Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The field present in the test zone of an antenna measurement range can be calculated from the range field measured on a spherical surface containing the test zone. Calculated test zone fields are accurate only within a spherical volume concentric to the measurement surface. This paper presents a technique for determining the probing radius necessary to create a volume of accuracy containing the test zone of the range. The volume of accuracy radium limit is caused by the spherical mode filtering property of the displaced probe. This property is demonstrated in the paper using measured field data for probes of differing displacement radii. This property is used to determine the volume of accuracy radium from the probing radius. This is demonstrated using measured far-field range data.
A technique for multipaction analysis based on finite element modeling of the electromagnetic fields within a device is demonstrated. A multipaction device is modeled with HFSS to determine the field solution for use in multipaction analysis. The resultant field magnitudes within the critical gap region were compared with the measured breakdown events for 4 different gap sizes of the device. The relationship between the scattering coefficient convergence and field solution convergence is examined, and some indicators of the latter are established. The correlation between the data and the predictions indicates that the technique represents s reasonable analytical tool for such analysis.
Progress in Georgia Tech's research in Near-Field Spherical Microwave Holography (NFSMH) is reported. Previously, the amplitude resolution of Spherical Microwave Holography (SMH) was defined and demonstrated. The definition of resolution has been altered to include phase resolution. The resolution of phase is shown to be equivalent to the resolution of amplitude, and both depend on the highest mode order used in the spherical wave expansion. Previous measurements showed that SMH can easily achieve x/2 phase resolution where X refers to free space wavelengths. Current measurements show that the X/2 resolution limit of planar microwave holography can be surpassed by using evanescent energy in the NSMFH technique. Measurements of small, closely spaced, insertion phase defects placed on a hemispheric ally shaped radome are used to demonstrate the improved resolution. The measurement of evanescent energy is achieved by using a specially designed small aperture probe and a small separation distance between small aperture probe and a small separation distance between the radome surface and the measurement surface. The relationship between measured and theoretical insertion phase of a known radome defect is shown. Given the defect size and the maximum mode order used in the spherical wave expansion, measured insertion phase can be used to predict the actual defects electrical thickness.
A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.
Antenna microwave holography is now a well established technique and has for a number of years provided a diagnostic tool for the evaluation and optimization of the electrically large reflector antennas used for satellite ground stations. Increasing interest is being shown in the use of the technique during the development of other complex antenna configurations in order to improve the design, minimize design cycles and, hence, reduce the overall cost. This contribution presents two examples of applications of the technique during the development of high performance antennas at ERA Technology LTD. For a corrugated slot-array antenna operating at 19.95 GHz, a clear improvement in the performance following design optimization based on the results obtained from microwave holography is shown for a 3 Am diamond reflector antenna for SATCOM applications operating at 14GHz, the technique provides a verification of distortions in the surface profile by mapping of the aperture phase distribution.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.
The presence of the sea surface has a powerful influence on the scattering characteristics of marine targets during radar cross section (RCS) measurements. To obtain accurate RCS measurements of a large, distributed marine target, the radar site must satisfy various requirements. The major requirement is to provide quality RCS data without strong multipath distortion of the target return signal. In this paper multipath effects on a large scatterer measured at both low-and high-elevation radar sites are summarized. It is observed that multipath effects contribute strongly to the RCS of the target measured at a low elevation radar site. The data show large RCS fluctuations of more than 15 dB when a scatterer is measured at difference altitudes or ranges. The quality of the data measured at a low-elevation radar site then becomes questionable, which creates difficulties in assessing the true RCS of the target. For diagnostic purposes, it may be necessary to change the target range or altitude several times to make a credible assessment of RCS. The same target measured at a high-elevation site has less multipath influence on the RCS data, making assessment of the true RCS feasible.
The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.
Aeronautical SATCOM systems for INMARSAT typically employ circular polarized electronically or mechanically steered multi beam antennas. Characterization of thee antennas requires extensive measurements that differ from conventional antenna pattern measurements. Some of these are: A. Multiple frequently CP gain, axial ratio, and discrimination measurements over a hemisphere for a large number of beams. B. Noise temperature and G/T measurements C. Carrier to multipath rejection D. Intermodulation characteristics E. Receiver and Transmitter system characteristics Details of instrumentation and procedure for these tests are presented with special emphasis on issues such as measurement speed, accuracy and processing of large amounts of data.
Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.
This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.
As antennas have become more sophisticated, the testing requirements have grown tremendously. Testing often adds significantly to the cost of the system. A need has developed for test equipment more advanced than the completely manual systems of the past and less expensive than the completely automated systems of today. An antenna pattern recorder which helps to minimize test time is presented. The instrument utilizes a use friendly touch screen which facilitates user interaction with the unit. The pattern recorder is capable of measuring up to five channels of data simultaneously as a function of angle, linear position, or time. The data is stored on electronic media and may be saved, retrieved, zoomed, plotted, analyzed by internal programs or exported for analysis by external programs. The user may customize the plot format for test reports, proposal information, and other data requirements.
The utility of array feeds for compact range reflector antenna applications is discussed. The goal is to feed a circular-aperture, offset parabolic reflector such that the central illumination is uniform and the rim illumination is zero. The illumination taper results in significant reduction of edge-diffracted fields without the use of reflector edge treatment. A methodology for designing an array feed requiring only two real excitation coefficients is outlined. A simple and cost effective array implementation is presented. The array beam forming network is realized as a radial transmission line which is excited at the center from a coaxial transmission line, and terminated at the perimeter with absorber and conductive tape. Energy is probe-coupled from the radial line to balun-fed dipole array elements. The required element amplitude excitation is obtained by adjusting the probe insertion depth, and the required element phase excitation is supplied by the traveling radial wave. Construction and test of an X-band array are summarized. The measured array patterns display a flat-topped beam with a deep null at angles corresponding to the reflector rim.
This paper briefly reviews existing compact range performance characterization methods showing the limitations of each technique and the need for an accepted and well understood technique which provides efficient and accurate characterization of compact range measurement accuracy. A technique known as the transverse pattern comparison method is then described which has been practiced by the author and some range users for the past several years. The method is related to the well known longitudinal pattern comparison method, however, comparisons are conducted in the transverse planes where the required span of aperture displacement is much smaller and does not exceed the dimensions of the quiet zone. This method provides several advantages for characterizing compact range performance as well as enables range users to improve achievable measurement accuracies by eliminating the impact of extraneous signal errors in the quiet zone.
Traditional Compact Range Antenna (CRA) applications are related to Antenna Pattern and RCS measurements. For these purposes, as a rule, CRA are installed within or outside of an anechoic chamber as stationary equipment. However, for some modern applications, such as Electronic Warfare development, radar tracking system testing, indoor RF environment simulation and others, where dynamic and pointing properties of an AUT are to be tested, the mobile and multi-beam CRA is of great importance, since it provides the designer with powerful simulation and testing capabilities. Such a CRA has been designed, built and tested at ORBIT ADVANCED TECHNOLOGIES, LTD. The design trade-offs, CRA analysis, test set-up and results are discussed in the presented paper.
The antenna analyzer is specifically designed to make use of measurement techniques that have been difficult to use until now The analyzer is an original vectorial receiver design, based upon the analysis of one of the sidebands of the marked RF measurement signal. Thanks to the RF marking process, the antenna analyzer is not the only equipment that allows characterization (in amplitude, phase or return loss) of all devices in a transmitting chain, including the high power elements, without cutting off the transmission. Originally introduced for the analysis of wired antennas in UHF-VHF bands, its use is now extended to microwave antenna measurements, especially printed circuit antennas. A special characteristic of the new analyzer, ESTAR 2110 is its capacity to measure the phase of RF signal with power levels as low as -120dBm. The analyzer is ideal for elaborate analysis of fundamental antenna parameters such as RF current distribution, close field, antenna pattern, impedance and phase balance of antenna network. The paper describes the marking principle and its use in making antenna parameter measurements.
The Precision Airborne Measurement System (PAMS) is a flight test facility at Rome Laboratory which is designed to measure in-flight aircraft antenna patterns. A capability which provides antenna pattern measurements for multiple VHF and UHF antennas, at multiple frequencies, in a single flight, has recently been demonstrated. A unique half space VHF/UHF long periodic antenna is used as a ground receive antenna. Computerized airborne and ground instrumentation are used to provide the multiplexing capability. The new capability greatly reduces time and cost of flight testing. The design, construction, and calibration of the half-space log-periodic ground receiving antenna is discussed and the ground and airborne segments of the instrumentation are described.
A modern outdoor test facility has been designed for comprehensive evaluation of antenna systems on full scale aircraft. The aircraft are mounted to a positioner/tower assembly in an underground handling facility, and are raised to a height of 25 meters by a hydraulically activated lift. A source site 1000 meters downrange provides illumination of a 7 meter radius test zone over a 0.1-18 GHz band. All source site functionality is remotely controlled from the operations center located near the aircraft support tower. The range is designed to provide the capability not only for conventional automated antenna pattern measurements, but also for the support of ECCM testing. This is accomplished by activating both fixed and mobile jamming transmitters available to illuminate the test zone using either CW or modulated waveforms. The FR Model 959 Automated Antenna Measurement Workstation is being enhanced to allow for control of the jammer sites as well as the primary range sited. The system design and operation is described.
A new multi-purpose antenna measurement facility was put into operation at the National Institute of Standards and Technology (NIST) in 1993. This facility is currently used to perform gain, pattern, and polarization measurements on probes and standard gain horns. The facility can also provide spherical and cylindrical near-field measurements. The frequency range is typically from 1 to 75 GHz. The paper discusses the capabilities of this new facility in detail. The facility has 10 m long horizontal rails for gain measurements using the NIST developed extrapolation technique. This length was chosen so that gain calibrations at 1 GHz could be performed on antennas with apertures as large as 1 meter. This facility also has a precision phi-over-theta rotator setup used to perform spherical near-field, probe pattern and polarization measurements. This setup uses a pair of 4 m long horizontal rails for positioning antennas over the center of rotation of the theta rotator. This allows antennas up to 2 m in length to be accommodated for probe pattern measurements. A set of 6 meter long vertical rails that are part of the source tower gives the facility that added capability of performing cylindrical near-field measurements. Spherical and cylindrical near-field measurements can be performed on antennas up to 3.5 m in diameter.
The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.