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Radar cross-section and scattering matrix measurements on microwave radar navigation targets
This paper presents both radar cross section and polarization scattering matrix measurements on microwave radar navigation targets. The polarization measurements are performed using a unique two-channel facility which allows for measuring the circularly polarized scattering matrix elements at X-band. For the same targets conventional RCS measurements are performed using an automated system comprising a network analyzer (HP-8510) and a desk top computer system (HP-236 or 310). This system allows wide frequency range measurements. Details of these measurement techniques, and results will be presented.
A Pulsed/CW RCS measurement system using the HP8510 network analyzer
This paper describes an automated, frequency-step, pulsed/CW Radar Cross Section (RCS) measurement system using the HP 8510 network analyzer. The system has been built using the concepts developed at Lincoln Laboratory (1) and is being utilized in an operational capacity. The unique features of this system are the use of (a) a dual-probe antenna for the transmission and reception of RF signals, and (b) a pulse system for separating the target-scattered signals from the incident and background signals. The single antenna configuration provides a true monostatic backscatter measurement. A polarization control circuit makes RCS measurements for all combinations of transmit/receive polarizations possible (linear and/or circular). The pulse system uses pin-diode switches capable of generating a 7-ns pulse width and a repetition rate up to 8 MHz. The pulse system effectively eliminates unwanted signals at ranges other than the target range. Therefore, the full dynamic range of the receiver can be used for the measurement of the target.
Antenna calibrations using pulsed-CW measurements and the planar near-field method
For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.
Low axial ratio circularly polarized compact range feeds
Recently, needs have arisen for low axial ratio feed horns for prime focus fed compact ranges. The compact range environment necessitates a feed possessing low back lobes to minimize extraneous radiation. Circular polarization demands dual orthogonal linear polarizations with symmetrical radiation characteristics. An iris loaded square waveguide section was developed to produce a quadrature phase shift in one linear polarization versus the orthogonal polarization. This 90 degree phase shifter was incorporated into a corrugated horn to achieve a 1 dB axial ratio or less over a full waveguide band. Theoretical and experimental data will be presented for several of these horns. Extensions to lower axial ratios (less than .5 dB) using a double tuned circuit approach will also be presented.
The Effects of an offset fed parabolic reflector on polarization
The offset fed parabola is one type of reflector used in compact radar ranges. Cross-polarization problems have been noted when a parabola is used in near field applications. A good understanding of the near field cross-polarization effects was needed to evaluate this type of reflector for a compact range. We found that the polarization vector was rotated differently at each location in the "quiet zone." The polarization vector rotation is due to the parabolic curvature. In addition, a mathematical model was derived that compares well with the data. A theoretical study of how the RCS measurements of a wing are affected is presented.
Formulation of proper standards in radar polarimetry
We have found several crucial inconsistencies in the basic equations of radar polarimetry which are rather common in the current literature on the subject. In particular, the pertinent formulations of the polarization state definitions given in the IEEE/ANSI Standards 149-1979 are in error. These and other inconsistencies and conceptual errors are analyzed very carefully in this presentation. We provide the correct formulae for the proposed revision of the polarimetric standards together with a well-defined and consistent procedure for measuring target scattering matrices in both, mono-static and bi-static arrangements. Further, the proposed procedure can be applied to an arbitrary measurement process in any general elliptical polarization basis.
Practical aspects of construction and testing of bicone telemetry and command omnidirectional satellite antennas
A bicone telemetry and command antenna is a stack of two physical antennas with toroidal patterns which have radiation patterns which are omnidirectional in the azimuth plane, perpendicular to the transfer orbit spin axis of the satellite, but are directional in the elevation plane. Each of the two physical antennas,, which operate at different frequencies and polarizations to avoid feedback, has two independent RF inputs (for redundancy) making it actually a four antenna configuration. Each physical antenna consists of three components, which are the feed input section with dual RF inputs, a circular polarizer and a radiation structure comprised of slots, in the circumference waveguide structure, which feed the circumferential conical horn necessary to obtain the required directivity in the elevation plane. The procedures and the problems encountered in constructing and testing each of these parts, as well as the components necessary to permit their testing as independent units is discussed. Because of the broad radiation patterns which characterized these omnidirectional C-Band and K-Band antennas, special consideration had to be given to the measurement of the antenna patterns. These problems and their solutions are highlighted in the paper.
Design, fabrication and measurement of an earth terminal antenna for 2-degree satellite spacing
Prior to 1965, there were no geostationary communications satellites. But since the success of Early Bird (INTELSAT I), in 1965, an explosion has occurred in the number of communication satellite (communication channels) in the geostationary orbit (GSO). Because of this increase of satellites in the fixed satellite service (FSS), coupled with the even greater demand for the more desirable positions above land masses and additional channels in the 4/6 GHz band, the FCC and CCIR are attempting by every possible means to increase the number of satellites and channels available. Interchannel interference, of course, must not be increased. Besides enlarging the frequency spectrum available, the use of orthogonal polarizations and closer inter-satellite spacings are under consideration as a means of increasing channel capacity. In 1981, the FCC proposed a decrease from 4 degrees to 2 degrees spacing for satellites operating at 4/6 GHz in the FSS. While many current users prefer larger "close" spacings (2.5 to 3 degrees), 2 degrees will probably become the required inter-satellite spacing for the FSS and is the currently accepted antenna design requirement. When satellites are more closely spaced, their ground terminal antennas must not only have narrow main beams but must also have very low side lobe levels to avoid interference with adjacent satellites. The CCIR has established a new reference pattern, 29-25*log(phi) (ref. CCIR Recommendation 465-1), shown as the overlay in Figure 5. Ground terminal antennas with a diameter to wavelength ratio greater than 100 must comply. This study set out to determine whether this specification could be applied to a carefully designed antenna with a diameter to wavelength ratio as low as 50, 3.5m for the 4/6 GHz band. The future market for such an antenna would be in a low cost earth terminal (LCET) intended for the 'smaller users', who require rapid, reliable communications. The small users may lease channels until the time when their increase in data transmission requirements and the decrease in the cost of earth terminals justifies acquiring their own small earth terminal.
High speed pattern measurements of a multi-port phased array
This paper describes the measurement requirements of a phased array comprised of three sub-arrays and the test system built to measure it. To evaluate the performance of the array, it is necessary to measure the radiation patterns of all three outputs at various azimuth scan angles. Because the relative phase and amplitude between the elements is an important performance parameter, if data is to be taken "on the fly", then high speed measurements are required. In addition, when taking elevation patterns through the peak of the beam, which has been scanned in azimuth, the polarization of the antenna under test changes with elevation angle. Consequently, since the patterns are to be measured to matched polarization, the transmit antenna polarization must be varied as a function of elevation angle. To complicate matters, this is a non-linear relationship. The test system architecture and resultant performance capabilities are presented.
A Wide band instrumentation radar system for indoor RCS measurement chambers
An instrumentation radar system suitable for collection of backscatter characteristics of targets in an indoor chamber was built and installed in the Ohio State University ElectroScience Laboratory. The radar is a pulsed system with continuous coverage from 2 to 18 GHz, and spot coverage from 26 to 36 GHz. The system was designed to have maximum flexibility for various test configurations, including complete control of the transmit waveform, H or V transmit polarization, dual receive channels for simultaneous measurement of like and cross polarization, greater than 100 dB dynamic range, and convenient data storage and processing. A personal computer controls the operation of the radar and is capable of limited data reduction and display functions. A mini-computer is used for more widely sophisticated data reduction and display functions along with data storage. This paper will present details of the radar along with measured performance capabilities of the system.
The Radar image modeling system
The characteristics of a unique indoor RCS modeling facility are described. The David Taylor Research Center (DTRC) has implemented an indoor, over-water radar cross section measurement facility. Major components of the facility are the DTRC Seakeeping Basin, an imaging radar, an underwater target mount and rotator, a calibration system, and video monitoring equipment. Initial operational capabilities include dynamic pulse-to-pulse polarization-agile measurements at X and Ku bands, elevation angles from grazing to 7 degrees, maximum target length of 50 feet, and simulated sea states adjustable between state 0 and state 3. Several data products are available, including high-resolution inverse synthetic aperture radar images. Eventual capabilities will include extended elevation angles up to 30 degrees, frequencies to beyond 100 GHz, and SAR imagery.
Parasitic multimode/corrugated (PMC) feed for a compact range
The radar cross section of large targets has previously been measured on large outdoor far field ranges. Due to environmental and security limitations of outdoor ranges, low cost indoor compact ranges are preferred. To optimize compact range performance and to minimize size, careful attention must be paid to the design of feeds which are required for the proper illumination of the reflector. This paper describes a new polarization diversified parasitic multimode/corrugated (PMC) feed for a compact range reflector. The performance attributes of the PMC feed are presented. The PMC feed provides several advantages over other known commercially available compact range feeds.
A Low cost, PC based far-field antenna range
A far-field antenna range has been assembled on the roof of the Electrical Engineering building at Virginia Tech. Antenna radiation patterns and polarization patterns can be measured. The system consists of two Scientific-Atlanta azimuth positioners, a Scientific-Atlanta 1711 receiver, a Scientific-Atlanta 1832A amplitude display unit, a DC motor controller, a synchro-to-digital converter, an IBM PC, and signal sources. The DC motor controller has been interfaced to the PC along with the synchro-to-digital converter, forming a closed loop positioning control system that can be used with either of the azimuth positioners. One of the positioners is used for the antenna under test while the other positioner controls the polarization of the transmit antenna. The receiver and amplitude display provide a 60 dB dynamic range for antenna measurements. The PC has been programmed in TURBO Pascal to control the antenna positioner, record antenna patterns, store pattern data on disk, and provide antenna pattern plots. This modular approach provides permanent storage on PC disk of all measurements as well as allowing many plot combinations including linear or logarithmic form and rectangular or polar format.
Antenna measurements for millimeter waves at the National Bureau of Standards
For the past two years the National Bureau of Standards (NBS) has been developing the capability to perform on-axis gain and polarization measurements at millimeter wave frequencies from 33-65 GHz. This paper discusses the error analysis of antenna measurements at these frequencies. The largest source of error is insertion loss measurements. In order to make accurate insertion loss measurements, flanges on antennas need to be flat and perpendicular to the waveguide axis to within approximately 0.001 cm (0.0005 in). In addition, waveguide screws need to be tightened with a device that supplies constant torque. For antennas with gains less than about 25-30 dB (probes) we can measure on-axis gains within an uncertainty of 0.14 dB in the 33-50 GHz frequency band and within 0.16 dB in the 55-65 GHz frequency band using the three-antenna technique on the extrapolation range. For antennas with larger gains we can measure on-axis gains within an uncertainty of 0.21 dB in the 33-50 GHz frequency band and within 0.24 dB in the 55-65 GHz band using the planar near-field technique. NBS in continuing development of its measurement capabilities, including measuring probe correction coefficients required in planar near-field processing, in order to provide accurate pattern measurements at these frequencies.
High volume testing in a near field facility
While near field antenna test techniques are well understood, published methods for high volume testing are rare. This paper addresses special requirements for production testing of satellites at the Hughes Aircraft Company Space and Communications Group facility in El Segundo, California. The El Segundo facility has the capability of testing antennas which employ multiple beams and polarization isolation for frequency spectrum reuse. It is required that the measurement techniques and equipment be able to test this type of antenna during a single traverse of the planar near field scanner. Serious demands are placed on the system to meet these requirements: * Maximum dynamic range and linearity must be maintained in an environment of rapidly shifting signal levels. * Isolation of signals must be maintained while allowing rapid switching for beam and polarization sampling. * Equipment settling time must be minimized to maintain scan rate at the highest possible speed. * RF interfaces must be repeatable, and capable of rapid reconfiguration. * Calibration and system checkout techniques must be accurate, quick, and capable of detecting malfunctions and costly setup errors. * Data transfer and processing must not be a limitation to the availability of the system for measurement. * System growth capability must be maintained, but not allowed to interfere with 'old and valued' customers. Some of the trades and pitfalls in meeting these requirements will also be presented.
Calibrating antenna standards using CW and pulsed-CW measurements and the planar near-field method
For over a decade the National Bureau of Standards (NBS) has used the planar near-field method to accurately determine the gain, polarization and patterns of antennas either transmitting or receiving cw signals. Some of these calibrated antennas have also been measured at other facilities to determine and/or verify the accuracies obtainable with their ranges. The facilities involved have included near-field ranges, far-field ranges, and compact ranges. Recently, NBS has calibrated an antenna to be used to evaluate both a near-field range and a compact range. These ranges are to be used to measure an electronically-steerable antenna which transmits only pulsed-cw signals. The antenna calibrated by NBS was chosen to be similar in physical size and frequency of operation to the array and was also calibrated with the antenna transmitting pulsed-cw. This calibration included determining the effects of using different power levels at the mixer, the accuracy of the receiver in making the amplitude and phase measurements, and the effective dynamic range of the receiver. Comparisons were made with calibration results obtained for the antenna transmitting cw and for the antenna receiving cw. The parameters compared include gain, sidelobe and cross polarization levels. The measurements are described and some results are presented.
Comparison of antenna boresight measurements between near-field and far-field ranges
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
Improvements in polarization measurements of circularly polarized antennas
A new measurement technique that is used to measure the polarization properties of dual port, circularly polarized antennas is described. A three antenna technique is used, and high accuracy results are obtained for all three antennas without assuming ideal or identical properties. This technique eliminates the need for a rotating linear antenna, reduces the setup time when gain measurements are also performed, and reduces errors for antennas with low axial ratios.
Quantifying simultaneous V and H coverage from an underside aircraft location
This paper presents the results of a measurement program conducted to evaluate antenna coverage from a constrained-space airborne platform. Two levels of coverage are reported: Level 1 details coverage without the aircraft, but over a small groundplane, while level 2 includes aircraft effects. Coverage and gain for each polarization was evaluated by quantizing the data and computer processing to obtain a measure of goodness vs frequency. In particular, horizontal polarization showed considerable changes, as expected, compared to level 1 performance. Vertical polarization showed smaller, but significant changes.
Design development and qualification of an advanced, large compact test range
To fulfill the future demand of highly accurate antenna-, RCS- and payload testing, MBB built a new antenna test centre at Ottobrunn (Ref. 1). This paper describes the development and qualification of the large, dual reflector Compact Range (CR) which has a plane wave zone of 5.5 x 5.0 x 6.0 m (w x h x d). It starts with the results of a detailed electrical trade-off study between different CR-concepts, followed by some mechanical/thermal construction aspects of the large, highly accurate reflectors. Finally, some qualification results are shown, covering the frequency range from 3.5 GHz up to 200 GHz (lowest frequency of operation approx. 2 GHz). The achieved plane wave performance (amplitude ripple ±5o, phase ripple ±5o, cross-polarization isolation > 40 dB) verifies the high quality overall system design.
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