AMTA Paper Archive
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A Microwave interferometer technique for RCS and phase measurements
The radar scatter matrix can be accurately characterized in magnitude, relative phase, and polarization for both far-field monostatic and bistatic conditions by means of a microwave interferometer. A separate transmitting antenna illuminates the target of interest while two adjacent receiving antennas measure magnitude and the combine in a phase comparator whose output is a phase differential caused by a changing target aspect angle. Using correct constants and scale factors this differential is integrated to provide target phase information. Different polarizations are obtained by switchable feeds. The technique can be used on an RCS range under static conditions or under dynamic conditions with a ground based radar and an airborne target. The advantage gained is that errors due to radar path length changes are eliminated.
Dynamic radar cross section measurements
There is a continuing need for radar cross section (RCS) measurements of targets of military interest. Such measurements are used in predicting detection performance of radars, in quantifying new radar system performance, in designing protective ECM envelopes of aircraft and ships, and in quantifying changes in RCS modification programs. There is, in addition, an interest in determining the actual radiated pattern of an avionic antenna installed on an airframe. While the system and techniques being described here have been used to support all those uses, the system was designed initially with only RCS measurements in mind.
Wideband radar cross section diagnostic measurements
This paper describes a diagnostic RCS measurement system which uses a low-power, wideband, linear-FM radar to provide RCS responses of targets as a function of frequency, range, cross range, and angle. Range and frequency responses are produced by using an FFT analyzer and a desktop computer to perform on-line signal processing and provide rapid access to final results. Two-dimensional maps of the target RCS distribution in range and cross range are obtained by offline processing of recorded data. The system processes signals resulting from a swept bandwidth exceeding 3GHz to provide range resolution of less than 10 cm. The various operating modes of the instrumentation provide a powerful tool for RCS diagnostic efforts in which individual scattering sources must be isolated and characterized. Several examples of experimental results and presented to demonstrate the utility and performance limits of the instrumentation. The examples include results obtained from measurements of a number of simple and complex shapes and of some commercially available radar absorbing materials.
High speed measurement receiver
This paper describes a digitally controlled receiver-recorder capable of time division multiplexing in the frequency domain at a 400 KHz rate and in the amplitude domain at a 20 MHz rate. Good sensitivity and interference rejection are other features of this receiver which operates over the 2-18 GHz band. It is utilized to obtain a measure of antennas performance as impacted by air frames upon which the antenna(s) are mounted.
VHF antenna range design
The design concept for outdoor antenna ranges operated at frequency 50 MHz is discussed. The antenna range is designed for test of VHF antennas mounted on a full-scale satellite mockup. Due to the large size of test objects, a tradeoff between cost and test accuracy among carious range configurations is addressed. Due to near-omni directional characteristics of test antennas, the multipath interference may be severe. The interference ground reflection, surface wave and multiple scattering are quantified and evaluated.
Polarization correction of spherical near-field data
This paper describes the relationship of probe polarization correction to probe-pattern corrected and non-probe-pattern-corrected spherical near-field measurements. A method for reducing three-antenna polarization data to a form useful for polarization correction is presented. The results of three-antenna measurements and the effects of polarization correction on spherical near-field measurements are presented.
A Broadband RCS measurement system
The fast fourier transform capabilities of the Hewlett-Packard 8510 Network Analyzer provide the basis for an RCS measurement system covering the 50 MHz to 26 GHz frequency range. When used in the broadband mode, fine range resolution is achieved. Vector subtraction and gating capabilities permit the acquisition of accurate data in the presence of strong range reflections. Combining this instrument with a high speed data collection and analysis system yields a powerful RCS measurement capability.
RCS Measurements with the HP8510 Network Analyzer
Paper not available for presentation.
Options and considerations for the design of computer aided antenna measurement systems
Rapid advances in digital and micro-computer technology have revolutionized automated control of most measurement processes and the techniques for analysis, storage and presentation of the resulting data. Present-day computer capabilities offer many “user-friendly” options for antenna instrumentation, some of which have yet to be exploited to their full potential. These range from vendor-integrated turnkey systems to innovative designs employing a multitude of subsystem components in custom-interfaced configurations. This paper reviews system and component choices keeping in mind their relative merits and trade-offs. Key design considerations are outlined with particular emphasis on: a) Integration and interfacing of different instrumentation, hardware and software subsystems. b) Upgrading and/or designing of completely new facilities. Various other problems, such as vendor package compatability, and those associated with the analysis and application of measured antenna data are discussed. In addition, suggestions are offered to promote the establishment of a mechanism to facilitate the interchange of data between different antenna measurement laboratories and analysis centres.
Extension of plane wave scattering matrix theory of antenna-antenna interactions to three antennas: A Near-Field Radar Cross Section Concept
This paper presents a three-antenna plane-wave scattering-matrix (PWSM) formulation and a formal solution. An example will be demonstrated in which two of the three antennas are electromagnetically identical (the transmitter and the receiver) and the third (the scatterer) has arbitrary electromagnetic properties. A reduced reflection integral-matrix will be discussed which describes the transmit, scatter, receive (TSR) interaction. An antenna scatterer spectral tensor Greens function is identified. In this formulation the transmit spectrum will be scattered by the third arbitrary antenna (target) and this scattered spectrum may be considered to have originated from a transmitting antenna. Near-field antenna measurement techniques are applicable with determine the electric (scattered) field spectral density function.1, 2 If a second deconvolution is applied, a transmit probe corrected spectral density function or scattering tensor can be determined in principle. In either case, a near- or far-electric field can be calculated and a radar cross section determined.
A High speed, high accuracy position indicator
One of the variables to be quantified when making antenna measurements is position. Without accurate and timely position information, the spatially dependent data cannot be correctly interpreted. Scientific-Atlanta’s 1885 Positioner Indicator and 1886 Position Data Processor offer several improvements in providing position information which can enhance an antenna measurement system. New position indicating techniques have been implemented to allow a higher degree of accuracy and speed than previously attainable. These have been combined with advanced features for automatic system flexibility to create a high performance instrument for many applications. This paper describes the capabilities of these two instruments and how they can be used to improve system performance.
Communication satellite antenna measurement
RCA-Astro Electronics in Princeton, N.J. designs, develops and tests multiple-beam offset reflector antenna systems in the C and Ku frequency bands for satellite communications. Antenna measurements are performed at the antenna subsystem and the system level and on the complete spacecraft to demonstrate that alignment and performance meet their specification. This paper discussed the antenna range designs and test techniques involved in data acquisitions for contour patterns, cross-polarization isolation and antenna gain characterization. A description of the software required to obtain, analyze and present the data will be included in addition to typical test results.
Satellite near field test facility
The paper describes a near field facility developed at Hughes Aircraft Space and Communications Group for the purpose of performing measurements on satellite antennas. The facility is designed for planar near field scanning with capability for adding cylindrical scanning. The facility has a scanner with a 21 foot square range and is capable of measuring large antennas with operating frequencies up to 15 GHZ. The measurement system is designed for testing multi-beam, multi-frequency antennas. Data collection, scan control and data analysis functions are all controlled by a single computer system. Growth plans include the addition of an array processor for the ability to perform Fast Fourier Transforms in near real time. Results for the antennas which have been measured will be shown along with far field range data for comparison.
The Determination of near-field correction parameters for circularly polarized probes
In order to accurately determine the far-field of an antenna from near-field measurements the receiving pattern of the probe must be known so that the probe correction can be performed. When the antenna to be tested is circularly polarized, the measurements are more accurate and efficient if circularly polarized probes are used. Further efficiency is obtained if one probe is dual polarized to allow for simultaneous measurements of both components. A procedure used by the National Bureau of Standards for determining the plane-wave receiving parameters of a dual-mode, circularly polarized probe is described herein. First, the on-axis gain of the probe is determined using the three antenna extrapolation technique. Second, the on-axis axial ratios and port-to-port comparison ratios are determined for both the probe and source antenna using a rotating linear horn. Far-field pattern measurements of both amplitude and phase are then made for both the main and cross components. In the computer processing of the data, the on-axis results are used to correct for the non-ideal source antenna polarization, scale the receiving coefficients, and correct for some measurement errors. The plane wave receiving parameters are determined at equally spaced intervals in k-space by interpolation of the corrected pattern data.
Effects of the alignment errors on ahorn's crosspolar pattern measurements. Application to L-SAT propagation package antennas.
When low crosspolar pattern measurements are required, as in the case of the L-SAT Propagation Package Antennas (PPA) with less than -36 dB linear crosspolarization inside the coverage zone, the use of good polarization standards is mandatory (1). Those are usually electroformed pyramidal horns that produce crosspolar levels over the test zone well below the -60 dB level typically produced by the reflectivity of anechoic chambers. In this case the alignment errors (elevation, azimuth and roll as shown in fig. 1) can become important and its efects on measured patterns need to be well understood.
Fourth generation indoor range
The measurement of microwave antennas indoors began with the advent of commercial absorbing material. The use of absorbers can be traced back to a 2 gHz material developed by the Dutch in the Thirties. During the Forties, considerable progress was made on absorbing materials, but even after World War II, security considerations limited the application. Some materials found use as indoor shields for antenna tests, but limited bandwidth limited the utility of these materials. When a broad band absorber was developed the antenna experts did not believe that this material would be made commercially because they presumed a limited market.
Performance criteria for RCS measurement systems
The purpose of an instrumentation radar is to characterize the Radar Cross Section (RCS) of a target as a function of target aspect and radar frequency. In addition, an instrumentation radar may be used to produce a high resolution radar image of a target which is useful in target identification work and as a diagnostic tool in radar cross section reduction. These purposes differ from those of a conventional radar, in which the objective is to detect the presence of a target and to measure the range to the target. Several different radars are currently used to perform radar cross section measurements. Common instrumentation radars may be classified as CW, Pulsed CW (Low-Bandwidth IF), Linear FM (FM-CW), Pulsed (High-Bandwidth IF) and Short Pulse (Very High-Bandwidth IF). These radars accomplish the measurement task in distinct manners, and it is sometimes difficult to determine where the strength or weakness of each radar lies. In this paper, a set of performance criteria is proposed for RCS measurements. The proposed criteria can be applied uniformly to any instrumentation radar independent of the type of radar design employed. The criteria are chosen to emphasize those performance characteristics that relate directly to RCS measurements and thus are most important to the user. Two instrumentation radars which have been designed at Scientific Atlanta, namely the Series 2084 (Linear FM) and the Series 1790 (Pulse), are used to illustrate the application of the performance criteria.
Extraction of narrow band responses for wideband RCS data
Wideband RCS instrumentation systems can provide a high degree of range resolution. By combining wideband RCS data with a synthetic-aperture or Doppler processing, the spatial distribution of radar reflectivity can be determined. These systems provide diagnostic capabilities which are useful for locating scattering sources on complex objects and for assessing the effectiveness of modifications. The Proceedings of the 1983 meeting included a paper which described a linear-FM system operating over a 3 GHz bandwidth capable of measuring RCS vs range, cross range, and frequency using a single measurement set-up. This paper analytically demonstrates a procedure for extracting CW RCS patterns from the wideband data obtained using the linear-FM system. By combining the latter and the former processing, it is possible to obtain from a single data array both wideband responses showing the spatial distribution of scatterers and narrowband responses which are the traditional CW RCS patterns. The paper includes experimental verifications of these assertions by comparing results of CW measured data with data extracted from wideband RCS measurements.
Near field RCS measurements
A planar surface, near-field measurement technique is presented for the near-field measurement of monostatic radar cross-section. The theory, system configuration and measurement procedure for this technique are presented. It is shown that the far field radar cross-section can be determined from the near field measurements. An associate near-field radar cross-section measurement technique is presented for the measurement of bistatic near field radar cross-section. The bistatic technique requires a plane wave illuminator in addition to the planar surface near field measurement system. A small compact range is used as the bistatic illuminator. Bistatic near-field measurements are presented for a simple target.
Broad band feeds for new RCS ranges
Recent construction of RCS ranges has involved paraboloidal reflectors ranging from a few feet to sixty feet in diameter. These reflectors have required broad band feeds because the typical radar illuminator-receiver is capable of operating over an octave in frequency. This paper will describe a series of feeds which cover any octave in frequency from 100 mHz to 8 gHz, with coaxial line inputs. In addition waveguide-port feeds will be described which cover all of the standard waveguide bands up to 18 gHz. The four basic requirements for all of these feeds are: a) capable of handling the radar power, b) VSWR less than 2 to 1, c) orthomode operation with a 30 db isolation between the two linear polarizations and d) a radiation pattern which is constant with frequency. A fifth problem, for the reflectors which are truncated, is that of providing an elliptical cross section beam over the frequency band.
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