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Back projection techniques have been used extensively in planar near-field ranges and to a lesser degree in spherical near-field ranges. Recently a back projection technique allowing back projection from spherical near-field data onto a planar surface has been published and implemented. This paper explores this technique further through the presentation of measured data for a large microstrip array antenna. The results demonstrate how the technique can be used to investigate anomalies in the feed structure of the array.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations ä z (x, y) can be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence. This paper describes a method for estimating the Zposition error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
We discuss the mitigation of truncation errors in spherical scanning measurements. The main emphasis is the spherical harmonic representation of probe transmitting and receiving functions; however, our method is applicable to nearfield measurement of electrically small antennas for which fullsphere data are either unreliable or unavailable.
We have measured the amplitude and the phase of the electric field on a planar area very near (about 0.3 wavelengths) to the aperture of a X-band standard gain horn antenna using a photonic sensor and transformed the aperture field distribution to the far field pattern. The measured aperture field distributions and antenna patterns agreed well with those calculated by the method of moments. Comparing the far field patterns by the photonic sensor and the conventional open-ended rectangular waveguide probe reveals that the antenna measurement using the photonic sensor has advantages over the conventional probe.
High precision near field measurement systems in dual polarization have stringent requirements on the probe performance in terms of radiation pattern shape, on-axis and off-axis polarization purity and port-to-port isolation. In general, these specific requirements can be fulfilled using single polarized probes for narrow frequency bands (about 10% relative bandwidth) and using mechanical rotation for polarization diversity. Consequently, several sealed probes and complicated procedures are necessary to cover the operational frequency band of most common antenna applications leading to inefficient and time-consuming measurement procedures. SATIMO has developed high precision wide-band, dual polarized near field probes covering the frequency range from L to Ka-band to overcome this problem. Two different probe technologies have been applied, each particularly well suited for the appropriate low (L to X-band) and high (X to Ka-band) frequency range. The low frequency probe design is based on a compact corrugated horn with capacitive orthogonal excitations. The high frequency probe design consists of an axially symmetric corrugated horn, a square to circular wave-guide transition, and a wide-band, high isolation ortho-mode junction (OMJ) exciting the orthogonal polarizations. Probes in C-band and Ku-band have been delivered to and tested by ALCATEL SPACE INDUSTRIES in their planar near field antenna test range in Toulouse. The C-band probes have operational bandwidths of 25% covering the entire commonly used transmit and receive frequency bands for C-band communication satellites. The Ku-band probes have operational bandwidths of 40% covering the entire commonly used transmit and receive frequency bands for Ku-band communication satellites.
A passive intermodulation (PIM) near-field XY-scanner for the GSM900 frequency band has been earlier constructed to localize distortion sources in antennas and in other open structures. However, the measured intermodulation level has been relatively high, around 90 dBm. The equipment should be able to measure distortion levels down to 115 dBm with an input power of 2x20W, since the noise floor of a GSM900 base station is typically around 110 dBm. The sensitivity is limited either by thermal noise or by residual intermodulation distortion depending on the sensor coupling. Various causes of residual intermodulation distortion in the PIM near-field measurement are considered and evaluated. Sensitivity measurements of the scanner have been carried out on two test devices. With a sensor coupling of 30 dB, sensitivities of 115 dBm and 105 dBm have been achieved with an electric and a magnetic field sensor, respectively.
A filter-based approach to software range gating is presented. Conventional approaches to range gating are widely used and include hard gates applied in the time domain and running average filters applied in the frequency domain. The potential problems with those methods are well understood and involve (1) sideloberelated distortion of the frequency-domain data caused by hard clipping in time and (2) the dual problems that arise from finite-duration smoothing kernels in the frequency domain. Herein, range gating is formulated as a digital filter design problem. We employ a type-II Chebyshev design, which has a maximally flat pass-band and a specified stop-band attenuation. User parameters include constraints on the smoothness of the passband and the width of the gate transition. Edge effects are minimized by filtering symmetrically extended copies of the measured data. The results are illustrated on data acquired by the JHU-APL compact range.
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
This paper presents an approach to experimental identification and investigation of the higher-order diffraction effects. The proposed technique allows one to determine parameters (particularly coordinates of the attachment and launching points) of the higher-order diffraction centers and can be considered as an extension of the Inverse Synthetic-Aperture Radar (ISAR) imaging technique.
Ultra-Wide Band (UWB) antenna descriptors have been used to design an optimal Log Periodic Dipole Array (LPDA) antenna for the Army Research Laboratory’s (ARL) BoomSAR. Although dispersive in nature, the LPDA design offers improvements over existing antennas with broader beamwidth, higher efficiency, and improved off-boresight polarization performance. In the UWB/A SAR application, it is important that the antenna polarimetric spectral response remains coherent and uniform over the aspect angles used in the image formation process. This paper describes the process by which these goals were achieved and presents measured results from the BoomSAR of trihedrals and cylinders that validate the approach.
A Ground-Based Synthetic Aperture Radar (GB-SAR) interferometer system operating at 17 GHz is used to monitor the movement of an active landslide. The selection of the optimal image formation technique for such an imaging system is addressed. The algorithms considered in this study are those previously developed for spaceborne and airborne SAR. A near-field algorithm that forms the image in the time domain is selected as the optimal solution. Furthermore, example results obtained in a measurement campaign in Schawz (Austria) are shown.
A portable handheld antenna array system (SARBAR) capable of generating high-resolution two dimensional spotlight radar images is designed and built. The design goals were to build a portable device with maximum sensitivity, that can generate zonal images of a target at close range, and produce live updates of the scene (goal of 10 image per second). To achieve the design goals, an array antenna setting with separate transmit and receive elements have been used. The radar system is based on conventional FM-CW homodyne radar. The novelty of the design, however, is that for each FM CW waveform, the signal is successively routed through all the transmit elements and received from the designated receive elements. The transmit/ receive switching is such that a complete scan over the entire frequency and aspect interval is obtained in less than 80 msec. This allows image update rates that make the SARBAR resemble a video camcorder.
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.
This paper will draw the attention to a revolutionary new, extremely mobile and flexible approach on a nearfield test facility concept for outdoor measurements. After addressing the current measurement dilemma, the potential measurement objects are indicated, covering application areas in telecommunication, defense, air traffic management, research and verification of outdoor antenna & RCS test facilities. Further an outlook will be given on the future and urgent necessity on measurements of the radiated performances of outdoor antenna installations. The presented antenna test facility is based on a remotecontrolled and floating platform, enabling probing of electromagnetic fields within relatively large air volumes of up to 100 x 100 x 100 meters. In combination with precise position techniques, accurate measurements of up to 20 GHz are considered to be achievable. The design philosophy and system concept will be explained. The paper concludes with a prediction on the system performance and with a brief realization schedule. The proposed ANTF concept will allow detailed radiation analyses in unprecedented depth and quality, representing a real breakthrough in characterizing electromagnetic fields in open air test sites (OATS).
Choosing the proper antenna range configuration is important in making accurate measurements and verifying antenna performance. This paper will describe the steps involved so the antenna engineer can select and specify the best antenna range configuration for a given antenna. It will describe the factors involved in choosing between near-field systems versus far-field systems, and the different scan types involved. It will explain the advantages of each type of antenna range and how the choices are affected by such factors as aperture size, frequency range, gain, beamwidth, polarization, field of view, sidelobe levels, and backlobe characterization desires. This paper will help the antenna engineer identify, understand, and evaluate the applicable characteristics and will help him in specifying the proper antenna range for testing the antenna.
The radiation properties of an antenna are defined in the far field, since this is the environment that they will operate. Creating far field conditions when testing a large aperture antenna is quite challenging. This is particularly true if testing occurs within the confines of an anechoic chamber, or if other complicating field characteristics (like angle-of-arrival simulation) are desired. Rather than attempt to generate a true planewave in the usual manner, we propose an instrument that creates a field distribution in the near field of a transmit array that is planewave-like in nature only over specified regions of interest (a region occupied by an antenna under test, for example); we do not require that the incident field be a true planewave at other locations. In these other locations the field is free to assume any value demanded by the governing equations of electromagnetics. By relaxing the requirement on the electromagnetic field in the test volume, we considerably reduce the complexity of the problem and define a tractable problem with a potential engineering solution.
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.
A calibration uncertainty analysis was conducted for the Air Force Research Laboratory’s (AFRL) Advanced Compact Range (ACR) in 2000 [1]. This analysis was a key component of the Radar Cross Section (RCS) ISO-25 (ANSI-Z- 540) Range Certification Demonstration Project. The scope of the RCS uncertainty analysis for the demonstration project was limited to calibration targets. Since that time we have initiated a detailed RCS uncertainty analysis for a more typical target measured in the ACR. A “more typical” target is one that is much larger with respect to wavelength than the calibration targets and characterized by a wide dynamic range of RCS scattering levels. We choose a 10’ ogive as the target due to the fact it is a large target, exhibits a wide dynamic range of scattering, and the scattering levels can be predicted using readily available CEM codes. We will present the methodology for the uncertainty analysis and detailed analyses of selected component uncertainties. The aspects of the uncertainty analysis that are unique to the “typical target” (i.e., a non calibration target) will be emphasized.
The on-orbit performance of communication satellites is measured for two reasons. First, the satellite’s compliance with specified performance must be evaluated by measurements conducted shortly after the satellite’s launch. Second, satellite performance over its lifetime must be monitored and diagnostic capabilities augmenting telemetry data must be provided if performance anomalies develop during the satellite’s lifetime. Necessarily, these measurements quantify the system level performance of the satellite with specially developed and calibrated test terminals.
The Antenna Branch of NAVSEA Crane was tasked to design and formulate a plan to pattern test a CWI Illuminator antenna in line of sight with a SATCOM antenna. The Navy has problems finding new places aboard ship to mount antennas without having interaction between them. The separation would be about 19 feet, within the near field zone of the Illuminator. The testing was performed on a 2000 foot outdoor range. A special test fixture was designed by Crane Engineering to mount both the Illuminator and the SATCOM to their proposed mounting locations. The unique characteristic of this measurement approach is the mounting of both the FCS antenna and the SATCOM antenna and radome on a test fixture, which allows complete pattern measurements with different antenna orientations relative to each other. A limited raster scan was used for collecting the data in a 3-D result. Baseline data files were collected without the SATCOM present for comparisons. A Matlab program written to evaluate the results. The proposed mounting location produced unacceptable results in the radiating pattern of the Illuminator antenna. Crane Engineering calculated a new mounting location from the results of the data taken in the Raster scan. Subsequent testing was done and proved to be a valid location for the Illuminators test requirements.