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Calibration

A Probe Station Based Setup for On-Wafer Antenna Measurements
K. VanCaekenberghe,K. Brakora, K. Jumani, K. Sarabandi, M. Rangwala, Y-Z. Wee, November 2006

A probe station based antenna measurement setup is presented. The setup allows for measurement of complex impedance and radiation patterns of an on-wafer planar antenna, henceforth referred to as the device under test (DUT), radiating at broadside and fed by a coplanar waveguide (CPW). The setup eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the DUT with a coplanar RF probe. In addition, the DUT is probed exactly where it will be connected to a transceiver IC later on, such that no de-embedding of the measured data is required. The primary sources of measurement errors are related to calibration, insufficient dynamic range (DR), misalignment, scattering from nearby objects and vibrations. The performance of the setup will be demonstrated through measurement of an on-wafer electrically short slot antenna (.0/35 × .0/35, 5 mm2) radiating at 2.45 GHz.

Demonstration of an Inverted Steward Platform Target Suspension System using Lightweight, High Tensile Strings
A. Buterbaugh,B. Kent, C. Mentzer, M. Scott, W. Forster, November 2006

This paper presents the design, development and testing of an inverted Stewart platform for suspending and positioning targets during RF antenna and signature testing. Previous string target support systems use multiple string attachment point configurations that do not allow the target roll or pitch to be modified during the azimuthal data collection. This presentation will discuss an in-house development of a scale model target support system that allows for high accuracy simultaneous target roll and pitch positioning. The inverted Stewart platform also offers unique stability of the target by damping out the torsional pendulum motion typically encountered in conventional string support systems. In this paper we will also discuss the advantages and disadvantages of the string support concepts and provide design guidance for a building an inverted Stewart platform support system. If possible, a simple squat calibration standard will be measured to assess the quality and precision of this novel support system.

A Portable Near-Field Scanner for Calibrating the AN/SPS-48E Antennas on LPD-17 Ships
D. Woollen,F. Closser, W. Slowey, November 2006

The AN/SPS-48E antenna is a three dimensional air search antenna that is currently installed on 27 US ships. Currently the 48E antenna is removed from the ship after five to seven years to be overhauled at NSWC Crane Division. The new San Antonio Class ships (LPD 17 – 25) have a new enclosed mast design, the Advanced Electromagnetic Mast/Sensor (AEM/S), in which the 48E antenna and others are installed inside the enclosed mast. The cost of removing the enclosed mast led to the decision that the 48E antenna systems (antennas and pedestals) will not be removed for overhaul and maintenance on these ships as is currently done for all other installations. As a result, new fixtures and procedures need to be developed to allow maintenance inside of the mast. The most challenging of the new fixtures is a near-field scanner, which will be used to re-tune the antenna and characterize the RF performance parameters. This paper discusses the design and development effort currently underway for this Enclosed Mast Antenna Calibration System (EMACS), most notably the mechanical design constraints placed on the scanner by the enclosed mast regarding equipment movement, installation, alignment and testing.

The RCS Calibration Uncertainty of Balloon Tethered Spheres For Outdoor RCS Measurement Systems
B. Kent,A. Buterbaugh, L. Cravens, T. Coveyou, W. Forster, November 2006

Hollow metallic aluminum spheres have been used for years for calibrating RCS measurement systems both indoors and outdoors. While many previous papers have identified the RCS calibration shortfalls associated with spheres [1,2], most of these papers have concentrated on indoor RCS measurement systems, where there exist a number of accurate calibration alternatives to spheres, including the so-called "squat cylinder" [3,4]. For outdoor free space RCS measurement systems, especially those designed to measure dynamically moving or changing targets, (i.e. the NASA Shuttle C-Band Debris Radar), calibration is a much tougher problem. Frequently, spheres are used to calibrate such systems, by releasing and tracking a sphere attached to a lighter-than-air balloon, or by tethering a sphere to a lighter-than -air balloon and allowing it to float through a fixed radar beam. Recently, the Air Force Research Laboratory Mobile Diagnostic Laboratory (MDL) had the opportunity to measure the clutter and uncertainty associated with balloon tethered Sphere RCS calibrations. Two spheres were measured suspended by various string types and a line under an 8 ft. diameter tethered Helium filled balloon. We will provide design guidance, signal processing techniques and measurement uncertainty to help minimize the clutter and error induced by balloon borne RCS calibration spheres.

Antenna Measurements Using Satellite Beacons
R.B. Dybdal, November 2005

Three methods are described to characterize ground antennas in frequency bands where satellite beacons exist. This measurement method is useful when the antenna to be tested cannot be easily measured using conventional general purpose facilities or radio source measurement techniques. The measurement methods are described, and the factors that result in measurement uncertainties are discussed. Key Words: Antenna Measurements, Gain Calibration, Ground Terminals

Calibration of GPS On-Orbit Monitor
R. Dybdal,M. Partridge, November 2005

The calibration of a prototype system to monitor the on-orbit performance of heritage and modernized GPS satellites is described. While the monitor can measure other GPS parameters of interest, the calibration to accurately determine the received signal levels is described here. The calibration determines the monitor’s receive antenna gain and relates the received power at the antenna terminals to the indicated output of the monitor’s receiver. Key Words: System Calibration, Error Budgets, Satellite Measurement

Proximity Effects in the Calibration of Microwave Radiometers
R. Wittmann,A. Cox, R. Direen, November 2005

Abstract— Microwave, total-power radiometers are cali­brated by observing sources of known brightness temperature. Free-space calibrations are usually performed assuming far-.eld interactions, although the actual calibration source may be well into the near-.eld region of the radiometer antenna. Here, we simulate the free-space coupling between radiometer and cali­bration target to explore the potential contribution of proximity effects to measurement uncertainty.

Obtaining High Quality RCS Measurements with a Very Large Foam Column
M.C Baggett,T. Thomas, November 2005

A large compact range facility required a foam column for RCS testing where the center of the quiet zone was six meters above the floor level. The RCS measurement after vector background subtraction, had to be accurate down to a –50 dBsm level from 1.5 GHz to 40 GHz. A foam column was constructed from a single billet of material. The foam column was evaluated as to its RCS level in both whole body and ISAR imaging modes. This paper describes the specification, construction and RCS evaluation of this column in the compact range facility. The column was evaluated at single frequencies and with RCS images from 2 GHz to 36 GHz using a gated CW radar. Data is presented that shows the effects of the column on the response of a calibration sphere and the response of the column itself. A study of the foam column imaging response used as the background for vector background subtraction is also described. Targets in the –60 dBsm range were successfully imaged with vector background subtraction of the foam column.

A Simple Probe Calibration Method of a New Compact Spherical Near-Field Measurement System for Antennas from 1 GHz to 10 GHz
M. Hirose,K. Komiyama, S. Kurokawa, November 2005

ABSTRACT We have developed a new compact spherical near-field measurement system using a photonic sensor as a probe and successfully measured the 3D antenna patterns of a double-ridged horn antenna from 1 GHz to 10 GHz. This system consists of a compact spherical scanner and a photonic sensor that is used for the probe of the spherical near-field measurements. In our system, only one probe can be used for the wide frequency range measurements and the probe compensation is not needed in the measurements. For the system, we propose a simple calibration method using a double-ridged horn antenna for our system. We calibrate the system by measuring the double-ridged horn antenna on the reasonable assumption that the antenna efficiency is 100 %. Comparing the absolute gain obtained by the proposed calibration method with the one decided by using three-antenna method at far-field range, we show that the agreement is good within 1 dB over the whole frequency range.

Angular Errors In Polarimetric Radar Cross Section Calibration Using A Rotating Dihedral
L Muth,C. William, D. Morales, T. Conn, November 2005

We examine how accurately the transmit and receive parameters of a radar cross section measurement sys­tem can be determined by use of a rotating dihedral as the polarimetric calibration device. We derive expres­sions for the errors due to misalignment in the angle of rotation. We obtain expressions for the angles a0,hv and a0,vh for which the measured cross-polarization ratios of a target vanish. Since the theoretical cross-polarization of a cylinder is 0, we can .nd the calibra­tion bias-correction angles. We use simulated and real data to demonstrate the robustness of this bias-angle correction technique. We derive expressions for the uncertainty in the polarimetric system parameters.

Effects of Array Panel Joint Discontinuities on RF Calibration
V. Tripp,D. Wright, G. Wilson, R. Hemphill, R. Parks, November 2005

In this paper is presented an experimental investigation of conventional array calibration in the presence of various kinds of joint discontinuities between array panels. Two rigid array panels were positioned such that the element lattice was continuous across a narrow joint. Three kinds of discontinuities were applied to the joint: (1) an angle, (2) a gap (including an edge), and (3) a step between panels. Each type was investigated for joints oriented in the E-plane and the H-plane. Each discontinuity was also varied in magnitude so as to observe parametric effects. Planar near-field-range (NFR) measurements were made in a conventional array calibration mode and a near-field pattern mode. Processing included separating the pattern component due to element transmission (impedance) change from that due to pattern shape change. Results show that conventional calibration methods quickly become inadequate to calibrate these discontinuities because they change element pattern shapes.

Method for Selecting Sources to Calibrate a Non-Rigid Phased Array
G. Brown,D. Wright, G. Wilson, R. Parks, November 2005

Abstract— A method is proposed that will optimally se­lect the placement of sources to aid in the calibration of a phased array of scalable panels that is mounted on a station­ary, ground-based, non-rigid frame. A cost function based on the Cramer-Rao Lower Bound is optimized through con­strained minimization. The array is constructed from ide­alized (non-deforming) subarray panels that have unknown perturbations in orientation and location. To demonstrate the proposed method, several case studies are investigated involving combinations of known calibration sources.

Influence of Truncation of Near-Field Data in Calibration of Phased Array Antennas
A. Boomstra, November 2005

In this paper, reduction of the near-field scanplane in calibration of phased array antennas is discussed. In general, truncation of near-field data can give a considerable reduction of acquisition time. This particularly applies in a larger extent to phased array measurements, where a high number of channels is measured in the calibration process. Also, relative small equipment can be used to measure relative large antennas, which can be cost-effective. In this paper, it is shown that under certain conditions the scanplane, and therefore acquisition time, can be reduced substantially. Based on an example, different scanplane sizes and reduction techniques are considered to investigate and estimate the influence of truncation size on the error in the calibration parameters.

Three-Antenna Method for Group Delay Calibration
P. Rousseau,F. Pisano, November 2005

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.

Antenna Measurements by Novel Optical Link System Using New Microwave-Optical Technologies
S. Kurokawa,K. Komiyama, M. Hirose, November 2005

We propose a novel microwave measurement system that consists of transmitting and receiving optical-fiber link systems. The system can measure parameters of S11 and S21 of an antenna under test (AUT) by the procedure of OSLT 1-pass and 2-port calibration, due to the simultaneous measurement of its relevant signals going into, reflected and transmitted from the AUT. It is shown by some experiments that the S11 and S21 of the two log-periodic antennas measured by the optical link system agree very well with those by a conventional system using metal coaxial cables. It is proved that the optical system can be used to evaluate the S11 and S21 of the AUT in broad frequency range without using coaxial cable.

S-Parameter Extraction of a Partially Filled Waveguide by Using the Finite Element Method and the Numerical TRL Calibration Technique
P. Barba,A. Bogle, L. Kempel, November 2005

Inversion of the material parameters for a sample usually requires that the sample fill the waveguide cross-section. Alternative methods require that a non-filling sample be aligned along the center-line of the waveguide. However, it is not known how errors in placement impact the accuracy of the inversion. Hence, a numerical simulation to assess these errors is beneficial to the community. The extraction of the S-parameters from a rectangular­dielectric-filled waveguide is conducted numerically by means of the Finite Element Method (FEM) and the Thru-Reflect-Line (TRL) calibration technique. Three different ratios of dielectric sample width (d) to waveguide width (a) are primarily studied. The results are then validated with experimental data on the X-band. An assessment of error with respect to position will be presented at the meeting.

The Calibration of Four-Arm Spiral Modal Measurements for Angle-of-Arrival Determination
J. Radcliffe,K. Pasala, November 2005

Direction Finding (DF) systems have long been an area of intense research within the Air Force Research Laboratory. There are presently two types of existing DF systems: wideband multi-mode antennas and interferometers. Wideband multi-mode DF systems allow for a large bandwidth but present a low resolution and high variance. Interferometers provide high accuracy and low variance but are narrow band and require a large number of single aperture antenna elements. An effort has commenced to incorporate a broadband DF system with high resolution using two multi-mode spiral antennas. Using an interferometer of multi-mode elements, we can provide high resolution and wideband operation without using numerous antennas. This paper presents the results of extensive wideband measurements carried out on a four-arm spiral antenna and the associated modeformer. These measurements are used to assess and validate the angle estimation capability of the multi-arm spiral antenna.

A Novel Method for Antenna Gain and Phase Calibration
I. Gupta,C-C. Chen, E. Newman, J. Krieger, W. Burnside, November 2005

Standard Gain Horns (SGH) are normally used as reference antennas in antenna measurements. Gain charts for SGH are provided by the supplier. These charts give the gain of the SGH in dBi versus frequency but do not provide any information on the phase variations versus frequency. For complete antenna calibration, one needs the phase as well as gain data for SGH over the frequency band of interest. To obtain the gain and phase data, one can use the three-antenna method which requires three independent measurements and, therefore, is more susceptible to measurement errors. Note that if one has access to two identical antennas, the three-antenna method reduces to a single measurement which is more desirable. In practice, however, one does not have access to two identical antennas. In this paper, a novel method which mimics measurements with two identical antennas is described. In the method, one performs S11 type measurements on the antenna of interest by placing the antenna in front of a large conductive flat plate. The late term in the S11 measurements is then used to obtain the boresight gain and phase of the antenna under test. The measured gain and phase data of several antennas obtained using the proposed method is presented and compared with the results obtained using the three-antenna method as well as with analytical results.

Conducted Emissions Testing for Electromagnetic Compatibility
M. Moy,D. Arakaki, November 2005

Operating frequencies in the gigahertz range is creating an increased need for electromagnetic compatibility (EMC) testing. In the United States, FCC regulations require conformance to radiated and conducted emissions specifications. An EMC laboratory was established at Cal Poly San Luis Obispo (screen room, test instrumentation, and software) and an experiment was developed to explore conducted emissions effects. This paper will describe the test configuration, explain the calibration procedure needed to acquire accurate measurements, and illustrate measurement techniques applied to two example systems. In addition, the data collection process is illustrated through software donated by CKC Laboratories (EMC specialists). To verify the functionality of the laboratory and to assess measurement accuracy, two 12V/15W switching power supplies are characterized for conducted emissions performance; one as supplied by the vendor (KGCOMP) and a second unit with the EMC filters removed. The noise spectrum for both units are plotted against frequency and compared to FCC specifications. The unaltered unit is shown to be in compliance, thus verifying the accuracy of the test procedure and instrumentation.

Study of Calibration Targets of Full-polarimetric RF Measurement
T. Van,B. Kent, B. Welsh, K. Hill, W. Forster, November 2004

Co-polarized and cross-polarized radar cross sections (RCS) are required to completely characterize a complex target. However, it is common for a RCS range to measure only the co-polarized RCS. This practice is primarily due to the inability to produce accurate cross-polarization analysis data for the calibration targets. The most commonly used calibration targets, spheres and cylinders, cannot be used to calibrate cross-polarized RCS due to lack of cross-polarized returns. In this paper, we consider objects that can potentially be used as calibration targets for cross-polarization measurements. Specifically, we numerically study the cross-polarized responses of the Tungsten rod, the grooved cylinder, and triangular dihedrals. Co-polarized measurement data are also included in this initial assessment. From this initial study, we find the counter-balanced dihedral to be a suitable calibration target for cross-polarized measurements.







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