AMTA Paper Archive
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Two Dimensional Scattering Analysis of Data-Linked Support Strings for Bistatic Measurement Systems
This paper reports the recent investigation of data-link strings as supporting structures for antennas used in a bistatic radar cross section (RCS) measurement system. Although several candidate strings exist, analysis of the strings’ scattering contribution needs a generic string model to make comprehensive comparisons. A simple theoretical two-dimensional (2D) dielectric coated cylindrical wire model is initially utilized to predict and compare scattering characteristics of various data-link string structures. In addition to the simple model, a non-destructive measurement method is proposed for extracting the material properties of the string material. Using the analytic 2D model of a dielectric clad wire as the generic string model, the unknown permittivity is computed from reflectivity measurements taken with a focused beam system. Extracted permittivity values are then used in a full-wave electromagnetic solver to validate the model. Measured and simulated results are shown to have excellent agreement for the 2D RCS, radar echo width, of different strings and polarization configurations.
An Integrated UWB Dual Polarized Tapered Chamber Feed Design Examply
Wave-launching in tapered chambers is often done by placing a commercially available antenna in the feed section. This approach has its own drawbacks: First, the physical sizes of these commercial antennas are often too big and cause the actual radiation center to be significantly away from the desirable apex point, resulting in poor measurement performance. Second, these antennas may need to be rotated when taking dual-polarization measurements or they may even need to be replaced completely when taking measurements at a different frequency band for which the existing antenna is not operational anymore. This antenna positioning in turn introduces another place for the uncertainty in the measurements. Previously, a novel integrated wave-launcher mechanism was presented by The Ohio State University-Electroscience Laboratory (OSU/ESL) researchers to overcome the problems stated above. In this work, a new integrated chamber feed has been designed employing new design ideas to address the issues encountered in this previous effort, such as transmitted power attenuation caused by waveguide cut-off at lower frequencies.
COMPENSATION FOR PROBE TRANSLATION EFFECTS IN DUAL POLARIZED PLANAR NEAR-FIELD ANTENNA MEASUREMENTS
In this paper a technique is described that allows for the determination and correction of probe translation during polarization rotation in planar near-field measurements. The technique, which relies on the independent translation of coordinate systems for the two orthogonally polarized data sets, has significance for mm-wave testing, where bulky RF components makes probe alignment difficult. Measured data is presented to demonstrate the success of the technique.
WIDE BAND DUAL POLARISED V/UHF PHASED ARRAY FOR INDOOR RCS MEASUREMENT
Phased arrays antennas have desirable features in terms of simplicity, compact dimensions and low weight for low frequency applications requiring dual polarization and medium gain such as RCS measurements. However, a fundamental problem with phased arrays technology in wide band applications is grating lobe limitations due to the grid topology of the phased array elements. The spacing of the array elements cannot be to close in order to limit element coupling and not to large to avoid grating lobes. Consequently, conventional phase array antenna applications are generally limited to a useable frequency bandwidth of 1:2. A unique grid topology has recently been developed to overcome this problem [1, 2]. By interleaving three separate phased arrays, each dedicated to a different subband with close to 1:2 bandwidth, the useable bandwidth of the combined phased array antenna can be extended to as much as 1:7 while maintaining the nice performance features of the basic phase array technology. Based on this technology a large dual polarized phase array antenna has been designed for indoor RCS testing in the frequency range from 140MHz to 1000MHz. The operational bandwidth of the array is split into three subbands: 140-260 MHz, 260-520 MHz and 520-1000 MHz. The array is 6.34 x 6m and weighs less than 250Kg. Due to the element spacing and topology the phased array is sensitive to excitation errors so the beam forming network (BFN) feeding the elements must be wellbalanced. A uniform amplitude and phase distribution for the array excitation coefficients has been selected to simplify the BFN design and minimize possible excitation errors throughout the bandwidth. This paper describe the antenna electrical design and performance trade-off activity, the manufacturing details and discuss the comprehensive validation/testing activity prior to delivery to the final customer.
Cross Polarization Uncertainty in Near-Field Probe Correction
The probe correction of near-field measured data can be considered as being composed of two parts. The first part is a pattern correction that corrects for the effects of the aperture size and shape of the probe and can be analyzed in terms of the far-field main component pattern of the probe. The second part is due to the non-ideal polarization properties of the probe. If the probe responded to only one vector component of the incident field in all directions, this correction would be unnecessary. But since all probes have some response to each of two orthogonal components, the polarization correction must be included. The polarization correction will be the focus of the following discussion. Previous studies have derived and tested general equations to analyze polarization uncertainty12. This paper simplifies these equations for easier application. The results of analysis and measurements for Planar, Cylindrical and Spherical near-field measurements will be summarized in a form that is general, easily applied and useful. Equations and graphs will be presented that can be used to estimate the uncertainty in the polarization correction for different AUT/Probe polarization combinations and measurement geometries. The planar case will be considered first where the concepts are derived from the probe correction theory and computer simulation and then extended to the other measurement geometries.
CIRCULAR POLARISED WIDE BAND FIELD PROBES
Dual polarized probes for modern high precision measurement systems have strict requirements in terms of pattern shape, polarization purity, return loss and port-to-port isolation. 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. As a consequence, the probe design is a trade-off between performance requirements and the usable bandwidth of the probe. For measurement applications in circular polarization the choice is between measuring the linear polarization components separately and derive the resulting circular polarized by computation or to measure directly with a circular polarized probe. Dual polarized probes in circular polarization with high polarization purity is difficult to achieve on a wide bandwidth. Dual linear polarized probe technology has recently been developed capable of achieving as much as 1:4 bandwidth while maintaining the high performance of traditional probe designs [1–7]. This paper describes the development, manufacturing and test of dual circular polarized probes with as much as 1:2 bandwidth as shown in Figure 1.
THE SOFTWARE DEFINED ANTENNA; PROTOTYPE AND PROGRAMMING
The goal of this research is to develop an unconstrained reconfigurable programmable array antenna that we call the Software Defined Antenna™. We create patch arrays using individual controllable pixels. The aperture of the array is made up of a large group of small (<1/10 ?) pixels. Each computer controlled pixel is a small piston made up of a metal top, a dielectric shaft, and a metal base. When a line of pixels is raised into the up position, a microstrip transmission line is created. A patch antenna is created when multiple pixels are raised into the up position to form a larger rectangle or other shape. In the final design, a set of feed lines and antennas can be created to form an antenna array within 1 millisecond. Under computer control, it is possible to change the beam direction, the beamwidth, the polarization, and the frequency of operation of the array. Theoretical results, experimental results and the implementation of a working prototype will be shown.
A Compact 6-Element Tri-band GPS Array
Small GPS antenna elements and arrays are of great interest for future smaller vehicles. The need to cover the lower L5 band presents an additional challenge. To that end, a compact 6-element GPS array is proposed to cover all three GPS bands, namely, L1 (1575 MHz), L2 (1227 MHz) and L5 (1176 MHz). This GPS array consists of six proximity-fed stacked patches (PFSP), each having 1.2’’ in aperture size (?/8 at L5). The overall GPS array has a diameter of 4.5’’, nearly 90% smaller in area size as compared to the standard 14’’ GPS array. A key feature of the GPS element is its single feed to realize the right-hand circularly polarized polarization (RHCP) via a quadrature phase microstrip line splitter. As a result, only 6 coax cables are needed to feed the entire GPS array. Design concepts and procedures are presented, followed by measurements and a performance assessment.
When To Use the Square Root of Two in Circular Polarized Calculations
Decomposing a signal of unknown polarization into combinations of linear and circular components can be very confusing, especially when one wants to relate them to a known phase and amplitude reference. Many publications have addressed parts of this issue, some employing the square root of two and some not. There does not seem to be a substantial consensus on this is-sue, experts being somewhat evenly divided. The prob-lem relates to both mathematical analysis and meas-urement analysis, which must be in agreement when comparing measurement to theory. This paper presents an abbreviated mathematical analysis involving depolarization of a wave incident upon a radome, yielding the magnitude and phase of both resultant circular components. The result is com-pared to well-established published formulas. However, derivations of the same components from measure-ments may reach a different conclusion depending on the procedure used. The difference is a factor of the square root of two. These two conflicting results are compared and a resolution proposed.
Evanescent Wave Electromagnetic Holography
Highlights of work at the Naval Research Laboratory in evanescent near-field electromagnetic holography (ENEH) will be presented. This work grew out of extensive experimental work in near-field acoustical holography at our laboratory that has been recognized formally by the Laboratory as one of the 75 most innovative technologies over the past 75 years. This new electromagnetic approach differs from the usual nearfield imaging in that it provides much better than halfwavelength resolution due to the inclusion of evanescent waves. Furthermore ”imaging” to a source surface provides a reconstruction of the surface currents, Poynting vector as well as the E and H field vectors. These quantities are derived from two measured holograms (phase and amplitude) of two polarizations of the electric and/or magnetic fields over a 2-D surface (the hologram). Experimental work in both low (100 Hz) and high frequencies (10GHz) is of interest, although we present here results of the latter along with the theory. Two approaches will be discussed for backtracking the measured fields: one that uses wave function expansions in plane, cylindrical or spherical geometries, highlighting the cylindrical geometry in this paper, and a second more general formulation that uses the field expanded using an array of equivalent dipole sources especially useful in arbitrary geometries. Both approaches represent inverse methods and are ill-posed and require regularization to stabilize the reconstructions. We hope that these methods will provide high resolution new diagnostic tools for antenna analysis, as well as diagnostics for applications in EMC and EMI among others. Currently we are seeking partnership with other laboratories and universities to direct this technology towards problems that could benefit from its unique diagnostic capabilities. Work supported by the Office of Naval Research.
Estimation of the Rician K-factor in Reverberation Chambers for Improved Repeatability in Terminal Antenna Measurements
An estimator of the RicianK-factor for reverberation chamber is derived in this paper using maximum likelihood estimation approach. This is done by reviewing the existing statistical model of the fields in the reverberation chamber. The functionality of the derived K-factor estimator is tested with the measurement data for the well stirred and unstirred (only platform stirring) chamber. Moreover, the impact of polarization of the antenna on the Rician Kfactor is also investigated. The Rician K-factor is found to be almost zero for a well stirred reverberation chamber whereas it is higher for unstirred (only platform stirring) chamber. It is also observed that the orientation of half wavelength dipole influence significantly the K-factor values.
Radome Theory Testing & Repair
The purpose of the nose radome has changed over the past twenty or so years. As the antennas and electronics become more sophisticated the radome becomes more important to the overall system performance. Electrical testing of the radome has become a necessary part of the radome repair process. In addition to Transmission Efficiency, radome test facilities must also test Boresight Error, Reflections, Sidelobes and Polarization. Radome repair is also becoming very sophisticated. As the performance expectations of the radome increase, the difficulty in making an electrically transparent repair increases significantly. This paper is a general overview of the radome testing process, range requirements that make radome test ranges unique from antenna test facilities. This paper also shows some examples of good and bad repair techniques and their effect on electrical testing.
A Measurement Setup for Characterizing Antenna on an Infinite Ground Plane from 1 to 18 GHz
Currently there is a lack of facilities capable of measuring the full upper hemisphere radiation patterns of antennas mounted on an infinite ground plane. Measurements performed with a finite ground plane suffer diffraction interference from the truncated edges. To circumvent this problem, a new measurement setup was developed at the Ohio State University ElectroScience Laboratory (ESL) for fully characterizing upper hemisphere radiation gain patterns and polarization for antennas up to 4” in diameter from 1-18 GHz. A probe antenna is positioned 46” away from the antenna under test (AUT). The ground plane end diffractions are removed using time-domain gating. The key design consideration is to position the probe antenna in the far-field region and yet shorter than the radius of the ground plane. This paper will present the calibration procedure necessary for the measurement system and it’s limitations due to ground plane probe antenna coupling at low elevation angles. In addition, the complete radiation pattern of a 4” monopole measured from 1-5.5GHz to demonstrate the systems capability for the lower third of the systems operating frequency range.
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.
RECONFIGURABLE ANTENNA ARRAYS USING PIXEL ELEMENTS
The goal of this research is to develop an unconstrained reconfigurable programmable array antenna. The concept is to build patch arrays using individual controllable pixels. The aperture of the system is made up of a large array of small (1/10 .min) pixels. Each pixel is a small piston made up of a metal top, a dielectric shaft, and a metal base. The pistons can be moved up and down under computer control. When all pistons are in the down position, a ground plane is created. When a line of pixels is raised into the up position, a microstrip transmission line (a metal line over a dielectric substrate) is created. A patch antenna is created when multiple pixels are raised into the up position to form a larger rectangle or other shape. In the final design, a set of feed lines and antennas can be created in any pattern within 1 millisecond. Under computer control, it is possible to change the beam direction, the beamwidth, the polarization, and the frequency of operation of the array. Design details, theoretical models, and the behavior of test fixtures and configurations will be discussed during this presentation.
Extracting the Polarization from Bi-polar Phaseless Nearâ€“Field Measurements
The polarization extraction in the phaseless near-field measurement is investigated. Sensing the antenna polarization based on the implementation of phase-retrieval methods like IFT (Iterative Fourier Technique) will not result to a unique solution. It is shown how a single extra point measurement can provide the complete vectorial representation of the field in a two-component representation. This means for the first time by the application of phaseless methods, one not only can get an understanding of the dominant polarization of the antenna in terms of linearity, ellipticity or circularity but also the true representation of the co- and cross polarized components in the far-field based on any definition (like Ludwig’s definitions). The applicability of the method is shown through a near-field measurement of a right-hand elliptically polarized antenna array in UCLA bi-polar near-field facility.
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