are made in electronics, signal processing, communication, and navigation technologies. In the past, antenna design requirements have focused on parameters such as gain, efficiency, input impedance, and radiation pattern (e.g., beamwidth and sidelobe level). For some new systems, the group delay characteristics of the antenna are important, where the group delay is proportional to the derivative of the insertion phase as a function of frequency. The group delay is required to stay within certain bounds as a function of frequency and pattern angle. Unfortunately, there are not well established methods or standards for calibrating antenna group delay like the standard methods used for gain and input impedance. This paper presents a method for calibrating the group delay of three antennas based on an extension of the widely used three-antenna gain and polarization calibration methods. No prior knowledge of the gain or group delay of the three antennas is required. The method is demonstrated by a measurement example where it is shown that multipath errors and time gating can be critical for calibrating the group delay.
A Modified Three-Antenna Polarization Measurement Technique Using a Least-Squares Curve Fit
This paper presents a modification to the standard three-antenna polarization measurement method. The new technique solves for the sense, axial ratio, and tilt angle utilizing a least-squared errors routine and multiple measurements of the response at different roll angles between antennas. The paper compares the results of this method to Allan Newell’s well known modified three-antenna polarization measurement technique. Four antennas were measured two at a time and in several different arrangements to get twenty-four measures of the polarization parameters for each antenna. The work shows this method had a more repeatable measure of the axial ratio than the parameters determined using Newell’s technique.
Development, Measurement and Analysis of a Sixteen Element Stacked Patch Microstrip Array for Remote Sensing Applications
A low-profile, high efficiency sixteen-element stacked patch microstrip array operating in the L-band frequencies of 1.26GHz and 1.413GHz was designed, fabricated and tested for use in applications to airborne sensors operating on small aircrafts. The array was optimized for element spacing, excitation amplitude taper, low cross-polarization and high beam-efficiency using Particle-Swarm Optimization (PSO) and Finite-Difference Time Domain (FDTD) methods. The design and measurement of sixteen-element array topology, stacked patch elements, and power-divider beam forming network are presented in detail. The study highlights the repeatability measurements and characterization of array with the effect of dielectric radomes in a spherical near-field test facility at UCLA. The results met the requirements of center-frequencies and frequencybands(1.26GHz ± 10MHz, 1.413GHz ± 15MHz), side-lobes, very good beam-efficiency (>90%) and low-cross polarization (<-40dB) in main-beam region of array. The measured results compared well with simulations for the two frequencies. Based on measurement results, the microstrip array design has a potential to be used as a feed for deployable mesh antennas for future spaceborne L-band passive and active sensing systems that can operate at integrated active radar (1.26GHz) and passive radiometer (1.413GHz) frequencies with dual polarization capabilities to study soil-moisture and sea-surface salinity.
An Open-Boundary Quad-Ridged Guide Horn Antenna for Use as a Source in Antenna Pattern Measurement Anechoic Chambers
The present paper introduces a new antenna design to be used in anechoic chambers. When measuring 3D patterns the receiving antenna in the anechoic chamber must be able to sense the two orthogonal components of the field that exist in the far field. This can be accomplished by mechanically rotating the source horn in the chamber. A better and faster approach is to use a dual polarized antenna and electronically switch between polarizations. This new design is a broadband (2-18GHz) antenna with dual polarization. The antenna is a ridged guide horn. The novel part is that the sides have been omitted. Numerical analysis and measurements show that this open-sided or open-boundary horn provides a better and more stable pattern behavior for the entire band of operation as well as good directivity for its compact design. The radiation and input parameters of the antenna are analyzed in this paper for the novel design as well as for some of the early prototypes to show some of the ill effects of bounded quadridge horn designs for broadband applications. Mechanically the antenna is built so that it can be mounted onto the shield of an anechoic room without compromising the shield integrity of the chamber.
Automated Ellipticity Measurements of Ultra-Wideband Circular Polarization Antennas
This paper describes an automated ellipticity measurement system for ultra-wideband (UWB) circular-polarization antennas. The system comprises a double-ridged horn (DRH) antenna, a high-precision polarization rotator, an antenna-under-test (AUT) positioner, a vector network analyzer (VNA), and a controlling computer. The liner-polarized DRH antenna typically rotates 360° in 5°-intervals controlled by the rotator. At each angle, the VNA sweeps an ultra-wide bandwidth to measure the path gain. The least squares method was employed to find the axial ratio (r >= 1) and inclination angle at each frequency by fitting the plots to an anticipated peanut shell curve. Since the conventional cross polarization discrimination (XPD) has been defined for narrowband antennas, we proposed the wideband XPD as a frequency integration of the square of the circular polarization ratio (x), where x = (r + 1) / (r - 1), embracing a certain bandwidth. The wideband XPD represents the total power ratio between co- and cross-polarizations in the bandwidth. We measured the ellipticity and the wideband XPD of an axial-mode helical antenna using this system.
Surface Wave Contributions in the RCS of a Squat Cylinder
While using squat cylinders for calibrations, we study the MoM-simulated data in terms of surface waves. We have found that the fine structures in both the amplitude and the phase are related to the target geometry. Key Words: RCS calibration, simulation, polarization
OTA Performance Testing of Wireless Devices with Multiple Antennas
OTA performance testing of active wireless devices has become an important part of evaluation and certification criteria. Existing test methodologies are extensions of traditional antenna pattern measurement techniques. A critical assumption of these methods is that the device under test utilizes a single active antenna. Advances in wireless technology continue to incorporate more complex antenna systems, starting with simple switching diversity and progressing to more advanced concepts such as adaptive arrays (smart antennas) and multiple-input multiple-output (MIMO) technologies. These technologies combine multiple antennas with various software algorithms that can dynamically change the behavior of the antennas during the test, negating the assumption that each position and polarization of an antenna pattern measurement represents a single component of the same complex field vector. In addition, MIMO technologies rely on the multipath interaction and spatial relationship between multiple sets of antennas. An anechoic chamber with a single measurement antenna cannot simulate the environment necessary to evaluate the performance of a MIMO system. New measurement methods and system technologies are needed to properly evaluate these technologies. This presentation will discuss the issues and evaluate possible solutions.
A Dual-Linear Polarization UWB Dielectric Rod Probe Design
A dual-polarization ultrawide bandwidth (UWB) dielectric rod antenna containing two concentric dielectric cylinders was developed for near field probing applications. This antenna features more than 4:1 bandwidth, dual-linear polarization, stable radiation center and symmetric patterns. The antenna begins with a tapered wave-launching section consisting of shaped conducting plates and resistive films. This launcher section is followed by a guided section where the excited HE11 modes are transported to the radiation section. The radiation section contains specially shaped dimensions and materials to generate similar E and H plane patterns with 3-dB beamwidths greater than 55° over 4:1 bandwidth (2 to 8 GHz).
Cross-Polarization Parameters in the Presence of Drift in Radar Cross Section Measurements
We use a rotating dihedral to determine the cross-polarization ratios of radar cross section measurement systems. Even a small amplitude drift can severely degrade the calibration accuracy, since the calibration relies on accurate determination of polarimetric data over a large dynamic range. We show analytically how drift introduces errors into the system parameters, and outline an analytic procedure to minimize the in.uence of drift to estimate system parameters with greater accuracy. We show that only very limited information about the drift is needed to provide measured system parameters accurate to second order in the error-free parameters. Higher-order accuracies can be achieved by using more detailed information about the drift. We use simulations to explain and illustrate the analytic development of this theory. We also show that, using cross-polarimetric measurements on a cylinder, we can recover the exact system parameters. These .ndings show that we can now calibrate polarimetric radar cross section systems without the large uncertainties that can be introduced by drift.
Comparison of RCS Measurement of a NASA Almond Using Classical Compact Indoor Facility and a new Phased Array Antenna
CEA-Cesta has developed a new phased array antenna for RCS dual polarization wide bandwidth measurement in V/UHF bands. This array enables us to enhance signal to noise ratio especially at low frequencies. It is composed of 3 sub arrays dedicated each to one frequency band. The innovative design allows installing it in one of CEA/CESTA RCS facilities called “CAMELIA”. In order to validate this array in the highest sub-band [700 to 2000MHz], we measured in both HH and VV polarizations the near field RCS of a 2.5m long NASA almond target. This canonical object has been made of polystyrene coated with conducting nickel varnish. It has been hung on an eight wires rotating positionner. The results are compared with the data acquired in a classical RCS compact range and with the output of the 3D finite element code called ODYSSEE developed at CEA.
Wide-Band Dual Polarized Probes for Near-Field Antenna Measurements
Dual polarized probes for modern high precision near field measurement systems have stringent performance requirements in terms of pattern shape, on-axis and off-axis polarization purity, return loss and port-to-port isolation. A further requirement to the probe is that the useable bandwidth should exceed the antenna under test. As a consequence, the probe design is often a trade-off between performance requirements and the usable bandwidth of the probe. Current high performance designs are based on corrugated horns with balanced capacitive orthogonal excitation achieving close to 25% bandwidth . This technology is well suited for near field probes in the L to Ka band range. Although attractive for compactness, simplicity and excellent performance, probes with external balanced feeding require high precision couplers and manual tuning that impact the overall complexity and manufacturing cost of the final probe. A reduction in cost and complexity can be achieved while maintaining the high performance standards. SATIMO has developed an innovative near field probe with self-balanced feeding maintaining high performance on a wide bandwidth. The overall simplicity makes the new technology very attractive for probe designs in the L to Ka band range.
Use of a Compact Range to Measure Satellite TV Reflectors And Low Noise Block Downconverter Feeds
Satellite TV reflectors for home use, provided to the public by service companies such as DIRECTV, have many features which must be adequately characterized prior to design release, including: • Multiple Beam Frequency Re-use • FCC Sidelobe Envelope Verification • Circular Polarization Isolation These features must be adequately tested at frequencies up to Ku band and beyond. The use of a far-field range is impractical, as some of the reflectors measure several feet in diameter, and thus requires a range length of several hundred feet at Ku band. Near-field testing requires a full scan to determine a single cut for evaluation of FCC compliant sidelobe performance. Thus, a compact range is a logical alternative for measurement of this class of antennas. The compact range can provide a quick assessment of multiple beam coverage performance and pass/fail analysis against FCC sidelobe curve specifications. In addition, the feeds for these antennas often use Low Noise Block (LNB) Downconverters that are built in as part of the feed assembly. Measuring the output of an LNB does not yield the phase information required to determine all polarization parameters. A spinning linear measurement with some unique processing was implemented on this range to determine the full polarization characterization, using some elementary assumptions about polarization sense. This paper describes the implementation of a compact range based measurement facility for satellite antenna testing, with emphasis on the circular polarization measurement of the LNB assembly, capability for comparison against FCC sidelobe levels, and measurement of offset beams featuring frequency re-use capability.
Novel Method for Antenna Material Characterization
This paper describes the method and hardware implementation of a test bed that was designed and built to characterize the reflection characteristics of various types of reflector materials. The system described measures reflection amplitude and phase from flat test panels relative to a metal panel standard at normal incidence and for dual linear polarizations simultaneously. The measurement’s theoretical concept is based on a focused free space technique with time domain gating to remove the effect of multi-path coupling between the test panel and the feed assembly. The system as a whole demonstrates a novel method for measuring the reflection from reflector materials and characterizing their potential impact on polarization purity. The measurement system consists of: 1) A fixed reflector, 2) An alignment fixture accommodating feed assemblies, which include corrugated horns that operate over a 40% bandwidth that may be swapped out in order to cover a continuous frequency band from 18 to 75 GHz and Orthomode Transducers (OMT) in order to measure dual linear polarizations simultaneously, 3) An additional alignment fixture for mounting the flat panels under test, and 4) A Vector Network Analyzer (VNA) and computer for data collection and processing. The system is assembled on a bench top and aligned utilizing a Coordinate Measurement Machine (CMM). Sample results demonstrating the measurement of various types of reflector materials including composite reflector lay-ups with graphite face sheets and mesh samples for deployable reflectors are presented.
EM Propagation in Jet Engine Turbines
There is interest in the propagation of EM signals inside jet engine turbines for a number of reasons. Applications include radar scattering phenomenology and jet engine plasma plume formation studies. In our research, we are interested in the communication channel characteristics for micro-size wireless sensors attached to the turbine blades that measure parameters such as strain and temperature. Propagation measurements were performed on both F-16 (F-110) and Boeing 747 (CF6-50) turbines. The frequency band extended from 2 to 20 GHz (wavelengths longer than the turbine blades to wavelengths shorter than the gap between turbine blades). Signals were propagated with both radial and circumferential polarization. Both transmission and scattering measurements were made from both the inlet and the outlet. We also used small probe antennas inserted in boreholes between turbine stages. A range of blade positions were included. We will show the propagation characteristics as a function of polarization, frequency and time (UWB time domain transformations). We will also show the internal radar reflection characteristics of the turbine as a function of various stator blade rotation angles. Comparisons with a hybrid mathematical propagation model will be given.
Measurement of Circular Polarized Antennas
In antenna measurements, the orientation of the antenna under test (AUT) is very important. The orientation here refers to the antenna placement in a plane perpendicular to the incident wavefront. For a linear polarized antenna, the antenna should be oriented parallel to the co-polarized component of the incident fields. A small error in the orientation can lead to a drop in the measured gain and an increase in the measured cross-polarization level. In the case of a circularly polarized antenna, it is not obvious how the antenna should be oriented. If the quiet zone fields (incident wavefront) have no cross-polarized component, then the orientation does not affect the measured data. However, when the quiet zone fields have a cross-polarized component, which is true for almost all test ranges, the measured gain and cross-polarized level can vary significantly with the antenna orientation. In this paper, the measured data is used to show the effects of antenna orientation on a circularly polarized antenna. The reason for the variations in the measured data with antenna orientation is discussed. A simple method to improve the measurement accuracy is presented.
Near field measurement errors due to neglecting probe cross-polarization
Calibration of planar near field probes is generally required to obtain accurate cross-polarization measurements of satellite antennas; however, probe calibration is costly and time consuming. One way to avoid probe calibration is to ignore the probe cross-polarization and use the probe co-polarized patterns alone for probe correction. Then the probe can be easily characterized by standard, in-house measurements or by analytical models. Of course, if the probe cross-polarization is ignored, additional errors are introduced in the co- and cross-polarized pattern measurements, but the errors can be manageable, depending on the probe and Antenna-Under-Test (AUT) polarization properties. Complete formulas and/or tables for near field measurement errors for three popular measurement configurations are presented, along with experimental verification of the error estimates for one case.
Indoor Spherical 3D RDC Near-field Facility
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor near field monostatic RCS assessment. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. The target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.
Evaluation of the Telia Scattered Field Measurement Method for Estimation of In-Network Performance of Mobile Terminal Antennas
In this paper we present and evaluate a method for estimation of in-network performance of mobile terminal antennas developed by the Swedish telecom operator Telia. The Telia Scattered Field Measurement (TSFM) Method is intended to give a better estimate of the performance of the mobile terminal antenna as in an in-network fading scenario. The parameter measured from the TSFM method is referred to as the Scattered Field Measurement Gain, SFMG, i.e. the Mean Effective Gain, MEG, measured relative to a half wave dipole antenna. MEG includes the radiation pattern of the mobile terminal antenna as well as an estimate of polarization and directional losses that occur due to the propagation environment. In this study it is found that the TSFM method provides a good measure of the in-network performance of the mobile terminal antenna. Furthermore, it is shown that the SFMG measured with this method is found to be well correlated with the Total Radiated Power Gain, TRPG, or radiation efficiency. This suggests that the Total Radiated Power, TRP, may be a good measure of the in-network performance of mobile terminal antennas if measured with proper adjustment to the antenna and propagation channel mismatch.
Planar/Spherical Near-Field Range Comparison with -60 dB Residual Error Level
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical near-field ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi. The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI. The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
A Method to Correct Measurement Errors in Far-Field Antenna Ranges
Now-a-days, far-field ranges are being used to measure antenna radiation patterns. Two main types of ranges used are used for these measurements: direct and indirect illumination. In either case, the accuracy of the measurement is dependent upon the quality of the range quiet-zone fields. In direct illumination, phase and amplitude taper cause discrepancies in the fields. For indirect illumination, only amplitude taper must be accounted for. Additionally, stray signals and cross-polarization will further distort the quiet-zone fields and lead to measurement errors. This new methodology starts with the measured antenna data and a priori knowledge of the incident fields and estimates an Effective Aperture Distribution (EAD). The EAD compensates for these sources of error and can be used to predict the far-field radiation pattern of the antenna under test. Analytical results are presented for taper and stray signal analysis.
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