AMTA Paper Archive
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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.
Coordinate System Plotting For Antenna Measurements
Antenna measurement data is collected over a surface as a function of position relative to the antenna. The data collection coordinate system directly affects how data is mapped to the surface: planar, cylindrical, spherical or other types. Far-field measurements are usually mapped or converted to spherical surfaces from which directivity, polarization and patterns are calculated and projected. Often the collected coordinate system is not the same as the final-mapped system, requiring special formulas for proper conversion. In addition, projecting this data in two and three-dimensional polar or rectangular plots presents other problems in interpreting data. This paper presents many of the most commonly encountered coordinate system formulas and shows how their mapping directly affects the interpretation of pattern and polarization data in an easily recognizable way.
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 Electromagnetic Holography in Conductive Media
This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.
Some Considerations for the Measurement and Analysis of Circularly Polarized Antennas Using a Linearly Polarized Feed/Probe
This paper examines the inter-relatedness between the polarization vector of the linearly polarized feed/probe, the analysis coordinate system used for the DL-to-CP transformation, and how the test antenna generates its circular polarization response. The measurement of the performance characteristics of circularly polarized antennas is often accomplished using a linearly polarized feed/probe to measure horizontal and vertical polarization components. The measured orthogonal linearly polarized components are combined using a mathematical technique to transform them to the orthogonal right-hand and left-hand circular polarization components of the test antenna. One of the difficulties in using this technique is insuring the proper orientation of the feed/probe for the measurement, the analysis coordinate systems used for the DL-to-CP transformation algorithm. Another is understanding the manner in which the circular polarization is being generated by the antenna under test. These factors are inherently related, and without proper care the wrong answer can easily be calculated.
HIGH PERFORMANCE BROADBAND FEEDS FOR ECONOMICAL RF TESTING IN COMPACT RANGES
Compact test ranges are worldwide used for real-time measurements of antenna and payload systems. The Compensated Compact Range CCR 75/60 and 120/100 of Astrium represent a standard for measurement of satellite antenna pattern and gain as well as payload parameter due to its extremely outstanding cross-polar behavior and excellent plane wave field quality in the test zone. The plane wave performance in the test zone of a compact test range is mainly dependent on the facilities reflector system and applied edge treatment as well as on the RF performance of the range feed. To provide efficient and economic testing and maintaining the needed measurement accuracy the existing standard set of high performance single linear feeds covering the frequency range from 1 - 40 GHz had been extended to operate simultaneously in dual linear polarization. In addition several customer specific range feeds had been developed and manufactured and validated. More detailed information and achieved test results for the new high performance range feeds will be presented.
The Effect of the Absorber Collar on Open Ended Waveguide Probes
This paper describes measurements performed at the National Physical Laboratory (NPL) and Near Field Systems Inc (NSI) on Open Ended Waveguide (OEWG) probes that are typically used for near-field measurements. The effect of the size and location of the absorber collar placed behind the probe was studied. It was found that for some configurations, the absorber collar could cause noticeable ripples in the far-field patterns of the probe and this in turn could affect the probe correction process when the probe was used in near-field measurements. General guidelines were developed to select an absorber configuration that would have minimal effect on the patterns, polarization and gain of the probes.
Portable RF Target Simulator
RF guided missile developers require flight simulation of their target engagements to develop their RF seeker. This usually involves the seeker mounted on a Flight Motion Simulator (FMS) as well as an RF target simulator that simulates the signature and motion of the target. Missile intercept engagements are unique in that they involve highly dynamic relative motion in a short period of time. This puts demanding requirements on the RF target simulator to adequately present the desired phase slope, amplitude, and polarization to the seeker antenna and electronics under test. This paper describes a newly installed RF Target Simulator that addresses these requirements in a unique fashion. The design utilizes a compact range reflector, dynamically rotated in two axes as commanded by the flight simulation computer, to produce the desired changing phase slope and an RF feed network dynamically controlled to produce the desired changing polarization and amplitude. Physical optics analysis establishes an accurate correlation between reflector physical rotation and resulting angle-of-arrival of the wave front in the quiet zone. The RF Target Simulator is self contained in a two-man portable anechoic chamber that can be disengaged from the FMS and rolled to and from the FMS as needed. Measurements are presented showing the performance of the RF Target Simulator.
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.
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