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Accurate gain measurements using any measurement technique require a calibrated gain standard, and the uncertainty in the gain of the standard is usually the largest term in the error analysis. To reduce the uncertainty, gain standards are often calibrated using a three- antenna measurement technique and the resulting gain values are generally certified to have an uncertainty of approximately 0.10 dB1-11. For near-field measurements, the gain standard may be the probe that is used to obtain the near-field data or it may be a Standard Gain Horn (SGH). Since the calibration of the gain standard is time consuming and often costly, it is desirable to verify that the gain of the standard is stable over long periods of time. This paper will describe tests to verify the gain stability of the standard and will also illustrate the terms in the error analysis that have the major effect on the uncertainty of any near-field gain measurement. With proper attention to the major error terms, the stability of the gain standard can be verified to approximate the original calibration uncertainty.
TRW, working with Nearfield Systems Inc., has installed a state-of-the-art near-field antenna measurement system1 to test the Astrolink payload antenna system. Astrolink is the next generation broadband satellite network that will deliver high speed Internet connections to the business desktop. TRW is building the Astrolink on-board communications payload which includes the antenna system. For this multi-reflector antenna payload, TRW has built a 40 ft. x 30 ft. horizontal near-field measurement system to operate from 1 to 50 GHz using NSI’s high speed Panther receiver and Agilent Technologies high speed VXI microwave synthesizers. The system is capable of performing conventional raster scans, as well as directed plane-polar scans tilted to the plane of a specific reflector. The range was completed in January 2001. This paper will describe the Astrolink Near-field range and installation, present test data and plots from this new 40x30 near-field range, show results of a NIST 18-term error assessment, compare raster vs plane polar scans summarize the error assessment process.
This paper investigates the effect of alignment errors on near-field cylindrical ranges at frequencies over 18 GHz. This is of particular interest because the small probe sizes and wavelengths above 18 GHz can make the alignment of the near-field system a difficult task. Previous probe alignment investigations have been done at frequencies below 18 GHz. This paper will determine if the conclusions from the previous work are valid at higher frequencies and will expand on that previous work. Measured data will be presented to demonstrate the effect of the probe axis not intersecting the azimuth axis as well as the probe not being orthogonal to the azimuth axis of rotation.
Earlier (AMTA'97, AMTA'98), we have proposed a new low-cost laboratory method named the quasi-optical waveguide modeling (QWM) method to study power and amplitude-phase scattering characteristics of objects, in particular the RCS of targets or their scale models, in the near millimeter (NMM) and submillimeter (SMM) wave regions. A specific feature of this technique in that an investigated object (or its scale model) is mounted inside a quasi-optical waveguide structure in the form of a hollow dielectric waveguide (HDW), in which the scattering characteristics of the waveguide dominant HE11 mode are determined. These characteristics are related to the wanted scattering characteristics of the test object in free space by definite relationships. At the same time the HDW serves several functions: it forms a quasiplane incident wave within the scattering area where test object is placed, performs the low-loss and low-distortion transmission of the scattered wave carrying information of the object being tested to the receiver, effectively filters the unwanted modes arising at the scattering on the test object, and insulates the measurement area from the ambient conditions containing parasitic sources. In this paper we consider the possibility of using the QWM method to study polarization backward scattering characteristics of physical objects, in particular the complex elements of the scattering matrix with relative phase (SMR). A quasi-optical polarimetric micro-compact range (PMCR) based on the circular HDW and quasi-optical devices has been developed and built. The measurement results of the SMR and backward scattering patterns of a reference object as a square metallic cylinder obtained in the PMCR for the different linear polarization basic sets at the 4-mm wave band are presented. The comparison between the experimental results for the reference object and the theoretical data calculated by the geometrical theory of diffraction have shown a good agreement, and demonstrated the possibilities of the QWM method, and its good perspectives for backward scattering polarization characteristics modeling in the NMM and SMM wave regions.
This paper summarizes some results of a recent NIST measurement effort. The purpose of this effort was to use a NIST-developed ultrawideband measurement system to assess the time- and frequency-domain characteristics of selected ultrawideband (UWB) transmitting devices. Brief descriptions of NISTdeveloped measurement systems are provided. Highfidelity time-domain waveforms are shown, along with associated amplitude spectra for two devices. Excellent results are obtained for both conducted and radiated emissions from UWB devices. Keywords: amplitude spectrum, anechoic chamber, conducted emission, frequency domain, radiated emission, time domain, ultrawideband
An accurate and reliable target positioning system is mandatory for a good antenna and/or radar cross section (RCS) measurement facility. Most measurements involve characterizing the radiation or scattering of the unit under test as a function of angle and frequency. Accuracy and repeatability become increasingly important in RCS measurements where background subtraction is utilized. Any error in target position will reduce the subtraction effectiveness. Wear and tear of existing equipment coupled with improvements in motion control technology may compel some measurement facilities to upgrade their positioning system. Doing so, while keeping the rest of the measurement system intact, poses integration challenges that cannot be over emphasized. Problems will inevitably be encountered. Their source could be the new positioning system, the old measurement system, or the communication between the two. Subtleties of how the motion control system works can be overlooked during the requirements definition phase of the project. Further idiosyncrasies can be missed during acceptance testing of the system. The Air Force Research Lab compact range has recently upgraded their target positioning system and will share the lessons learned as a result.
A fundamental part of the work of the BAE SYSTEMS Advanced Technology Centres Materials Group at Towcester (UK) is the microwave characterisation of the electromagnetic parameters of lossy materials. This paper describes a Quasi-optical microwave system for the free space measurement of material parameters in the frequency range 5 GHz to 18 GHz. The system employs two spherical reflectors which are illuminated from the side by gausian beam forming antennas. This produces a well defined parallel beam between the reflectors. The 5 GHz ro 18 GHz frequency range is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that the beamwaist diameter (or illumination area) is essentially the same for each of the three frequency bands The measurements are taken using a vector network analyser under computer control. The parallel beam enables a “Through,Reflect,Line” calibration technique to be used. After calibration the sample under test is placed in the beam (mid way between the reflectors) and the four microwave ‘S’ parameters are recorded automatically in complex form. The permittivity, permeabilty or lumped admittance (if the sample is very thin <ë/50) for the material are then determined from the ‘S’ parameters. The operation and performance of the system is discussed and some material parameter measurement results are given.
Spacecraft payload testing with full power necessitates to prevent radiation over areas reserved to personnel. Moreover, the increase of spacecraft Radio Frequency power needs to install high power absorbers in front of the RF flux. The acceptable limit temperature of such absorbers is provided by the manufacturers. This so being, the temperature behavior of such absorbers have to be correlated with RF Power Flux Density in order to define the spacecraft test set up. Using a well known flight spacecraft antenna, Alcatel Space Industries have performed a non destructive characterization of the temperature behavior of a high power absorber versus PFD. First, the total transmitted power by the antenna was computed using TICRA Grasp8 [1] software. Then the predicted PFD was correlated to the measured one and the temperature of the wall was recorded with an infrared camera, and related to this measured PFD. Such a result gave a simple relation between PFD and temperature. The relation is now used for temperature predictions on the absorbers during high power spacecraft tests, and helps to manage test set up and air cooling installation. The available results are limited to Alcatel Space Industries needs, but could be extended to any type of absorber, in a wide temperature and frequency range.
1. Introduction 2. IM2-a new unknown criteria for dual-band systems 3. Theoretical approach 4. Comparing measurements of IM2 and IM3 5. Devices under test (DUT) 6. Measurement results 7. Discussion of results 8. Basic technologies to minimize generation of intermodulation 9. Silver surface 10. Solid inner and outer conductors 11. Solderjoints to corrugated copper cables 12. Summary
Standard gain horn antennas are commonly used as reference antennas in establishing absolute gain levels of antennas under test. However, their high sidelobes and backlobes can interact with the structure surrounding the horn in the measurement setup. These interactions degrade the accuracy of the gain values. Thus, while the gain of the horn may be carefully calibrated in free space, its gain value in the measurement environment can differ from its free space value. Examples will illustrate this problem and ways are described to reduce the sensitivity to the environment.
This paper will explore the effects of the source antenna's axial ration on the apparent gain of an Antenna Under Test (AUT). A technique will be given to correct these errors. Finally, experimental test results will be given.
Radio source measurements of antenna G/T are well established, and can be anticipated to have more applications in the future. These techniques, however, sometimes have limitations that are overlooked and are not universally understood. Other valuable information about the antenna's performance can be determined. The measurement procedure is reviewed and common limitations are described.
A prototype broadband ground reflection range, to be used for measuring antenna patterns on full sized aircraft, was built and evaluated. The range was designed to evaluate an antenna at several arbitrary VHF/UHF frequencies simultaneously. This is a follow on to two previous papers that explored the design of such a range using numerical modeling and optimization by the use of genetic algorithms.
We present a preliminary evaluation of a microwave measurement system that has been designed to determine electromagnetic fields in the quiet-zone of an antenna measurement range and to produce an image of the sources, intended and unintended, of the incident radiation. This information is of potential value in the processes of improving range perfor mance, correcting pattern results for non-ideal illumination , and evaluating measurement uncertainty.
This paper describes the initial pointing control possible with large deployable mesh reflectors that use a control mechanism and gives the results of a control experiment. In the experiment, the pointing error is measured by an RF sensor and reduced to within designated value by altering the support structure. The result indicates that such alteration can appropriately reduce the initial pointing error for a large deployable reflector.
The design of a compact range facility in the National University of Singapore is presented. The range is designed for antenna and RCS measurements from L band to Ka-band and for test objects up to about 2 metres in size. The reflector in the range is parabolic in shape with a focal length of 3.5 metres. The instrumentation is standard measurement equipment with some purpose-built controllers for the positioners and the scanner.
Quiet zone field probe data of a recently built compact range system is presented. The com pact range uses an optimally designed blended rolled edge reflector to operate from 800 MHz to 18 GHz. The absorber on the walls, floor and ceiling of the chamber is also designed and placed for optimal performance. It is shown that the range is free of any significant stray signals over the whole frequency band.
The requirement to calibrate and test large active pulsed planar array RADAR antennas, such as the one developed for the advanced synthetic aperture radar (ASAR), places certain requirements on the measurement facility and analysis software that are perhaps not encountered in other areas of application. This paper gives a brief overview of ASAR and an introduction to some of the difficulties encountered during the test and measurement campaign. Results are presented that compare measurement with theoretical prediction. Good agreement has been obtained for both far and near field data.
Near-field ground-to-ground imaging systems are widely used to discover damage that could degrade the radar signature of low observable vehicles. However, these systems cannot presently assess the impact of this damage on the far-field signature of these vehicles. We describe progress made on a method to accurately project the near-field data from these to the far field. Near-field data for the algorithm development is provided by the hybrid finite element/integral equation RCS computer code SWITCH. The near-field data is processed to extract the near-field scattering centers using imaging. The imaging algorithm used differs from the usual far-field imaging formulation in that it incorporates some near-field physics. The processing algorithm, which incorporates a modified version of the CLEAN technique, verifies that the scattering centers that were extracted reproduce the original data when illuminated in the near-field. These near-field scattering centers are then illuminated by a plane wave to produce far-field data. This procedure was tested using VHF band scattering data for a full size treated planform. The near field data was projected to the far-field and then compared to data from a far-field SWITCH computation.
Microwave holography is a popular method for diagnosis and alignment of phased array antennas. Holography, commonly known in the near-field measurement community as "back transformation", is a method that allows computation of the primary (aperture) fields from the secondary (far-zone) fields. This technique requires the far-zone fields to be known over a complete hemisphere and adequately sampled on a regular spaced grid in K-space. The holography technique, while known to be mathematically valid, is subject to errors just as all measurements are. Surprisingly, very little work has been done to quantify the accuracy of the procedure in the presence of known measurement errors. It is unreasonable to think that the amplitude and phase of the array elements can be trimmed to better than the uncertainty of the back-transformed amplitude and phase. This makes it difficult for an antenna engineer to determine the achievable resolution in the measurement and calibration of a phased array antenna. This study reports the results of an empirical characterization of known errors in the holography process. A numerical model of the near-field measurement and holography process has been developed and many test cases examined in an effort to isolate and characterize individual errors commonly found in planar microwave holography. From this work, an error budget can be developed for the measurement of a specific antenna.