AMTA Paper Archive
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PROCEDURE AND PROCESS OPTIMIZATION FOR REDUCTION OF MEASUREMENT UNCERTAINTIES IN RF TEST FACILITIES
The measurement accuracy of state-of-the-art RF test facilities like near-field or compact test ranges is influenced due to applied system hardware as well as operational facts which are influenced by human errors. The measurement errors of near-field test facilities were analyzed and published in the past times and are based on the 18-term error model of Newell . For compact test ranges and especially for the cross-polar free compensated compact range a similar error model was established at Astrium GmbH within a study for the satellite service provider INTELSAT  in order to define possible facility performance improvements and maximum achievable values for the measurement accuracy. It has to be remarked, that test programs for space applications require very stringent adherence to procedures and documentation of process steps during a test campaign. Within this paper, recommendations for process optimizations and procedures will be presented to guarantee the adherence to the valid error budgets and to minimize the Human Factor. A description of main error contributions in the Compensated Compact Range (CCR) of Astrium GmbH will be performed. Furthermore, the error budgets for pattern and gain measurements and achievable performance improvements will be given.
Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna
In this paper, three possible approaches for definition of a highly accurate reference pattern of a reference antenna are described and their pros and contras are discussed. Following the most reliable approach, a dedicated measurement campaign was planned and carried out in 2007-2008 for definition of the highly accurate reference pattern of the VAST12 antenna. In planning the campaign, conclusions from the first comparison campaign with the VAST12 carried out within the ACE network in 2004-2005 were taken into account and these are also presented and discussed. Some typical measurement errors and uncertainties are listed and briefly discussed.
Characterization of measurement systems through extensive measurement campaigns
Within the European Union network "Antenna Center of Excellence" – ACE (2004-2007), a first intercomparison campaign among different European measurement systems, using the 12 GHz Validation Standard (VAST12) antenna, were carried out during 2004 and 2005. One of the challenges of that campaign was the definition of the accurate reference pattern. This was the reason why a dedicated measurement campaign for definition of the accurate reference pattern was hold during 2007 and beginning of 2008. This second campaign is described in the companion paper “Dedicated measurement campaign for definition of accurate reference pattern of the VAST12 antenna”. This dedicated measurement campaign was performed by Technical University of Denmark (DTU) in Denmark, SAAB Microwave Systems (SAAB) in Sweden and Technical University of Madrid (UPM) in Spain. This campaign consisted of a large number of measurements with slightly different configurations in each of the three institutions (2 spherical near field systems and one compact range). The purpose of this paper is to show the process to achieve the reference pattern from each institution and the evaluation of the accuracy. The acquisitions were performed systematically varying in applied scanning scheme, measurement distances, signal level and so on. The results are analyzed by each institution combining the measurement results in near or far field and extracting from these measurements: a “best” pattern, an evaluation of possible sources of errors (i.e. reflections, mechanical and electrical uncertainties) and an estimation of the items of the uncertainty budget.
EVALUATION OF ISOFILTER™ FIDELITY IN SELECTED APPLICATIONS
The IsoFilterTM technique was originally demonstrated to operate by rejecting secondary signals that derive from reflections off of a nearby metallic object – namely, the ground plane surface supporting a small pyramidal horn.[1,2] The aperture of the horn was located several wavelengths above the ground plane and the sidelobes and backlobes of the horn illuminated the ground plane itself. The success of this demonstration has been sufficient to encourage us to pursue further the question of how well the IsoFilterTM technique will work to suppress other types of secondary signals– such as signals coming from other elements of an array antenna or another individual first-order primary radiator nearby. Here we report on some of the results of that investigation. We have calculated the far-field patterns of a sparsely populated array and applied the IsoFilterTM technique. The goodness of the suppression is judged by how well the “IsoFiltered” result agrees with the calculated pattern of the individual radiator.
CONICAL NEAR-FIELD ANTENNA MEASUREMENTS
A near-field measurement technique for the prediction of asymptotic far-field antenna patterns from data obtained from a modified cylindrical, or plane-polar, near-field measurement system is presented. This technique utilises a simple change in facility alignment to enable near-field data to be taken over the surface of a conceptual right cone [1, 2], or right conic frustum [3, 4] thereby allowing existing facilities to characterise wide-angle antenna performance in situations where hitherto they would perhaps have been limited by truncation. This paper aims to introduce the measurement technique, describe the novel probe-corrected near-field to far-field transform algorithm which is based upon a cylindrical mode expansion of the measured fields before presenting preliminary results of both computational electromagnetic simulations and actual range measurements. As this paper recounts the progress of ongoing research, it concludes with a discussion of the remaining outstanding issues and presents an overview of the planned future work.
On the Robustness of Planar Phaseless Near-field Measurements to Probe Positioning Errors
The Phaseless techniques have gained considerable attention during the past two decades in the antenna measurements community. The removal of the phase measurements has some immediate advantages over the common vectorial measurements. They are cost effective, well-adapted for higher frequencies and insensitive to phase instabilities. The phaseless techniques have been discussed in the antenna measurements community and the theories behind these techniques are well explained in the literature. Unfortunately the issue of the noise and the presence of measurement errors are not investigated in details to provide strong impetus to the importance of phaseless measurements. In this paper the near field of a number of different types of antennas with high, medium and low side lobes is simulated to create as realistic case as possible. The effects of the probe positioning errors are investigated by injecting random errors in the position of the probe samples along x-, y- and z-axis. It is also illustrated how the positioning errors can distort the phase distributions. Through detailed characterizations of the constructed far field patterns, robustness of the Iterative Fourier technique even at the presence of very high probe positioning errors is demonstrated. It is shown how the utilization of the phaseless techniques will significantly reduce the probe positioning error effects when it is compared to the commonly used amplitude and phase near field measurement techniques.
USE OF VECTOR ISOMETRIC ROTATIONS IN THE MEASUREMENT OF IMPERFECTLY ALIGNED ANTENNAS
Most traditional antenna measurement techniques presume that the antenna under test (AUT) is accurately aligned to the mechanical axes of the test range. Sometimes, however, it is not possible to achieve such a careful antenna alignment . In these cases, standard post processing techniques can be used to accurately correct antenna-to-range misalignment. Alternatively, similar results may be obtained by approximation in the form of piecewise polynomial interpolation. When carefully employed, this method will result in only a small increase in uncertainty, but with a significant reduction in computational effort. This paper describes this far-field alignment correction method, which is closely related to standard active alignment correction methods . This paper then proceeds to use numerical simulation as well as actual range measurements to demonstrate the effectiveness of this method. Finally, the utility of this technique in the presentation of far-field antenna pattern functions is illustrated.
Using the phase center of a Standard Gain Horn in antenna gain determination
In this paper, it is shown that using the phase center as distance reference point is equivalent to applying the proximity correction in determining gain of antennas with a SGH at finite distances. Clear guidelines for calculating the phase center location of a SGH, for application to antenna gain determination, are presented and explained. The phase pattern necessary for the calculation can be obtained either from measurement data or from computer simulations of the SGH. The region of validity of this approach is outlined and the residual error is quantified.
Measurements of a combined radiation pattern generated by a few low-profile antennas mounted on a Moon probe
This paper describes joint studies of Wroclaw University of Technology and Denmark Technical University on optimizing placement and performance of low-profile antennas on small satellite, such as ESMO Moon orbiter. After comprehensive electromagnetic studies with use of numerical analysis, a spacecraft mockup modeling its conductive surfaces was developed. Two to four antennas were mounted and several placement configurations were investigated. For verification purpose of numerical analysis and formulating design guidelines to an actual Moon probe, precise measurements of combined radiation pattern were performed at the Near-Field Antenna Test Facility, Denmark Technical University.
A Standalone RF System for Solid-State Phased Array Antenna Measurements
Lockheed Martin MS2 has a long history of utilizing antenna ranges for calibration, test and characterization of the phased array antennas. Each range contains an integrated RF receiver subsystem for performing antenna measurements, typically on the full array. For solid-state phased array testing, what is often needed, however, is a test station capable of performing complex S-parameter measurements on a subarray or subset of the full antenna system without incurring the expense of a test chamber. To address this requirement, Lockheed Martin, working with Nearfield Systems, has developed a portable standalone RF measurement system. The standalone system consists of an Agilent PNA, automated transmit/receive unit (TRU) and a waveform generation (WFG) subsystem for interfacing to the phased array beam-steering computer. This paper will discuss the capabilities of the Standalone RF System including the TRU and WFG subsystems. The TRU is used to tailor the RF signal by automated switching of amplifiers and programmable step attenuators for various test scenarios. The WFG is an automated pattern generator used to present many digital waveforms in arbitrary sequences to the phased array beam steering computer. The design features of the standalone RF system will be presented along with the COTS hardware utilized in assembling the station.
Novel method for characterizing filtering antennas
Ultra Wide Band systems show promises in high data rate radio communications. However, these systems interfere with other communication protocols like the WLAN. To solve this problem, current works propose the insertion of thin half wavelength slots in the antenna structures, thus creating filtering antennas. Many criteria were used to characterize the filtering of these antennas, like the measurement of the VSWR, the return loss, or the maximum gain of the antenna versus frequency. In this paper, we show by studying the 3D gain pattern of these antennas that their filtering is not just dependent on the frequency but it also depends on the radiation direction, due to the radiation of the slots. Then we propose efficiency measurement as the best way to quantify the filtering of these omnidirectional antennas independently of the radiation direction. Two filtering antennas were characterized using a modified wideband Wheeler Cap efficiency measurement method.
Field Measurements of a Low Frequency HF Mobile Antenna
This measurement paper presents the methodology used in the characterization of a short, broadband, High Frequency (HF) mobile antenna operating in the frequency range from 1.8 to 30 MHz. The antenna patterns were measured while the antenna was mounted on a High Mobility Multipurpose Wheeled Vehicle (HMMWV). Pattern and performance predictions were made using the Numeric Electromagnetic Code (NEC) Pro 2 Version 5 before the field tests. Stimulation of the AUT was done with the use of a large magnetic loop to minimize AUT pattern perturbations. An attempt was made to measure gain profiles by comparisons to a monopole, ground based, resonating at the center of HF band (15 MHz). Measurement results provided the performance of the antenna in a mounted profile and the pattern distortions produced by the host vehicle.
RF Material Design, Measurement and Instrumentation: Optimization of an X-band Composite Reflector through Material Characterization and Testing
A recent project to develop and optimize the RF reflectivity of a Composite/nano-material, X-band reflector was pursued by a team lead at AFRL – RF Technology Branch. This was accomplished by iterative testing and signal pattern measurements performed at the AFRL-Rome Laboratory that provided critical feedback affecting the laminate design and configuration of the composite reflector. Testing of multiple configurations of composite reflectors provided data that was key to successful progression throughout the product engineering cycle, and accomplished the following milestones: . Initial performance verification . Characterization of influential material constituents . Optimization of design Eclipse Composites Engineering, along with metallic nano-material supplier Metal Matrix Composites worked to develop a composite dish with reflective properties identical to the baseline metallic reflector model. Through various methods of testing, the reflective influence of each material constituent was characterized and the relative effect within the overall composite laminate was modeled. Based on initial measurements, prototype articles were fabricated for testing and comparative evaluation. This project demonstrates the critical feedback loop between measurement/testing and design/development leading to the successful production of a segmented composite reflector that is lighter weight, more durable, with increased performance in the field to support today’s military and commercial operations.
APERTURE FIELD INVESTIGATION OF PLANAR 2-D LEAKY-WAVE ANTENNA DESIGNS USING THE INVERSE FOURIER TRANSFORM
The near-field aperture distribution excited on the guiding surface of various planar leaky-wave antenna designs is examined. The investigated antennas (for millimeter wave applications) are realized by circular, straight and elliptical metallic strip gratings on a high permittivity dielectric substrate. With such straight and curvilinear grating configurations, analytical determination of the near-field, and hence the leaky-wave phase and attenuation constants along the guiding surface, can be mathematically intensive. To assist in such complex characterizations, the near-field/far-field extrapolation techniques can provide insight and thus illustrate such 2- D aperture field distributions. Specifically, by taking the inverse Fourier transform of measured 2D far-field beam patterns, the near-field distribution along the aperture can be estimated.
In-situ Measurement of the Antenna Pattern for the Haystack Auxiliary Radar utilizing a Ground Based Recording System
Measurement of the antenna pattern of the Haystack Auxiliary Radar (HAX), an experimental Ku band radar system developed by the Massachusetts Institute of Technology Lincoln Laboratory for deep space experimentation, was recently carried out utilizing a ground based, mobile recording system. The HAX radar system uses a 12.19 m parabolic antenna placed inside of a radome which is located on Millstone Hill in Westford, Massachusetts. The recording system, which includes a Ku-band analog front end and a high-speed digitizer with 500 MHz instantaneous bandwidth and long duration recording capability, was located at the summit of Mt. Wachusett, 36.1 km southwest of HAX. Several azimuth and elevation antenna pattern cuts were acquired by transmitting towards a wide-band ground based recording system placed down range while rotating the HAX antenna. Throughout these pattern measurements the radar was operated in a reduced power pulsed CW mode. Continuous wide-band recordings from the slowly scanned pattern measurements were taken and the data was processed to detect individual pulses, retaining only the portions of the recordings containing detected pulses. Post-processing of the pulsed CW data allowed for measurement of the antenna pattern with a significant dynamic range, characterizing both the mainbeam of this antenna and the far-out sidelobes.
A State of the Art Anechoic Chamber for Air vehicle Testing at Alenia Aeronautica
Electromagnetic Anechoic Chamber has recently been built by Alenia Aeronautica at Caselle South Plant: The Anechoic Chamber is a full anechoic chamber, and it has been designed to carry out electromagnetic vulnerability tests mainly on fighter and unmanned aircraft. In addition measurement can be carried out on many different vehicles that can be brought into the chamber through the main access door. A system to extract exhaust gas was installed in order to carry out tests on a wide variety of vehicles. The Anechoic Chamber has been designed to carry out both HIRF/EMC test and High Sensitivity RF measurement: in particular HIRF/EMC tests in the frequency range 30MHz ÷ 18GHz with the capability of radiating a very high intensity electromagnetic field and High Sensitivity RF measurement, including antenna pattern measurements on antennas installed on aircraft in the frequency range 500MHz ÷ 18GHz. During the design phase a 1/12th scale model of the chamber had been fabricated to assess the desired electromagnetic performance. In this phase of design the model was tested at the scale frequencies for Filed Uniformity, Site Attenuation and Free Space VSWR results. This study was published at the AMTA 2004 meeting. In addition to the physical model, during the construction phase, various computer simulations were performed to further define the detailed internal absorber layout and to define test acceptance methods for procedures not covered by the standards. The computer model analysis was conducted to identify areas of scattering that could be treated with higher performance absorbers to improve the chambers quiet zone performance. The identified “Fresnel Zones." have been treated with high performance absorbers optimized to provide improved performance at microwave frequencies. The absorber optimization was reported at the AMTA 2006 meeting. This optimization has allowed validation of the chamber according to the requirements of CIRSP 16-1-4 2007-02 in the range of frequency 30 MHz - 18GHz. The size (shield to shield) of chamber is 30m wide, 30m long and 20m high, and the 18m wide by 8.5m high main door allows the SUT access. The shielded structure is a welded structure of 3mm-thick steel panels which guarantees shielding effectiveness of more than 100 dB in the frequency range 100 kHz to 20GHz. The chamber includes a 10 meter diameter turntable to rotate a 30 ton SUT with an angular accuracy of ± 0.02° and a pathway to allow SUT access. Both the pathway and the turntable are permanently covered by ferrite tiles. A hoist system permits lifting of the SUT (max 25 tons) up to 10 meters from the turntable centre enabling EMC testing on aircraft with the landing gear retracted.
Range Length Requirements for Notebook Computers
Wireless network adapters are now standard in most notebook computers. These network adapters are typically compliant with at least IEEE 802.11a/b/g and often include IEEE 802.11n. This requires that the antenna subsystem of the notebook computer operate at both 2.4 GHz and 5.25 GHz. The antennas used in the wireless system of a notebook computer are themselves small, but they are incorporated into a much larger device. It is unclear exactly what range length is required in order to make accurate pattern and radiated power measurements. This paper reports on a series of measurements made at different range lengths with the goal of determining the minimum range length required for acceptable measurements of radiation patterns and total radiated power (TRP).
Performance Considerations for Pulsed Antenna Measurements
Previous AMTA papers have discussed pulsed antenna measurements and the importance of parameters such as pulse width, pulse repetition frequency (PRF) and receiver dynamic range in determining the appropriate technique for performing pulsed measurements. Typically, the pulse width and PRF determine the IF bandwidth required of the instrumentation receiver to achieve a specific level of receiver performance. Less emphasis has been given to the receiver timing and synchronization required to achieve optimum performance for a full pulsed antenna measurement scenario. This paper will discuss receiver timing considerations and show examples of scan time performance during high-speed pulsed measurements. Inter-pulse and intra-pulse measurements will be compared with respect to their impact on measurement time. Pulse profile measurements will be examined to show the importance of a fast synchronous receiver for sub-microsecond pulse characterization. Pulsed antenna pattern results will also be presented and compared with CW measurements.
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.
Radiation Pattern Measurement of Passive UHF RFID Tags
Measuring the radiation pattern of passive ultra-high frequency (UHF) radio frequency identification (RFID) tags is challenging due to the special characteristics of RFID systems. In this paper we presented a comparison of radiation pattern measurement techniques for passive UHF RFID tags. Intermodulation-based, backscattered power-based and threshold power-based techniques are used to measure the E and H plane radiation patterns of a dipole-type and slot-type tags. In this study, we have measured the radiation pattern of RFID tags in active mode, i.e. when the tag is characterized in operation with the microchip.
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