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.)
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.
Precision rotary vane attenuator calibrations are required in both the planar near-field method for determining antenna parameters and in the extrapolation method for determining on-axis gain of standard gain horns and probes. These attenuator calibrations are used to measure the linearity of the receiving systems and also to provide a precise offset capability used in insertion loss measurements. The Antenna Metrology Group of the National Bureau of Standards has utilized the i.f. substitution method to calibrate millimeter wave precision attenuators using equipment available in their measurement laboratory. The technique will be described along with the problems encountered. Results will be presented. In addition to mm wave attenuator measurements, the first calibrations of mm wave antennas and probes has resulted in tests to determine waveguide flange to flange connection errors for insertion loss measurements where repeated connections are necessary. The effects of these measurements on the overall error budget for the determination of the gain of an antenna will be presented and the effects of methods to reduce these errors will be discussed.
Millimeter band measurements require that care be exercised in the connection and handling of the waveguide flanges and their contact surfaces. When properly connected these flanges can provide many years of reliable and repeatable measurements. Improper use will limit the flange life to just a few connections, and cause measurement errors. These misuses are especially acute in situations requiring repeated connecting and disconnecting of small waveguide flanges, such as in antenna or insertion loss measurements. Some examples of misuse are: 1. using the flange to support heavy devices, 2. rocking the flange to get it on or off, 3. over-torquing multiple sides, and 4. using flanges with non-uniform surfaces. The effect of these misuses is that the flange is no longer usable for measurements requiring repeatability and this results in calibrations with unsatisfactory error bounds. NBS is currently addressing these problems by developing a Mechanical Millimeter Flange Alignment Fixture. The fixture indicates deficiencies in the contact area which need to be corrected. The fixture is then used to ensure that the flanges are mated correctly and repeatably. No twisting, rocking or angular mating of the flanges can occur. The fixture relieves the weight of the device on the flange and makes a versatile mounting fixture for almost any device where repeated connections must be made. The fixture and its use will be discussed in detail.
This paper presents the results of a study conducted to determine the effects of reflector surface errors on compact range performance. The study addressed only the reflector surface accuracy and not edge scattering, reflector illumination or reflector size. The study showed that low spatial frequency sinusoidal surface errors are significant contributors to amplitude ripple in the quiet zone field. Simple equations are presented for estimation of quiet zone amplitude ripple due to reflector surface errors. The study also presents measured surface error for two manufactures of reflector panels. The spectral (plane wave) components of the reflected field are displayed for a compact range reflector composed of a collection of these panels. *This work supported by the U. S. Army Electronic Proving Ground, Ft. Huachuca, AZ and the Joint Services Electronics program
This paper describes the mechanical as well as electrical measurement of doubly curved reflector antennas. The techniques developed for measurement of the new Canadian RAMP Primary Surveillance Radar antenna are described. Instead of a conventional full size template fixture to measure the antenna contour accuracy, an optical twin-theodolite method is used. The problems of the method are discussed and a new simplified analysis for calculating reflector error of doubly curved antennas is presented. Reflector errors are calculated and displayed concurrent with the actual measurements. The measurement of primary and secondary patterns for such antennas are described. Included are brief descriptions of the improved Andrew pattern test range and anechoic chamber facilities.
Over the long periods of time needed to acquire spherical near-field data, thermal drift of the system can cause errors in the measurement. The effect of thermal-drift can be removed, if it is monitored during the scanning process. This is accomplished by periodically returning the probe to the near-field peak during acquisition. The same point is re-measured upon each return; and the variations in phase and amplitude are used to produce a correction factor which is applied to each point in the near-field data file. This paper describes the return-to-peak method and the correction algorithm. Experimental results will also be presented.
A novel technique for improved accuracy of sidelobe measurement by planar near field probing has been developed and tested on the modified near field scanner at the National Bureau of Standards. The new technique relies on a scanning probe which radiates an azimuth plane null along the test antenna’s mainbeam steering direction. In this way, the probe acts as a mainbeam filter during probe correction processing, and allows the sidelobe space wavenumbers to establish the dynamic range of the near field measurement. In this way, measurement errors which usually increase with decreasing near field signal strength are minimized. The probe also discriminates against error field which have propagation components in the direction of mainbeam steering, such errors may be due to multipath or scanner Z-position tolerances. Near field probing tests will be described which demonstrate measurement accuracies from tests with two slotted waveguide arrays—the Ultralow Sidelobe Array (ULSA) and the Airborne Warning and Control System (AWACS) array. Results show that induced near field measurement error will generate detectable far field sidelobe errors, within established bounds, at the –60dB level. The utility of te probe to detect low level radar target scattering will also be described.
In this paper we describe the implementation of an antenna pattern recorder using a desktop digital computer to replace the conventional analog electro-mechanical element. This means that all pattern recorder front-panel controls and charts are displayed on and accessed via the computer’s CRT, keyboard and peripherals. It has all the regular features, e.g. choice of scales, pen up/pen down etc., plus a multitude of additional features, obtained owing to the use of a digital computer, which will later be outlined in detail. In spite of the numerous options available, the instrument is very easy to master, requires no preliminary knowledge of computer operation and programming. It is entirely menu-driven and designed to trap most operator errors while maintaining a user-friendly environment suitable for technician-level operation.
Measurement of complex permittivity (er) and permeability (µr), both vector quantities of absorptive materials, has gained increasing importance with expanding use of the RF and microwave spectrum, particularly in communications and electromagnetic countermeasure applications. In addition, the network analyzer has seen increasing use in non-destructive measurements to determine the chemical composition of a sample dielectric material. The method described here is suited for the measurement of complex permittivity and permeability of ansorptive materials. These measurements have been made for years using numerous methods. A conventional technique involves a two-step process using a slotted line or network analyzer. First, the sample is backed up by a short circuit and the input impedance is measured. Next, the short circuit is moved ¼ ? from the sample to simulate an open circuit termination (where ? is the incident signal wavelength), and a second measurement is made. The results of these two measurements are used to solve simultaneous equations for er and µr. This procedure is repeated for each frequency of interest. Uncertainties in the measurement include test set-up frequency response, mismatch, and directivity errors, as well as the uncertainty in the physical position of the short circuit.
This paper describes the equipment, mechanics and methods of one of the outdoor ranges at Teledyne Micronetics. A computer controlled microwave transceiver uses pulsed CW over a frequency range of 2-18 GHz to measure the amplitude, phase and polarization of the signal reflected off the target. The range geometry, calibration and analysis techniques are used to optimize measurement accuracy and characterize the target as a set of subscatterers.
This paper describes a new planar near-field measurement technique in which near-field data is collected in a hexagonal rather than a rectangular format. It is shown that the hexagonal method is more efficient than the rectangular technique in that a lower sampling density is required and the hexagonally shaped measurement surface is more compatible with most antenna apertures than the conventional rectangular measurement surface.
For approximately 10 years the National Bureau of Standards has used the Extrapolation Technique (A. C. Newell, et al., IEEE Trans. Ant. & Prop., AP-21, 418-431, 1973) for accurately calibrating transfer standard antennas (on-axis gain and polarization). The method utilizes a generalized three-antenns approach which does not require quantitative a priori knowledge of the antennas. Its main advantages are its accuracy and generality. This is essentially no upper frequency limit and it can be applied, in principle, to any type of antenna, although some directivity is desirable to reduce multipath interence.
The following features have been added to the spherical near-field software set which is available for the Scientific-Atlanta 2022A Antenna Analyzer. Gain Comparison Measurement Probe Pattern Measurement and Correction Thermal Drift Correction Spherical Modal Coefficient Analysis Far-Field, Radiation Intensity, and Polarization Display The addition of the probe pattern correction permits antenna measurements to be made at range lengths down to within several wavelengths of touching. The addition of probe polarization measurement permits three antenna polarization measurements to be made and analyzed as well as two antenna polarization transfer measurements. Correction for phase and amplitude errors attributable to thermal drift is accomplished by the return-to-peak method. Reduction of antenna patterns to spherical modal coefficients is an essential feature of spherical near-field to far-field transforms and is offered as an augmentation to antenna design. Far field display features permit the far fields of antennas to be presented in both component and radiation intensity formats, in circular, linear and canted linear polarization components.
This presentation will focus on the recently revised ANSI C95 RF Radiation Exposure Standard. Some of the research background for the new standard will be given, and its impact will be explained. Instrumentation guidelines for measuring potentially hazardous fields will be presented. The possible damaging effects of non-ionizing RF radiation is receiving increased attention in the public eye, and it behooves the practicing antenna engineer to be aware of the potential dangers to health and safety from exposure of RF energy.
The following features have been added to the spherical near-field software set which is available for the Scientific-Atlanta 2022A Antenna Analyzer. Gain Comparison Measurement Probe Pattern Measurement and Correction Thermal Drift Correction Spherical Modal Coefficient Analysis Far-Field, Radiation Intensity, and Polarization Display The addition of the probe pattern correction permits antenna measurements to be made at range lengths down to within several wavelengths of touching. The addition of probe polarization measurement permits three antenna polarization measurements to be made and analyzed as well as two antenna polarization transfer measurements. Correction for phase and amplitude errors attributable to thermal drift is accomplished by the return-to-peak method. Reduction of antenna patterns to spherical modal coefficients is an essential feature of spherical near-field to far-field transforms and is offered as an augmentation to antenna design. Far field display features permit the far fields of antennas to be presented in both component and radiation intensity formats, in circular, linear and canted linear polarization components.
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.
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.
An innovative technique has been developed for accurately measuring very low Sidelobe Antenna patterns by the method of planar near field probing. The technique relies on a new probe design which has a pattern null in the direction of the test antenna’s steered bean direction. Simulations of the near field measurement process using such a probe show that -60dB peak side-lobes will be accurately measured (within established bounds) when the calibrated near field dynamic range does not exceed 40 dB. The desireable property of the new probe is its ability to “spatially filter” the test antenna’s spectrum by reduced sensitivity to main beam ray paths. In this way, measurement errors which usually increase with decreasing near field signal level are minimized. The new probe is also theorized to have improved immunity to probe/array multipath and to probe-positioning errors. Plans to use the new probe on a modified planar scanner during tests with the AWACS array at the National Bureau of Standards will be outlined.
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.
There is a growing interest, for developing large, high performance communication antennas for use in space. Such antennas employ many new technologies and are very expensive to design, build, and deploy. These high risk projects require thorough ground testing before becoming operational. Unfortunately, accurately measuring the far field pattern of a large, structurally weak, high performance antenna on the ground is a difficult problem. The antenna’s extraordinary characteristics place severe tolerances on an antenna measurement range. This paper examines many of the problems encountered with measuring the far field pattern of these antennas. Several possible techniques are reviewed and the errors, tolerances, and limitations associated with each technique are analyzed.