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NPL has been providing antenna gain standards since the late 1970's, initially to service internal needs for microwave field strength standards. To meet the increasing industrial demand for the calibration of microwave antennas in areas such as satellite communications and radar, NPL has developed an antenna extrapolation range. The current facility, which is due to be replaced by the end of the year, is used to measure the gain of microwave antennas in the frequency range 1 to 60 GHz, often with a gain uncertainty as low as ± 0.04 dB. Axial ratio, tilt, sense of polarisation and pattern measurements can also be made in the same facility, while for larger antennas a planar near-field scanner is used. Of the many measurement techniques for determining the gain of an antenna, the most accurate is the three antenna extrapolation technique [1,2] which was developed at the National Institute of Standards and Technology (NIST) at Boulder, Colorado, and is the method used at NPL. This is an absolute method as it does not require a prior knowledge of the gain of any of the antennas used. Since calibration data is often required across a wide frequency band, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, the cost and the need for interpolation between measurement points.
The European Space Agency (ESA) began serious investigations into the implementation and exploitation of near field antenna testing techniques already in the early 1970s where all three near field measurement geometries were considered (1). Spherical near field scanning was selected by ESA as being the most promising alternative to even larger conventional outdoor ranges. In the meantime, work was underway at the Technical University of Denmark (TUD) on spherical wave theory and its application to near field antenna measurements (2,3). As work began under ESA contract to demonstrate the technique, the most important aspect, the transformation algorithm and software was developed allowing dual polarized probe pattern and polarization corrected spherical near field measurements to be implemented (4).
A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.
Antennas characteristics can be measured in two ways. lfrequency Domain Method (FDM) is more widely known. The main measuring instruments: Microwave Generator and Receiver. In Time Domain Method (TDM) measurements are fulfilled by using superwide band pulses. The main measuring instruments: Pulse Generator and Sampling Oscilloscope. TDM shows a number of advantages but for narrow-band antennas TDM is difficult to apply and FDM is required. At the testing polygons aimed to measure various antennas we set equipment allowing to use both measurement methods. For TDM we used a two channel sampling converter SD200 of Geozondas production with bandwidth 0-18 GHz. To unify measurements we developed a 3-channel sampling converter SD303 allowing besides pulse to measure sine wave amplitude and phase difference in dynamic range 100 dB. The third channel is used for synchronization. Thus the same instrument assures antenna measurements both in TDM and FDM. At 100 m distance the following characteristics are obtained in Time and Frequency Domains Measurements: Bandwidth 1- 18 GHz. Antenna pattern dynamic range 60 dB Gain measurement accuracy 0.5 dB Phase difference between 2 antennas error 0.5 - 3° (depends on frequency). Hardware, software and digital signal processing algorithms are considered.
High-speed receivers for near-field antenna and RCS measurements have traditionally been one-of-a-kind, expensive, difficult to interface and lacking in software support. Advances in digital signal processing, computer technology and software development now provide the means to economically solve these problems. NSI offers a high speed receiver subsystem, the Panther 6000 series, that allows multiplexed beam and frequency measurements at a rate of 80,000 independent amplitude and phase measurement points per second. The Panther 6000 receiver directly digitizes the 20 MHz IF test and reference input channels, and includes a high speed beam controller (HSBC) to sequence the measurement process. The HSBC receives an input trigger to initiate a measurement sequence of user-defined frequencies and beam or pol states. NSI also offers a multi-channel all-digital receiver subsystem, the Panther 6500, to interface directly with Digital Beam Forming (DBF) antennas. The Panther 6500 allows up to 16 channels of l and Q digital input (16 bits each) with 90 dB dynamic range per channel. The all-digital DBF receiver reduces the cost, complexity and performance limitations associated with conventional instrumentation in DBF antenna measurement applications. All Panther series receivers are fully integrated with the NSI97 antenna measurement software and operate with existing microwave sources, mixers and IF distribution equipment.
The development and implementation of novel measurement systems for rapid electromagnetic (EM) field testing by using linear arrays of modulated scattering elements is presented and discussed. The measurement systems employ the Advanced Modulated Scattering Technique (A-MST) to accomplish rapid sampling of the incident electromagnetic field along the length of the linear probe array at rates that are faster than conventional mechanical scanning of a single probe by a factor of 10 to 1000 or more. The A-MST probe array may be located in the nea r-field (NF) or far-field (FF) of the EM sources.
For multi-frequency measurements, improving source frequency agility will greatly reduce measurement times. This paper introduces a new microwave source with a frequency agility 50 times faster than traditional sources. A description of the source, its specifications, and the unique features that make it well suited to antenna measurements is provided, as well as information on integrating the source with an antenna range. Actual antenna test scenarios are provided, and the corresponding measurement times are compared to the test times achieved with traditional sources.
We explore new methods for the evaluation of absorber-lined chambers in the 30-1000 MHz frequency range. Current domestic and international standards recommend chamber evaluations using CW site attenuation measurements. While these satisfy regulatory requirements, they are of little value in actually understanding a chamber environment. A technology, that circumvents the limitations of CW test methods, is currently under development at NIST. In order to explore the potential of this new methodology, we have developed a two-dimensional finite-difference time-domain model for a fully anechoic ferrite tile chamber. The results obtained are quite promising and demonstrate the potential of time-domain measu rements in chamber evaluations. A new chamber figure of merit is proposed that permits a more direct evaluation of the installed absorber system.
Characterization of radiated EMI by means of near zone measurements is examined by computer simulations. Electric field radiated by a test structure is expanded in spherical wave modes. The influence of the number of spherical wave modes on the accuracy to predict the maximum far-field magnitude and the total radiated power is examined. The examinations of this paper support the electric field measurements of small equipment at small measurement distances in the standard radiated EMI frequency range 30 - 1000 MHz. Results are presented as a function kr0, where k is the wave number and r 0 is the radius of the minimum sphere which fully encloses the EUT. Results of this paper give valuable guidelines for choosing an appropriate number of measurement locations for predicting the far field by means of a near-zone measurement.
The tapered chamber has been used for more than 35 years for mainly lower-frequency antenna measurements. The basic design of the tapered chamber has not changed significantly since its inception. Tapered chambers provide better quiet zone fields by placing the feed antenna's phase center at the vertex of the tapered walls, virtually eliminating reflections from the side walls. Recent innovations that better chamber versatility include an ultra-wideband SBH feed antenna [1,2], a less visible rotating support structure for the AUT and a new Chebyshev-based absorber treatment [3]. Utilizing these new features, a tapered chamber has been designed to have a large bandwidth, yet have an overall structure that is small enough to allow portability. This paper describes a chamber that operates from 400MHz to 40GHz and has an overall length less than 30' long. Structure, components, and field results are presented in this paper.
Errors arising in the measurement of reflection coefficient are identified and analyzed. The presence of multiple reflections due to poor connectors, transmission line discontinuities, and terminal loads is described, modeled and applied. Various measurement scenarios are analyzed, and measured results are presented as a guide for laboratory troubleshooting and as a validation of the measurement models. Improvements to Vector Network Analyzer calibration methods are proposed, including computer corrected calibration for one-port radiating elements and elementary improvements to two-port TRL calibration. An extensive error evaluation of the somewhat forgotten slotted line measurement is finally presented as a robust alternative, and computer automation, acquisition, and calibration of this measurement is outlined.
Over the last ten years, MRC has designed, fabricated, and installed a number of compact range reflector systems. This paper presents such reflector programs illustrating a variety of alternatives for reflector composition. Such programs include the MRC Scattering Measurements Lab (SML), the Air Force Research Lab's Advanced RCS Measurements Range (ARMR), Honeywell's Antenna Measurements Range, the new GE/NT Compact Range, and the new TRW Compact Antenna Test Range. Variations within these programs include single or dual-reflector configurations, single piece to panelized designs, and all composite to all aluminum construction. All approaches present excellent alternatives for various compact range needs.
The increase of output power for telecommunication satellites give constraints on design and manufacturing of antenna reflectors. Any non-linearity, such as a junction between two conductive materials, is a potential generator of inter-modulation products (PIM's). ALCATEL SPACE INDUSTRIES implemented a PIM test bench for reflectors. The objective is to validate, as early as possible in the satellite program, the reflector design, with regards PIM specification. The test principle consists in two separate transmit channels, each one having a single carrier at a well selected frequency. This configuration avoids the generation of PIM's by the bench itself. A basic conditions relevant to the output power for the test is that the flight conditions must be covered, in terms of Power Flux Density (PFD), on the reflector surface. The post-processing of the test results is based on a model whose parameters allow the following correlation : - variation of a given PIM order versus transmit power - variation between two consecutive PIM odd orders for a given transmit power. The model allows to correlate the reflector performance in terms of PIM to the flight conditions and to the customer's specification.
A fully-polarimetric compact range operating at 524 GHz has been developed for obtaining Ka-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, C W , X-band synthesizer driving dual X 4 8 transmitmultiplier chains and dual X 4 8 local oscillator multiplier chains. Software range-gating is used to reject unwanted spurious responses in the compact range. A motorized target positioning system allows for fully automated sequencing of calibration and target measurements over a desired set of target aspect and depression angles. A flat disk and a dihedral at two seam orientations are used for both polarization and R C S calibration. Cross-polarization rejection ratios of better than 45 d B are routinely achieved. The compact range reflector consists of a 1.5m diameter aluminum reflector fed from the side to produce a 0. 5 m diameter quiet zone. Targets are measured in free-space or on a variety of ground planes designed to model most typical grou nd surfaces. A description of this 524 GHz compact range along with 30 ISA R measurement examples are presented in this paper.
The development and measured performance of a hologram type compact antenna test range (CATR) for submillimetre wavelengths are presented. A 60 cm diameter hologram has been designed for the 310 GHz CATR. The instrumentation used for the compact range performance verification is described. This includes a millimetrewave vector network analyzer with alternative source oscillator configurations. Finally, future improvements to the hologram CATR such as a dual-reflector feed system are discussed.
We have designed and are building a 30 foot diameter spherical near-field range with some unusual and useful features. The range is designed to test up to 10 GHz. The range design is a double gantry arm type, the RF probe is moved and the antenna is normally stationary. The antenna is mounted to an anti-spin bearing (on the azimuth arm) located coaxially over an azimuth positioner. The antenna can be held stationary or rotated to check for room reflections. The azimuth positioner rotates a post supporting an elevation positioner, which in tum rotates a counterweighted elevation probe arm. Holding the antenna stationary means the cables and waveguides are not moved or twisted during testing. Testing in a stationary position is more accurate when gravity or thermal loads are significant. High power RF testing is safer and cheaper with a stationary antenna.
Saab Ericsson Space and Ericsson Microwave Systems have recently completed the installation of a new efficient test facility. The facility is a fully automated test range designed for high th roughput of measurements. The facility is mainly used for tests of antennas for satellites and for mobile com munication. It is used as a far-field range for small antennas or as a spherical near field range for directive antennas. The frequency range covered is 0.8 - 40 GHz. A design driver for the facility was the logistics of measurements, short test time and easy access to the AUT during measurements. To achieve this, high speed positioners and easy access to the AUT via a drawbridge in the anechoic chamber were introduced. The computer controlled RF system allows the use of automatic mode switching to test the AUT in either receive or transmit mode and to change frequencies and mixers without operator intervention.
RATSCAT has pursued a wide gamut of technical enhancements and upgrades to its Mainsite and RATSCAT Advanced Measurement System (RAMS) locations. Acquisition of three radar systems has provided RATSCAT with the most capable radar systems available. RAMS is capable of acquiring full scattering matrix (FSM) data from 120 MHz to 36 GHz. Mainsite is capable of acquiring bistatic FSM data from 2 GHz to 18 GHz and monostatic FSM data from 1 GHz to 36 GHz. RATSCAT is pursuing unparalleled background levels through the acquisition of new pylon technology at RAMS and is expanding its target handling capability via construction of additional target storage as well as the addition of a mobile target handling shelter and new 50' and 14' pylons at Mainsite. RATSCAT has acquired a full feature data processing capability at both sites that uses a reflective memory interface between data acquisition and data processing resulting in faster validation of data cuts. Through acquisition programs and partnership with industry RATSCAT has improved their RCS test capability to become the technical leader in outdoor static RCS testing.
The Boeing Near Field Test Facility (NFTF) in St. Louis, MO was constructed in 1991 to conduct near field RCS measurements of production parts, models, and full-scale operational aircraft. Facility upgrades were identified in 1997 to support operational aircraft testing, such as the F/A-18 E/F. Target rotation mechanization, measurement antennas, and the test radar were identified as requiring upgrades. The target rotation hardware was upgraded to a 40-foot diameter turntable capable of handling production fighter aircraft. Antennas were mounted in an elevation box, which also contains the radar and an absorber aperture. The elevation box translates vertically, and pitches in elevation for different view angles. A new Lintek Elan radar, with a frequency range of 2ml8 GHz, 200 Watt Traveling Wave Tube (TWT) amplifiers, and Programmable Multi-Axis Controller cards (PMAC), controls all motion in the facility. In addition, modifications to the facility were completed to improve efficiency and ergonomics.
In this paper, a novel means of assessing the RF performance of modern helicopters at Aviation Applied Technology Directorate (AATD) is presented through a newly built facility called the Modular Helicopter Communications Measurement Facility (MHCMF). The MHCMF provides AATD personnel the ready ability to perform a wide range of communications measurements on a number of different helicopters. Based around a novel process of utilizing compressed air as a means to "float" the vehicle or helicopter under test (HUT) over a smooth concrete surface abutting the Felker Army Airfield at Fort Eustis, VA, the HUT is rotated under computer control and the radio frequency (RF) characterization of the HUT is acquired and displayed for the operator and antenna engineer as well.