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A Technique is presented that allows for broadband nondestructive material electrical parameter measurements. Electrical parameters of a large number of materials are not readily available over extremely broad bandwidths (multiple octaves as an example). This information is required for accurate modeling of microwave circuits and antenna(s). These parameters consist of complex permittivity and complex permeability that result in loss due to the types and thickness of materials to be used. A Method is required that allows for fast, accurate and low cost measurements of the materials under test. The technique of using dual coaxial probes provides a solution that can be applied to numerous materials including thin films. It takes advantage of the full frequency extent of the network analyzer. This measurement uses dual coaxial probes, as compared to the implementation of cavity resonators, coaxial lines, waveguides and free space measurements, and performs the measurement in a 2-port calibration procedure. The resultant analytical solution is a transcendental equation with complex arguments. The Coaxial probes are described and can be easily made with available components where the only limitation is the valid component frequency bandwidth. Several material examples show the expected accuracy versus frequency range of this measurement technique.
Modern radome measurements often involve scanning the radome in front of its antenna while the antenna is actively tracking an RF signal. Beam deflections caused by the radome are automatically tracked by the antenna and its associated positioning system, which is typically a two-axis (pitch & yaw) gimbal. The motion required to accurately track the beam can be very demanding of the gimbal. High structural stiffness, zero drivetrain backlash, and extremely accurate angle measurement are all necessary qualities for radome beam deflection measurement. This paper describes a new, advanced, two-axis gimbal that embodies those qualities. The new gimbal incorporates direct-drive motors to achieve zero backlash. The motors are mounted directly to the rotating gimbal elements, thereby eliminating the usual causes of drivetrain compliance. Rated torque of the motors is not high, and the antenna is therefore fully counterweighted. Each of two optical encoders is mounted on the same rotating gimbal element as its associated motor. The encoders are directly mounted; no flexible coupling is used. The antenna is mounted to those same rotating elements. Antenna positioning error due to windup of the structure and drivetrain is virtually eliminated. Eccentricity of the encoder disk, which is the primary source of direct-drive encoder errors, is adjusted by virtue of a remarkable in situ process.
A technique is presented for determining the insertion phase of array elements directly from time domain measurements. It is shown that the Inverse Discrete Fourier Transform (IDFT) commonly used in swept frequency time delay measurements may yield unreliable phase results. A compensation to the IDFT is proposed which allows the phase of an array element to be accurately estimated from time domain data without gating and without taking a second DFT. The technique is applied to determine the insertion attenuation and phase of the elements in a linear L-band phased array. Compared to conventional array calibrations, the removal of extraneous range reflections implicit to the time domain technique resulted in a significant improvement in measurement accuracy.
The restriction of ? 2D2 R = is a commonly employed criterion for the minimum required separation between the range antenna and the Antenna Under Test (AUT) in a Far-Field (FF) antenna test range. However, this criterion, which is suitable for most common and simple cases, may not be adequate for more specialized test applications. Direction-finding (DF) interferometer antenna array testing is one such example. In a DF interferometer antenna array the phase difference between any two antennas serves as an Angle-Of-Arrival (AOA) discriminator for the radiation impinging on the array. At the system level, the array must be tested in order to calibrate its AOA discrimination function and to evaluate its accuracy, which, in many cases is done using a FF test range. In this paper, interferometer array FF testing is analyzed and an expression is developed for estimating the required separation between the range antenna and the array under test, in order to satisfy certain angle discrimination accuracy requirements. The results are compared with the common FF criterion and with restrictions imposed by other considerations.
This paper deals with different aspects of the on-ground antenna pattern characterization of the MIRAS radiometer for ESA’s SMOS mission. Various technical challenges of the project are briefly described. Special attention is given to the effect of multiple reflections between the antenna under test and the measurement probe. A series of antenna measurements of the MIRAS radiometer antennas is now on-going at the DTU-ESA Facility. The main objectives of these are to investigate the accuracy of the forthcoming antenna characterization, to find solutions to the already known problems, to identify new possible difficulties, and to establish an optimal measurement strategy, which should allow for the tight error requirements and minimize the overall time of the measurement campaign.
Bistatic radar cross sections of targets are computed from field measurements on a cylindrical scan surface placed in the near field of the target. The measurements are carried out in a radio anechoic chamber with an incident plane-wave field generated by a compact-range reflector. The accuracy of the computed target far field is significantly improved by applying asymptotic edge-correction techniques that compensate for the effect of truncation at the top and bottom edges of the scan cylinder. The measured field on the scan cylinder is a “total” near field that includes the incident field, the field of the support structure, and the scattered field of the target. The background subtraction method determines an approximation for the scattered near field on the scan cylinder from two measurements of total near fields. The far fields of metallic sphere and rod targets are computed from experimental near-field data and the results are verified with reference solutions.
We introduce a new calibration standard geometry for use in a static RCS measurement system that can simultaneously offer multiple “exact” RCS values based on a simple azimuth rotation of the object. Called the “cam,” the new calibration device eliminates the problem of frequency nulls exhibited by other resonantsized cal devices by shifting the nulls through azimuthal rotation. Furthermore, the “cam” facilitates the use of dual-calibration RCS measurements without the need to mount a second cal standard. The “cam” is practical to fabricate and deploy; it is conducting, composed of flat and constant-radius singly-curved surfaces, and is compatible with standard pylon rotator mounts. High-accuracy computational results from moment-method modeling are presented to show the efficacy of the new standard.
This paper describes the installation and implementation of a Cylindrical Near-field Test Facility at Chelton Radomes Ltd, Stevenage, (formerly British Aerospace Systems and Equipment Ltd.), in the UK for the testing of large radome/antenna combinations. Test site commissioning and validation activities to determine measurement accuracy & repeatability for the radome performance parameters of transmission loss and boresight error, are discussed. Test data from actual measurements are presented.
While probe arrays are now recognized to allow rapid and accurate near-field measurements, the interaction with the Antenna Under Test (AUT) is still sometimes considered as a potential limitation, especially for electrically large directive antennas [1]. Based on numerical simulations, this paper reports the results of a thorough investigation of the interaction mechanism and analyses its impact on the far-field pattern accuracy. The most often, interaction effects can be maintained at an acceptable level, thanks to an appropriate design of the probe array element and structure. However, the efficiency of a posteriori compensation schemes has also been investigated. The Pattern Coherent Averaging Technique (PCAT) [2], which is well known for compensating plane wave deviations in the quiet zone of antenna far-field test ranges or interactions from single probe near-field facilities, also proved very efficient to reduce the interaction effects with a probe array.
In this paper I demonstrate how our current technology now very readily permits a standard of accuracy and utility to be realized, that was formerly available only in research laboratories. This is accomplished with standardly available positioning equipment and standardly available software. Accurate alignment of the range is enabled by a tracking laser interferometer. This composite nearfield scanning antenna range has afforded us the opportunity to compare readily, far-field results from the classic planar, cylindrical and spherical coodinate systems. Comparison data are presented.
This work is part of the UK Ministry of Defence initiative to examine causes of uncertainties in RCS measurements and to establish a network of certified facilities. Having developed a ‘best practice’ guide where causes of uncertainty were listed, the effect of polar diagram was selected as a priority topic. Correction algorithms for RCS measurements require knowledge of the beam shape and resolution in crossrange of the significant scatterers. Accordingly, the accuracy of polar diagram measurement, the effect of amplitude ripple and the applicability of the correction algorithms to near-field data were addressed. Measurements were made on two targets, a long cylinder and a small aircraft. Two antennas and two ranges were used to achieve 1dB, 3dB and 6dB illumination tapers across the cylinder. The 6dB taper situation was modelled for three different numbers of points. The work demonstrated that polar diagram effects are significant for point scatterers or simple targets, like the cylinder; however, for the small aircraft with a large number of distributed scatterers, the overall effect is less significant.
One basic Direction Finding (DF) technique for Radar is Amplitude Based Comparison DF. Multiple directional antennas are placed around an aircraft to get a 360 deg view of the area. By placing these antennas on the aircraft, the antennas are subjected to reflections from the aircraft, which distorts the antenna characteristics. This antenna distortion causes errors in the measurement of the angle of arrival. The work presented here describes the measurement of the antenna characteristics of a cavity backed spiral antenna both by itself and attached to the nose of an MH- 47A helicopter nose measured in an anechoic chamber. The spiral antenna’s pattern was changed when it was measured on the helicopter. The effect this change in pattern has on the DF accuracy is discussed.
ORBIT/FR-Europe is in the process of finishing a new Compact Antenna Test Range for Ericsson Microwave Systems (EMW) in Sweden. The design was presented at AMTA 2001. Here the progress in the project is presented. Most subsystems are now installed, and system level acceptance will follow in the near future.
Sensor Concepts, Inc. has developed the SCI-Xe Portable Microwave Imaging System prototype for use in the assessment of the low observable (LO) characteristics of fielded military platforms in their native environments. The SCI-Xe is a single man deployable suitcase-size system that employs a small linear rail in order to acquire Linear Synthetic Aperture Radar (LSAR) data in the 8-18 GHz frequency range. Data collections are performed via a single button push and the data is stored on a removable harddrive for comparison to an existing database for analysis. Recent deployment of the SCI-Xe prototype has provided excellent feedback on the viability of performing repeatable field measurements using alignment techniques that do not significantly affect the overall system size and weight. The SCI-Xe employs a video camera and uses video image algorithms such as edge detection, thresholding, and overlay masks to provide a simple coarse alignment to a stored baseline position. Once positioned, a single LSAR collection is performed to provide the radar data necessary for analysis, which includes a robust image registration algorithm to first, perform a quantitative assessment of the positioning accuracy and second, align the data for further image filtering and statistical processing.
Contoured multi-beams achieved by multi-feed reflector antennas, realized in modern communication satellites, like Intelsat VIII and IX generations satellite, require an economic measurement of their antenna characteristic. Further, highly accurate, but also fast and therefore real-time measurements are assumed to be applied for the testing of the antenna performance. For that aim, the Compensated Compact Range CCR 75/60, applied at e.g. Space Systems Loral (SSL) in Palo Alto (USA), at ALCATEL in Cannes (France), at the MISTRAL facility in Toulouse (France) and at Astrium GmbH (Germany) was developed and installed by Astrium GmbH. In order to optimize the measurement accuracy of the CCR, detailed error analyses and investigations for improvement measures were performed. Within this paper, the accuracy analyses and improvement steps will be presented in order to establish accuracy values, which can be realized in state-of-the-art compact range test facilities.
The NUWC/NPT Overwater Arch Antenna Range consists of a 70 ft radius measurement arch located over an elevated 90 ft x 65 ft salt water pool. This facility, located outdoors, presents mechanical and electrical challenges. Measurement accuracy and precision are a function of environmental parameters (including unwanted signals), physical plant and instrumentation characteristics. Measured data variation will be presented along with techniques which could be employed to improve range performance.
This paper describes a novel system that overcomes the inaccuracies in antenna radiation pattern measurements caused by multipath propagation. The system operates by specifically compensating for the effects of unwanted signals rather than by attempting to remove, or minimize, their effects through the use of screens or baffles or an anechoic chamber. Compensation is achieved through the use of an equalization technique, the parameters of the equalizer(s) being determined from a special measurement of the antenna range under consideration. The method is generally applicable; it may be implemented ab initio in new indoor or outdoor ranges, or retrofitted to existing ranges to improve accuracy. Most importantly, however, the basic idea leads to the design of a completely new type of real-time 3-D range in which sensors are placed on the surface of an imaginary sphere surrounding the antenna under test (AUT), and an anechoic chamber is not required.
With the continued growth of mobile communications and the emergence of wireless LAN and personal area networks (PAN), there is an increased need to accurately measure the antenna properties for omnidirectional antennas and antenna systems. Furthermore, it is very desirable that antenna measurement systems be flexible to support a variety of antenna configurations and form factors. In this paper, we assess the performance of two measurement configurations utilizing dielectric positioners. These configurations comprise a traditional roll-over-azimuth antenna positioner and an arm-overturntable system such as that used in ORBIT/FR’s Advanced Spherical Cellular Near-Field (ASCENT) product. The results show that both configurations offer demonstrable improvements over conventional metallic positioners, and the arm-based system provides the highest accuracy for omnidirectional antennas.
The need for a well-defined accuracy estimate in antenna measurements requires identification of all possible sources of inaccuracy and determination of their influence on the measured parameters. For anechoic chambers, one important source of inaccuracy is the reflection from the absorbers on walls, ceiling, and floor, which gives rise to so-called stray signals that interfere with the desired signal. These stray signals are usually quantified in terms of the reflectivity level. For near-field measurements, the reflectivity level is not sufficient information for estimation of inaccuracy due to the stray signals since the near-to-far-field transformation of the measured near-field may essentially change their influence. Moreover, the inaccuracies are very different for antennas of different directivity and with different level of sidelobes, and for different parts of the radiation pattern. In this paper, the simulation results of a spherical near-field antenna measurement in an anechoic chamber are presented and discussed. The influence of the stray signals on the directivity at all levels of the radiation pattern is investigated for several levels of the chamber reflectivity and for different antennas. The antennas are modeled by two-dimensional arrays of Huygens' sources that allow calculation of both the exact near-field and the exact far-field. The near-field with added stray signals is then transformed to the far-field and compared to the exact far-field. The copolar and cross-polar directivity patterns are compared at different levels down from the peak directivity.
In a conventional manner, a majority of compact ranges are currently used between 2 GHz and 200 GHz. Mechanical stiffness limits compact ranges at high frequency and diffraction effects are dominant at low frequency. However, CNES has installed a single reflector with dedicated serrations to perform accurate measurements between 800 MHz and 2 GHz. These serrations are 2 meters long and minimize the ripple in both amplitude and phase within the quiet zone. In order to further improve its measurement capabilities at lower frequencies, CNES has installed, in co-operation with SATIMO, a spherical near field measurement system directly inside of its compact range building. The goal is to measure antennas within the frequency range 80 MHz – 400 MHz with a relatively good accuracy. The spherical near field measurement facility has been tested and validated with four antennas that had been previously measured in the compact range of CNES and other external ranges. This paper focuses in this smart approach, which allows to extend the lower frequency domain of compact ranges. This paper describes in details the measurement facility, the test and the validation of the system.