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ABSTRACT A new probe compensated near-field – far-field transformation technique with planar spiral scanning is here proposed. It is tailored for quasi planar antennas, since an oblate ellipsoid instead of a sphere is considered as surface enclosing the antenna under test. Such an ellipsoidal modelling is quite general (containing the spherical one as particular case) and allows one to consider measurement planes at a distance smaller than one half the maximum source size, thus reducing the error related to the truncation of the scanning surface. Moreover, it reduces significantly the number of the needed near-field data when dealing with quasi planar antennas. Numerical tests are reported for demonstrating the accuracy of the far-field reconstruction process and its stability with respect to random errors affecting the data.
A novel approach using Artificial Neural network (ANN) is proposed to identify the number of faulty elements present in a uniform linear array consisting faults in multiple elements. The input to the neural network is amplitude of deviation pattern and output is the number of faulty elements. In this work, ANN is implemented with three different algorithms; Radial Basis Function neural network (RBF), Generalised Regression neural network (GRNN) and Probabilistic neural network and their performance is compared. The network is trained with some of the possible faulty deviation patterns and tested with various measurement errors. It is demonstrated that the method gives a success rate of 93.4%.
Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed at CEA for indoor near field monostatic RCS assessment. The experimental layout is composed of a motorized rotating arch (horizontal axis) holding the measurement antennas. The target is located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allows full 3D near field monostatic RCS characterization. Two bipolarization monostatic RF transmitting and receiving antennas are driven by a fast network analyser : - an optimised phased array antenna for frequencies from 800 MHz to 1.8 GHz - a wide band standard gain horn from 2 GHz to 12 GHz. This paper describes the experimental layout and the numerical post processing computation of the raw RCS data. Calibrated RCS results of a canonical target are also presented and the comparison with compact range RCS measurements is detailed.
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Once this RCS is acquired it may be very interesting to calculate RADAR pictures of these targets because RADAR picture allows emphasizing the bright points. Until now, CELAR produced images in two dimensions, but these pictures have shown their limits in order to locate problems in altitude. This article fills this gap while proposing two methods in order to get an image in three dimensions: a method using a three-dimensional Fourier transform and a method based on interferometry.
This paper describes the details of a specialized data acquisition system developed at the David Florida Laboratory. The system acquires, monitors, records and performs post measurement analysis of passive intermodulation (PIM) and multipaction events observed during RF testing. This characterization of components and systems carrying radio frequency signals is an important element of space qualification of satellites and other space faring systems. A National Instruments PXI chassis equipped with a PXI-4462 acquisition card and a LabView based software application was implemented to digitize the resulting data. A second application provided by InfoBright permits the compact storage of hours of measured data in its entirety (multiple channels each sampled at over 200,000 samples per second) using a specialized real time data compression scheme. The application also permits quick retrieval of relevant data segments using SQL query processing. Performance of this solution is presented along with its effectiveness in detecting details of PIM and multipaction events.
A new method is here proposed to accurately evaluate the Specific Absorption Rate (SAR), e.g. of a mobile phone, through free-space measurements. The method takes advantage of the simple yet powerful plane-wave spectrum (PWS) representation of the electromagnetic (EM) field. The emitting device is tested in an anechoic chamber, where the two tangential components of the electric field are measured (amplitude and phase) and expanded into their PWS. These experimental data are subsequently fed to an equivalent transmission-line representation of the planar stratified structure composed by stacking the half-space made of free-space and the stratified flat phantom. Numerical simulations have shown that this method allows to accurately reconstruct the E field distribution inside a homogeneous phantom, with a worst-case error of 26 % in the estimation of the peak E field [1,2]. Furthermore, the proposed method is the first practical procedure for assessing the SAR in a stratified phantom, where the standard approach of moving a probe inside a liquid-filled phantom is no more feasible.
If an acquired RF field data set captures all the radiated energy, transformations will have minimal errors. However, it is sometimes impractical to capture the complete radiated field. In this case, some sort of edge treatment is required before transforming the data set. Usually, a function such as a cosine taper is added to the edge to minimize transformation errors. Unfortunately, these functions may be discontinuous to the measured data and its’ derivatives. This paper will present a method of truncation which matches the measured data and its’ derivatives. It will then transform the RF field data to the compact range reflector surface and compare the results of several truncation methods.
The present paper introduces a lower frequency design for the open boundary quadridge horn (OBQH) introduced in [1]. This new horn cover the UHF band and it is usable up to 6GHz. It exhibits a fairly uniform radiation pattern at the upper end of its range as well as a fairly flat gain as was the case with the higher frequency design. The increased frequency band up to 6GHz is accomplished by the use of a ferrite filled cavity that maintains a good VSWR even when the feed cavity is reduced to avoid higher order modes that cause the main beam of the pattern to split. As with the higher frequency design this horn can be used as a source in antenna pattern measurement chambers and even reflectors. As a second part to the paper some data is presented on the use of the S to Ku Band OBQH as a feed for reflectors used in Radio-Astronomy [2]. The results show that by placing the OBQH in an absorber lined cavity the pattern improves and the feed becomes more effective.
In order to ensure proper measurements in the compact range, the reflector needs to be aligned within the range. Unfortunately, the reflector does not have any direct method of leveling or locating such as straight edges or fiducials at known locations. The only known reference is the ideal point cloud. As the point cloud is given, it is oriented correctly in the range. So by centering the point cloud in the range, the compact range reflector can be aligned to the range by minimizing its deviation from the ideal point cloud. This paper will go through the mathematics used to accomplish this alignment in the translation along and rotation about the three primary axes. In addition, it will give a method of determining reflector twist. The method is sufficiently generic that it can be applied to other shapes and figures of merit.
The 2004 AMTA paper entitled “The “Cam” RCS Dual-Cal Standard” introduced the theoretical concept of the “cam,” a new calibration standard geometry for use in a static RCS measurement system that could simultaneously offer multiple “exact” RCS values based on simple azimuth rotation of the object. Since that publication, we have constructed a “cam” to further explore its utility. The device was fabricated to strict tolerances and its as-built physical geometry meticulously measured. Utilizing these characteristics and moment-method analysis, a high-accuracy computational electromagnetic (CEM) “exact” file required for calibration was produced. Finally, the “cam” was evaluated for its efficacy as a single device that could be utilized as a dual-cal standard. This development was conducted with a particular focus on the hypothesized improvements offered by the new standard, such as the elimination of frequency nulls exhibited by other resonant-sized calibration devices, and improved operational efficiency. In this follow-on paper, we present the advantages to and challenges involved in making the “cam” a viable RCS dual-cal standard by describing the fabrication, modeling and performance characterization.
In the Flight Model (FM) of the PLANCK telescope, the feed horns are connected to either HEMTs or bolometers operating at cryogenic temperatures to detect the Cosmic Microwave Background radiometric signal. For the purpose of an overall alignment verification at ambient temperature, reflectivity measurements will be performed using an auxiliary feed horn that is terminated with a switching diode. This verification test will be conducted at 320 GHz, to benefit from the narrow beam and a high sensitivity to misalignment. To perform the reflectivity measurements, an additional “circulator” with low loss and high isolation between transmit and return channels had to be developed. Besides that, the circulator co-locates the phase centres of both Tx and Rx range antennas on the focal point of the CATR, which allows monostatic reflectivity measurements. Quasi-optical techniques have been used to design a circulator that meets these requirements. The assembly has been developed, tested and used for reflectivity measurements.
ACC has developed for the ESA-ESTEC CATR a compact but highly versatile 5-axis positioner. It is composed of a roll axis, upper azimuth, elevation, translation and lower azimuth axis. The clearance between the floor and the translation stage is designed to pass over a 12” walkway absorber while the roll axis height is only 155 cm (~5 feet). The standard configuration for medium or high gain antennas is the roll-over-azimuth or elevation-overazimuth configuration with a vertical interface for the AUT. For omni-directional antennas and RCS measurements, the positioner can be configured as a low profile azimuth positioner with a horizontal interface without a blocking structure behind the AUT. The positioner can also be configured for bistatic RCS measurements and Spherical Near Field. With the addition of a linear scanner, the Quiet Zone can be scanned in a polar way but also planar scanning is possible. Other key parameters are: angular accuracy: 0.01°, accuracy of the translation axis: 0.01 mm, load capacity 100 kg.
Solid state diode based multipliers and sub-harmonic mixers are enjoying increasing popularity as frequency extenders for the mm-and sub-mm wave frequency bands. Nowadays, models are available with 40% bandwidth, thus covering full frequency bands, with a reasonable amount of output power. When driven by a frequency synthesizer, they exhibit excellent frequency and phase stability, in contrast to tube devices like a Backward Wave Oscillator (BWO). Drawbacks of the multipliers and sub-harmonic mixers (SHM’s) include their low efficiency, requiring high power amplifiers (HPA’s) to drive them, and the difficulty of achieving broadband impedance matching, which makes it hard to get a constant performance level over the band. For the transmit module, a single HPA driving the multiplier directly turned out to be a satisfactory solution. On the receive side, a feedback circuit regulating the LO power amplifier was introduced. This circuit is based on pilot tone injection in the IF channel of the SHM. The modules have been breadboarded and tested.
Current means to improve position accuracy in antenna ranges are often expensive, consume important CPU time, and/or limit data acquisition speed. By taking advantage of axes with good repeatability, higher multi-dimensional positioning accuracy can be achieved directly by a controller to ease complexity and achieve real-time position correction. A product family of controllers brings this capability to fruition. Comparison analysis of field data demonstrates improved accuracy with no measurement speed degradation. Results indicated a considerable accuracy improvement limited by axis repeatability. Existing and new antenna ranges can benefit from this simple cost-effective approach to improved position accuracy.
Boumans [1] has introduced an alternative to the classical (Advanced) Antenna Pattern Correction (A)APC method by moving the range feed in the focal plane of a Compact Antenna Test Range (CATR) instead of moving the Device Under Test (DUT) around in the Quiet Zone (QZ). The advantages are clear: it is easier (cost and accuracy wise) to implement a feed scanner than a DUT scanner; the method can be used for azimuth and elevation patterns and it can even be implemented using multiple feed horns to get to the same measurement time as with a single range feed. The capabilities of defocused measurements in the Compact Payload Test Range (CPTR) at ESA/ESTEC have been previously assessed [2] and they revealed a triply reflected ray [2] and a QZ ripple induced by periodic surface inaccuracies [3]. This paper focuses on verifying the performance of the Focal Plane AAPC method for these effects. Use has been made of the well known DTU-ESA VAST-12 antenna [3].
Modern day remote sensing spacecraft often feature multiple payloads sharing a common bus (spacecraft platform). RE02 emission testing (1, 2) characterizes the emission signature of a given payload in order to assess electromagnetic compatibility with respect to other payloads (i.e. “victims”) on the bus. Typically, a simple path loss model based on 1/r2 power variance (ref: Friis path loss equation) is used to account for the distance between the emitting and victim payloads using measured amplitudes taken during RE02 measurements. RE02 measurement technique (2) dictates that emissions testing take place at a fixed radial distance of one meter from the radiating instrument. At certain frequencies, however, this measurement takes place in the near field of the emitter. In general, power density amplitudes are greater in the near field than its far field counterpart. This paper investigates any potential error incurred by not accounting for this effect. A simple math model for a common mode radiator is developed to estimate this error and attempt to better understand the field relationships at lower frequencies where the near field predominates.
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
A new antenna diagnostics technique has been developed for the DTU-ESA Spherical Near-Field Antenna Test Facility at the Technical University of Denmark. The technique is based on the transformation of the Spherical Wave Expansion (SWE) of the radiated field, obtained from a spherical near-field measurement, to the Plane Wave Expansion (PWE), and it allows an accurate reconstruction of the field in the extreme near-field region of the antenna under test (AUT), including the aperture field. While the fundamental properties of the SWE-to-PWE transformation, as well as the influence of finite measurement accuracy, have been reported previously, we validate here the new antenna diagnostics technique through an experimental investigation of a commercially available offset reflector antenna, where a tilt of the feed and surface distortions are intentionally introduced. The effects of these errors will be detected in the antenna far-field pattern, and the accuracy and ability of the diagnostics technique to subsequently identify them will be investigated. Real measurement data will be employed for each test case.
Accurate antenna measurements at sub-millimeter frequencies are very challenging. Especially the phase measurement accuracy is usually limited by the mechanical accuracy of the measurement equipment. The measurement techniques used, and the measurement results of a dual reflector feed system (DRFS) at 650 GHz are presented in this paper. Planarity error compensation technique was used that enabled accurate correction to the measured phase pattern without accurate pre-existing information of the planarity error of the planar near-field scanner. The measured DRFS beam agrees well with the simulated and the achieved measurement accuracy is good.
In this paper we present and evaluate a method for estimation of in-network performance of mobile terminal antennas developed by the Swedish telecom operator Telia. The Telia Scattered Field Measurement (TSFM) Method is intended to give a better estimate of the performance of the mobile terminal antenna as in an in-network fading scenario. The parameter measured from the TSFM method is referred to as the Scattered Field Measurement Gain, SFMG, i.e. the Mean Effective Gain, MEG, measured relative to a half wave dipole antenna. MEG includes the radiation pattern of the mobile terminal antenna as well as an estimate of polarization and directional losses that occur due to the propagation environment. In this study it is found that the TSFM method provides a good measure of the in-network performance of the mobile terminal antenna. Furthermore, it is shown that the SFMG measured with this method is found to be well correlated with the Total Radiated Power Gain, TRPG, or radiation efficiency. This suggests that the Total Radiated Power, TRP, may be a good measure of the in-network performance of mobile terminal antennas if measured with proper adjustment to the antenna and propagation channel mismatch.