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Traditional ISAR imagery measures a rotating target as a function of frequency and angular orientation - typically azimuth at a fixed elevation, with attitude accurately instrumented. For dynamic measurements, knowledge of angular motion (or attitude in general) may be lagging, insufficient or absent. K-space itself, combined with an image focal quality indicator, provides a unique geometry for separating the multi-dimensional problem of estimating rotational motion coefficients into individual estimations localized to separate regions of k-space. In this paper, a polynomial representation is reformulated to separate the annular width of the subtended aperture from a series of control points which effectively isolate regions of influence over k-space. Angular motion terms are estimated to tune subapertures of k-space such that overall image focal quality is maximized. These posited polynomials are inverted, and the associated RCS data are linearized via bandlimited resampling. Both blind and semi-blind (insufficiently instrumented angular motion) cases are addressed.
Many periodic structures are often built up of multiple layers to improve the electromagnetic performance such as the frequency bandwidth. Two approaches can be employed to analyze multi-layer structures: one is to formulate and analyze a multi-layer structure in its entirety; the other is to compute the generalized scattering matrix (GSM) for each layer and then obtain the total GSM of the structure by simple matrix calculations. The second approach is more flexible and efficient to practical problems where several layers may be cascaded in arbitrary sequences. This paper describes an efficient procedure to analyze multi-layer periodic structure using the finite difference time domain (FDTD) method. Based on the constant horizontal wavenumber approach, the procedure first computes the GSM of each periodic layer. The scattering parameters of the entire multi-layer structure are then calculated using the cascading formulas. The validity of this algorithm is verified through several numerical examples including frequency selective surfaces (FSS) with different periodicities and under different incident angles. The numerical results of the developed approach show good agreement with the results obtained from the direct FDTD simulation of the entire structure, while the new procedure saves the computational time and storage memory.
In this paper we explore how placing the same amount of RF absorber in different locations within a reverberation chamber can have different loading effects. This difference can have a significant impact on measurement reproducibility, both for measurements in the same chamber and measurements between chambers (i.e., round robin style testing). We begin by discussing some of the theories behind this and show some experimental results from different absorber placements in a reverberation chamber. Our experimental results will be presented in a fundamental format and in a practical sense (RMS-delay spread). We conclude with some suggestions on how to ensure that absorber is placed consistently.
Antenna miniaturization will continue to be a key issue in wireless communications, navigation, sensors, and RFIDs. For instance, each cellular tower is often populated with many antennas to cover different angular sectors and different frequency bands. Each modern notebook computer is likely embedded with multiple antennas to provide service in WWAN (824 MHz to 2170 MHz) and WLAN (2.4 GHz and 5.5 GHz), Bluetooth, etc. Also automobiles, vessels, and aircrafts will require more antennas to compete for very limited real estate. This dire situation is changing antenna designer worldwide with a goal to develop a new generation of physically small antennas that multi-bands or wideband. This paper presents several generic miniaturize antenna design examples that applies the concept of artificial transmission line concept for artificially control phase velocity and impedance. This miniaturization approach can be applied to reduce the size of both narrowband and wideband antennas using minimal amount of materials. Thus improves antenna’s efficiency, and reduces its cost and weight.
This paper discusses a measurement technique for accurately characterizing low cross polarization level of antennas, and associated sensitivity and errors. The technique involves two-antenna transmission (S21) measurement that includes an AUT and a reference antenna that has low cross polarization level. This technique needs two far-field transmission data from two different relative roll angles. The cross-polarization sensitivity is determined by SNR of cross-polarization component and cross-polarization of the reference antenna. The cross-polarization error is related to roll angle uncertainty and receiver noise.
This paper investigates the relative roles of the forward and return links in determining the operational range of passive UHF radiofrequencyidenti.cation(RFID) systems. The relativeimportanceof thelinksarediscussed to .rst-orderwith establishedfree .eld models. Then,weinvestigateboresight transmission and scattering with a dipole target in a storage room by comparing losses in the forward link against measured returnlinklosses based on measurementsdescribedby ISO 18047-6. Results across 895 to 945MHz normalized against semi-anechoicdata showeddisagreementbetween return link fading and the square of forward link fading by up to8dB within a measurement range of2m.
RF power harvesting enables controllable and simultaneous wireless power delivery to many RF devices. Devices built with this unique technology can be sealed, embedded within structures, or made mobile, thus eliminating additional service for battery. A key component of this technology is the “rectenna”, which is composed of antennas and rectifying circuitry to convert RF energy into DC power. Typically, multiple antenna elements are used to produce sufficient power for reliable device operation. This paper compares two different rectenna architectures (see Fig.1) for maximum RF-to-DC power conversion efficiency due to non-linear characteristics of the rectifying circuitry. A simple rectenna design example containing a 2x2 planar antenna array will be presented to demonstrate such RF power harvesting technology. The quantity, Rectenna Topology Indicator (RTI), is introduced for performance comparison.
Spherical near-field to far-field transformation techniques allow for the reconstruction of the complete radiation pattern of the antenna under test (AUT) from the knowledge of the tangential electric field over a spherical surface [1-2]. However, in practical spherical near field measurements there are zones on the measurement sphere where data are either not available or less reliable. When the spherical wave coefficients (SWC) are calculated from incomplete near-field data by setting to zero the unknown samples, the abrupt discontinuity in the field values at the edge of the scan area may lead to erroneously large values of the higher-order spherical harmonic coefficients. Different solutions have been proposed to circumvent this problem [3-4] and have been demonstrated effective for small truncation areas [3]. In this paper a novel approach is proposed for the reduction of the truncation error in spherical near-field measurements. The method is based on a proper filtering of the SWC in accordance with the extent of the minimum sphere enclosing the AUT. More specifically, it consists in iteratively imposing the matching of the near-field with the measured samples and performing a spectral filtering in the spherical harmonics domain, based on the knowledge of the physical extent of the AUT [5-8]. The procedure has been tested on synthetic as well as measured near-field data and has proved to be effective and stable against measurement errors. The approach has shown to be effective even for increasing truncation areas.
A unique wideband, dual-beamwidth, X-Band antenna has been developed by STAR Dynamics Corporation in support of a Ground-to-Air Radar Signature (GTARS) measurement system. The GTARS radar system provides precision dynamic RCS measurement and radar imaging capabilities for maneuvering in-flight aircraft. This specialized antenna and radar system provides the capability to track and measure dynamic radar target signatures and parameters that are not practical to measure on a static ground-based RCS measurement facility. The GTARS radar requirements posed significant challenges for the antenna design, among which are the capabilities to measure and track targets in wide and narrow fields of view (FOV) with simultaneous linear co- and cross- polarizations. Precision target tracking is required during dynamic measurements to maintain an accurate beam centered on the target during its flight. Consequently, STAR Dynamics has developed an offset reflector antenna with dual polarized five-aperture eight-port feed network that maintains the antenna beam precisely centered on the maneuvering target. The dual beamwidth functionality is achieved by two separate reflectors while each reflector provides multiple channels for simultaneous radar signature measurement and monopulse tracking using the eight-port feed array.
Hardware gating has been widely used to eliminate stray signals in the test range for single antenna. While testing the antenna on satellite, several issues should be considered to obtain accurate result. The difference come from several new conditions such as complicated electro-magnetic circumstance, desired stray signals from the satellite and varying of time delay due to antenna rolling. The width of hardware gating pulses and time delay of these two pulses are carefully set to ensure the measurement accuracy. Several methods are presented in this paper. These methods have been used in several test of antenna with satellite, which prove to be very efficient.
The design of a specialized reflector antenna set that supports dual polarization, dual beam widths, and an integrated wideband monopulse tracking capability in the X-band range is described in this paper. The reflector antenna code available at The Ohio State University has been used as the design tool. The design of such an antenna has posed several challenges in the feed and reflector assemblies. The requirement for an integrated wideband monopulse has resulted in a feed array that contains 5 rectangular feed elements with a center-to-center spacing of 1" and a diamond configuration. The 5 feed design has been selected to enable a shared feed array and reflector surface for both transmit and receive beams that eliminates the need for a high-power wideband receiver protector in the radar system. The center feed element is used for transmit waveform and the 4 outer elements are used as receive elements only. Each feed element operates with horizontally and vertically polarized waveforms, requiring a total of 8 waveguide input ports. In this paper, the challenges of the dual beam widths, dual polarized, integrated RCS and tracking antenna are delineated and the tradeoffs among several design configurations are shown. The final design is selected based on the performance predictions using The Ohio State University Reflector Antenna Code. The performance of the antenna has been validated at the OSU compact range for pattern and gain. Both the design and measurement data are presented in this paper.
Millimeter-wave measurements on spherical near-field scanning systems present a number of technical challenges to be overcome to guarantee accurate measurements are achieved. This paper will focus on the affect of mechanical alignment errors of the spherical rotator system on the antenna’s measured performance. Methods of precision alignment will be reviewed. Sensitivity to induced mechanical alignment errors and their affect on various antenna parameters will be shown and discussed. Correction methods for residual alignment errors will also be described. The study includes 38 and 48 GHz data on the Alphasat EM model offset reflector antenna measured by TeS in Tito, Italy on a NSI-700S-60 Spherical Nearfield system, as well as a 40 GHz waveguide array antenna measured by NSI on a similar NSI-700S-60 Spherical Nearfield System at its factory in Torrance, CA, USA.
This paper presents the results of radiation pattern measurement of small reconfigurable slotted ultra wideband (UWB) antennas. The measurements were conducted by using RF measurement and instrumentation facilities, software tools available at WCC of Universiti Teknologi Malaysia. The proposed slotted UWB antennas are having band notched frequency at Fixed Wireless Access (FWA), HIPERLAN and WLAN bands. Band-notched operation is achieved by incorporating some small gaps instead of PIN diodes into the slot antenna. It is found that by adjusting the total length of slot antenna to be about a half-wavelength or less at desired notched frequency [1-3], a destructive interference can take a place, thus causing the antenna to be non-responsive at that frequency. It was also observed that the measured radiation patterns, H-planes, are omni-directional with slightly gain decreased at boresight direction for measured frequencies. There are also more ripples occurred in the measured pattern compared with the simulated one.
A bi-lateral comparison of power gain for a V-band (50 to 75 GHz) Cassegrain antenna and a standard gain horn antenna has been performed between KRISS and NIST. Measurement parameters for this comparison are the power gain and complex reflection coefficient of the traveling standards, and their measurement frequencies are 65 GHz for the high-gain antenna and 50, 65, and 75 GHz for the horn antenna. All participants used the planar near-field scanning method for characterizing the high-gain antenna and the three-antenna extrapolation technique for characterizing the horn antenna. This paper summarizes the comparison and its measurement results with uncertainties. Generally, the agreement between results in all measurements is within the uncertainty of each participant except for the gain result of the horn antenna at 65 GHz.
This paper analyzes the applicability of near field scanners into existing compact test ranges. The analysis is motivated by creating multi-purpose test chambers having the advantages of both, near field systems and compact test ranges. This contribution comprises the discussion of near field scanners at several positions inside a typical compact test range. A ray tracing analysis is presented taking these positions into account in the assessment of near field errors due to multi-path reflections. It is presented how reflections from the absorbers and reflectors are differently impacting near field measurements of low, medium and high gain antennas. The impact is quantified in terms of error levels used in common near field error budgets. It is shown that the combined approach is realizable for specific configurations only.
Instruments for Earth observation working from W-Band up to mm-wave frequencies mainly use quasi-optical feed feeds (QOF) to illuminate the corresponding reflector antennas. The final design of the QOF for the Cloud Profiling Radar System (CPR) for the EarthCARE satellite has been finalized. The QOF is designed to perform polarization and frequency tuning, as well as the separation of transmit and receive channels. The final design verification was performed theoretically by Astrium with QUAST, a new add-on to the GRASP software, especially developed by TICRA for a fast and accurate set-up and analysis of quasi-optical networks. Within the paper, at first a short description of the QOF working at 94.05 GHz will be given. Secondly, the modeling of the QOF will be explained. At last the RF test setup will be described and comparisons between resulting calculated and measured antenna pattern will be shown.
The Global Precipitation Measurement (GPM) mission is a satellite based Earth science mission that will study the global precipitation from rain, ice and snow. A critical part of this satellite is the multi-frequency radiometer system that covers frequencies up to 183 GHz. Beam pointing and beam efficiency must be measured very accurately to calibrate the radiometer response. This paper will focus on the measurements of the offset reflector antenna operating up to 183 GHz using a Nearfield Systems Inc. (NSI) planar near-field measurement system and the special challenges that this presents. Results will be presented and the uncertainty in beam pointing will be discussed.