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Automation of Radar Image Processing of Airborne Targets
We present innovations based on pattern recognition technology that significantly reduce the level of human intervention and increase data throughput when processing radar images of airborne targets. Time consuming operator intervention is normally required to insure that images are centered and non-aliased and wireframe overlay drawings are properly registered with the target image. We have developed techniques that produce high-quality images without operator intervention. These include a template registration algorithm that can reliably orient the outline drawing with a radar image even in the presence of image artifacts such as jet engine modulation (JEM). In addition, we have developed methods that remove the average Doppler responsible for crossrange image displacement or aliasing and methods that resolve downrange ambiguities. Examples are shown which illustrate these processes applied to images of a jet aircraft in flight.
Low Cross-Polarized Compact Range Feeds
Compact antenna test ranges intended for low cross polarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
On Design Aspects of Compact Antenna Test Ranges for Operation Below 1 GHz
Compact Antenna Test Ranges are eminently suitable for obtaining the far-field patterns of various types of antennas provided that the frequency is not too low. Typically, a low frequency limit of 1 or 2 GHz is realizable. There are, however a number of important applications between 500 and 1000 MHz for antenna diameters between 1 and 3 meters. The far field distance R = 2D2/l is just too large for an indoor far field range. It is generally accepted at present that a good solution for an indoor range for these kind of measurements is very difficult to realize. In this paper the low frequency performance of single and dual-reflector Compact Antenna Test Ranges will be investigated. It will be shown that with carefully designed serrations and feeds, excellent antenna measurements can be carried out at frequencies below 1.0 GHz for a large number of applications. For purposes of comparison, low frequency performance of a compact range with so called blended rolled edges will be presented as well.
Near Field Range Error at Off-Probe-Calibration Frequencies
Proper operation of a planar NFR (near field range) includes probe correction as part of the processing of the measured data to result in accurate far field angle patterns, particularly for low cross polarized patterns. The far field transform of the near field data produces the angular spectrum which is the product of the plane wave transmission coefficient pattern of the AUT (antenna under test) with the plane wave receiving coefficient pattern of the probe. Probe correction consists of dividing the angular spectrum by the complex probe angle pattern resulting in the pure far field pattern of the AUT . For best accuracy of co and cross polarized AUT patterns one needs to use accurately measured probe complex co and cross polarized patterns in probe correction for each NFR test frequency. The most accurate probe measurements are usually obtained from specialized test laboratories. However, if the number of frequencies is large, this may create problems due to cost or schedule. Because of this it is typical to procure probe calibration at only a few frequencies spanning the test band for each AUT even though pattern measurements are needed at several additional frequencies falling between the calibration frequencies. A typical strategy at any given test frequency is to perform probe correction using the nearest-neighbor-frequency probe calibration data. This strategy produces some unknown error in the processed probe corrected far field patterns of the AUT at each non-calibrated frequency. Inthis paper we will show a method for estimating the non-calibrated frequency probe correction error for co and cross polarized patterns with examples.
Algorithm to Reduce Bias Errors in Planar Near-Field Measurements Data, An
A bias error in planar near-field measurement data comes from receiver crosstalk or leakage effects [1, 2, 3]. The bias error is a complex constant added to every near-field data sample. After transformation from the near-field to the far-field, the bias error becomes an easily identifiable spike located at the center of k-space. If one is measuring a horn, then the bias error produces a small bump or spike at the center of the far-zone pattern (i.e. at the center of k-space). If one is measuring a highgain antenna with the antenna beam pointed away from the center of k-space, then the bias error causes an erroneous sidelobe spike at the center of k-space. The bias constant is difficult to estimate be cause it may be more than 60 dB below the peak near-field level. Nevertheless, if the effect of the bias error can be seen in the far zone pattern of the test antenna, then it can be estimated and removed from the measured data. An algorithm is presented that is used to estimate the bias constant directly from the near-field data, then the bias constant is simply subtracted from the data. Examples using measured data are used to illustrate how the algorithm works and to show its effectiveness.
Application of Non-Rectilinear Co-ordinate Systems in the Characterisation of Mis-aligned Space Antennas, The
Traditional measurement methods assume that very accurate antenna to range alignment of the antenna under test (AUT) is convenient or possible. It has recently been shown that the use of non-rectilinear co-ordinate systems are of particular use for the purpose of correcting antenna to range misalignment. Additionally, this misalignment correction can be used to construct an extended composite measurement plane from a series of mis-aligned scans that themselves can be considered as constituting a polyhedral measurement surface. This paper describes the additional processing that is required to yield corrected near and far field data from an acquisition of a mis-aligned AUT. This technique is then illustrated with example results. The agreement of the corrected results is determined via the application of image classification techniques which correlate antenna patterns in a reduced vector pattern space in terms of their overall global features.
Evaluation of Dual-Band Compact Cassegrain Antenna Characteristics Using Near-Field Measurement
A compact cassegrain antenna has been designed for dual-band satellite communications, operating at 20GHz and 30GHz. The antenna consists of a parabolic reflector, a hyperbolic sub-reflector, and a dual-band choke feed. The cassegrain structure has been optimized considering theoretical and measured feed patterns using different software packages, for maximum antenna efficiency with minimum sidelobe levels for a compact design objective. Experimental studies have been carried out in the near-field chamber of the University of Manitoba. The knowledgenof the near-field is helpful in order to adjust different components of the cassegrain antenna. After adjustment, results in terms of gain and radiation patterns are computed by Fourier transform using near-field data, and compared to the measurements realized in the compact range of the University of Manitoba. Comparisons are also made with the results obtained by simulation.
Performance Requirements for a Microwave Cable to be Used in a Near-Field Antenna Range
A near-field antenna range will often utilize a flexible microwave cable assembly as a means to transport the sampled signal from the moving sample antenna to a receiver as part of the measurement system. The performance of that cable directly impacts the quality of the final far-field pattern. It has been observed that the cable had been exhibiting a flex life much shorter than anticipated. Analysis of a failed cable revealed that the problem was the result of non-uniformities in the extruded jacket, which produced sites of high stress. These sites ultimately caused the cable conductors to work harden and fracture. A cable which utiized a woven expanded Polytetrafluoroethylene (ePTFE) fiber as an outer jacket was substituted, resulting in a threefold improvement in flex life to date, with the cable still in operation at this writing.
Cellular Handset Testing with a Simplified Head Mockup and an Internal Antenna
Two types of cellular handset testing are presented. The first studies models of a cellular handset near the human head. A comparative analysis is done between simulation and measurement of an inexpensive head mockup compared to a more expensive head mockup. Peak gain values have good agreement within about 1 dB. The second type of cellular handset testing is for a PCS band PIFA antenna integrated to a cellular handset. This paper describes the design and experimental study of the radiation patterns of a PCS band (1850-1990MHz) cellular handset with an internal PIFA. The PIFA described in this paper has good gain, impedance matching, and reduced sensitivity to human body interaction. This PIFA is a good cellular internal antenna.
Real-Time Spherical Near-Field Antenna Test Range for Wireless Applications
SATIMO has recently installed a spherical near-field antenna measurement system for ALLGON MOBILE COMMUNICATIONS, the market leader in the field of antennas for mobile telephones. This spherical near-field system, as shown in Figure 1, allows for real-time measurements of antennas and will among other be used for the measurements of the radiation characteristics of mobile telephones and satellite terminals in the presence of the human operator. The system consists of a circular of 4m diameter containing 64 dual polarized measurement which are electronically scanned giving a real-time near-field pattern cut over 310° in elevation. A full sphere measurement including near-field to far-field transformation is available in seconds with a single +/- 90° azimuth rotation. The paper will present the measurement system and the results of the final acceptance tests. The acceptance tests are based on both range inter comparisons and also by measurement of key terms in the overall error budget.
Antenna Calibrations at NPL
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.
TUD-ESA Standard Gain Horn Facility, The
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).
Time and Frequency Antenna Measurement With One Signal Receiver
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.
New Look at the Luneburg Lens Antennas: Design, Optimization and Measurements, A
Design optimization and measurement of the Luneburg lens antennas are the focus of this paper. One of the important design aspects of an optimal Luneburg lens antenna is to construct a high performance lens with as low number of spherical shells as possible. In a uniform Luneburg lens, the gain is decreased and unwanted grating lobes are generated by reducing the number of shells. This deficiency in the radiation performance of the uniform lens may be overcomed by designing a non uniform lens antenna. The optimized non-uniform spherical lens antenna is designed utilizing the dyadic Green's function of the multi-layered dielectric sphere integrated with a Genetic Algorithm (GA)/Adaptive Cost Function optimizer. Additionally, a novel 2-shell lens antenna is studied and its performance is compared to the Luneburg lens. Finally, measured results for far field patterns and holographic images are shown for the Luneburg lens antenna using the UCLA's bi-polar near field facility.
Evaluation of a Broadband Ground Reflection Antenna Range Technique
A prototype broadband ground reflection range, to be used for measuring antenna patterns on full sized aircraft, was built and evaluated. The range was designed to evaluate an antenna at several arbitrary VHF/UHF frequencies simultaneously. This is a follow on to two previous papers that explored the design of such a range using numerical modeling and optimization by the use of genetic algorithms.
Antenna Range Imaging
We present a preliminary evaluation of a microwave measurement system that has been designed to determine electromagnetic fields in the quiet-zone of an antenna measurement range and to produce an image of the sources, intended and unintended, of the incident radiation. This information is of potential value in the processes of improving range perfor mance, correcting pattern results for non-ideal illumination , and evaluating measurement uncertainty.
Portable Far Field Chamber, A
Composite Optics Inc (COI) has designed and constructed a Portable Far-Field Antenna Test Chamber to complement their Large Compact Range. The need for this chamber arose after COI won a contract to design, build, and test hundreds of small broadband antenna elements. Because of the portability requirement, COI chose to procure and modify an industrial container, suitable for transportation on a standard flatbed trailer. This paper discusses the design, fabrication, and installation of a chamber, suitable for pattern measurements of small (<2 feet) antennas in the 6-18 GHz frequency range.
Application of the NIST 18 Term Error Model to Cylindrical Near-Field Antenna Measurements
This paper describes error analysis and measurement techniques that have been developed specifically for cylindrical near-field measurements. A combination of analysis and computer simulation is used to show the comparison between planar and cylindrical probe correction. Error estimates are derived for both the pattern and probe polarization terms. The analysis is also extended to estimate the effect of position errors. The cylindrical measurement geometry is very useful for evaluating the effect of room scattering from very wide angles since scans can cover 360 degrees in azimuth. Using a broad beam AUT and scanning over a large y-range provides almost full spherical coverage. Comparison with planar measurements with similar accuracy is presented.
Anechoic Chamber With Easily Removable 3D Radiation Pattern Measurement System for Wireless Communications Antennas
In this paper, a versatile indoor antenna measu rement facility in Nokia Resea rch Center is presented Two measurement systems have been implemented into a rectangular, shielded anechoic chamber having dimensions of 10 m * 7 m * 7 m. The first configuration is an in-house developed 3D radiation pattern measurement system that uses a rotating elevation arm. The primary application of this system is characterization of terminal antennas including the effect of a test person or a human body phantom. The elevation arm can be easily removed and the chamber then used as a conventional 5-m far-field range. This configuration is applied mainly for directive antennas. The facility has been found out to be very useful in research and development of wireless com munications antennas. The 3D spherical scanning system opens up a much wider perspective than before on how the human body interacts with different kinds of terminal antennas and what are the radiation and receiving performance characteristics under realistic usage conditions.
Technique for Improving Omnidirectionality of Automotive Conformal Antennas, A
The automobile antenna industry is facing a rapidly growing trend leading to the incorporation of effective, low cost, conformal antenna designs. There are many situations where an omnidirectional azimuth pattern is desired for a conformal antenna on a vehicle. Conformal antennas, however, are typically restricted to mounting locations on the side of vehicles where the vehicle itself obstructs the signal. It is very difficult to obtain omnidirectional performance in these cases. A technique to substantially improve omnidirectionality of automotive conformal antennas is described. This technique is based on the use of dual symmetric antennas connected to a common junction point using equal length cables. Experimental results of implementing this technique using a dual sideUte film antenna on a commercial vehicle in the FM frequency band are presented. It is shown that the dual sidelite conformal antenna is an effective, low cost solution for achieving omnidirectional performance in FM automotive applications.
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