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In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
The National Bureau of Standards (NBS) has been measuring antennas and dealing with the problems of leakage for the past twenty years. This paper will discuss the various methods of detecting leakage, typical sources of leakage, how to correct leakage problems, and the effects that leakage can have on calibration.
It is common practice in antenna measurements to use a moderately directive source (or 'probe') antenna, to minimise the effect of reflections. The illumination of the test antenna then exhibits a degree of non-uniformity. A correction scheme has previously been proposed for spherical near-field measurements. This paper describes a new probe-correction algorithm that can be used in conjunction with spherical near-field or 'conventional' measurements. It is operable with a minimum amount of measured data (for either the test antenna or the probe). It may also be used for probe-correcting calibration measurements using a gain horn.
Results of an experimental study of the interactions between a scattering target and the absorber-coated walls and ceiling of the OSU Compact Range Anechoic Room are reported. A 6 ft. square flat metal reflector was mounted in the quiet zone and oriented at selected angles non-orthogonal to the range symmetry axis. In theory, this target (when non-orthogonal) has a relatively low backscattering signature, and a strong planar bistatic scattering beam which can be pointed at several regions and absorber types in the room. By processing, the bistatic iteration terms can be separated form the plate backscatter, and frequency domain spectra and/or transient response signatures of the different mechanisms produced. Th paper will present calibration information on the actual performance of the bistatic scattering beam of the plate, and measurements of both backscattering and bistatic scattering of the absorber-coated walls in the ESL chamber. Suggested guidelines for use of this as a standard anechoic room diagnostic test will be discussed.
Three test methods have been developed and validated for characterizing materials at VHF and UHF in an indoor environment. The first method employs a resonant strip-line cavity for the independent determination of permittivity and permeability from .15-2 GHz. The planar field geometry and sample configuration permit evaluation of material anistropy. Measurements are taken on an Automatic Network Analyzer (HP 8510 ANA). The second method measures the reflection/transmission (R/T) of planar material samples at UHF. This is a free space measurement performed in an anechoic chamber. Data is taken from .2-2 GHz using two dual ridged horn antennas and the ANA. A calibration method has been developed for the ANA to correct for measurement errors. Off-set shorts and thru delays are used in this technique. The third technique evaluates reflection performance of materials from 150-250 MHz. This technique employs a custom designed corner reflector antenna. Only one such antenna is needed due to the calibration technique. These methods allow a synergistic approach to material development. Candidate material can be evaluated using the cavity or R/T systems. Material designs can then be tested on either the UHF and/or VHF systems.
The performance of an outdoor, ground-plane RCS measurement range can be degraded by fluctuations in the atmospheric reflectivity N. These fluctuations can introduce error into RCS measurements, particularly when they do not manifest in the radar return from the secondary calibration standard. A propagation anomaly study at the RATSCAT RCS range compares the N-fluctuations -- obtained from meteorological instruments and separately from RF receivers -- at several levels above the ground. The fluctuation mechanisms are discussed in terms of temperature lapse rates, "constant-N" cell sizes, wind velocity, and rough ground effects. The optimal RF sensor height for propagation anomaly indications is found to depend on the cell size. This has implications for the positioning of secondary calibration standards.
It is common practice to window RCS data prior to inverse Fourier transformation into an image. Windowing reduces image sidelobes at the expense of some loss of resolution. When the window shape is adjusted to give the best resolution-sidelobe tradeoff for the given application, however, the apparent RCS of features in the image varies unless the correct calibration of normalization is applied. This paper discusses the proper calibration and normalization techniques to use with RCS imaging. These techniques permit efficient generation of images that accurately depict the RCS of significant target features, independent of the data window shape.
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.
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.
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.
Near-field testing of a very low sidelobe, L-band, 32-element, linear phased array antenna was conducted. The purpose was to evaluate testing and calibration techniques which may be applicable to a much larger, space borne phased array antenna. Very low sidelobe performance in a relatively small array was achieved by use of high precision transmit/receive modules. These modules employ 12-bit voltage controlled attenuators and phase shifters operating at an intermediate frequency (IF) rather than at RF. Three array calibration techniques are discussed. One technique calibrates the array by means of a movable near-field probe. Another method is based on mutual coupling measurements. The last technique uses a fixed near-field source. The first two calibration methods yield substantially the same results. Module insertion attenuation and phase can be set to 0.02 dB and 0.2 degrees, respectively. Near-field measurement derived antenna patterns were used to demonstrate better than -20 dBi sidelobe performance for the phased array. Application of increasing Taylor array tapers showed the limitations of the measurement systems to be below the -35 dBi sidelobe level. The effects of array ground plane distortion and other array degradations are illustrated.
A way has been found to utilize the reflector return in a compact range as a source of continuous drift compensation. This is performed by translating receive polarizations 45 degrees with respect to the transmit polarizations to ensure returns in co- and cross-polarizations. An added benefit is the simplicity of alignment for the polarization calibration standard.
This paper describes methods commonly used by anechoic chamber manufacturers to characterize chamber performance. Test procedures depend first on the purpose of the test; second on the purpose of the anechoic chamber and third on the amount of information required. Most anechoic chambers are built for a specific use. In order to prove its design, the test will be done accordingly. In most anechoic chambers one measures the reflectivity level because this is a measure for the accuracy on future measurements when the chamber is in operation. Anechoic chambers can vary from Antenna Pattern Test Chambers to Radar Cross Section Test Chambers, Electronic Warfare Simulation Chambers and Electro Magnetic Compatibility Test Chambers. Each type of chamber will have its specific evaluation technique. Some techniques can be done by the chamber user himself. Other methods need some special equipment that will or can only be used for that particular test method. Some customers want to do their own calibration on a regular basis. They can purchase this special equipment from the chamber manufacturer, if necessary. More complicated methods make use of computer controlled equipment. The data required can be taken in the chamber. This can be done relatively fast. All sorts of information about the chamber characteristics can be obtained in a later stage in a different format by use of the right software. This paper gives possible evaluation methods for different types of anechoic chambers. Detailed information about each method can be obtained from Emerson & Cuming.
A pulsed, coherent radar system was used in the inverse synthetic aperture radar mode to obtain 1-way high resolution images of simple antennas. These high resolution images display the amplitude and phase distribution of the received wave. The images were then edited and reconstructed using System Planning Corporation's Image algorithms contained in the SPC RPS software package. The 2-D (range vs. cross range) image data is very useful for detecting defects in antennas and can also 0be applied to modification of illumination conditions such as wavefront sphericity (phase taper) and/or amplitude variabilities (taper, ripple). This technique offers an alternate approach to near field/far field transformation. The technique involves rotation of the antenna under test at a controlled, uniform rate. The antenna port is connected to the radar receiver and the radar transmitter attached to an illuminating antenna. The radar transmits a step chirp wave form. The received signal is recorded to tape and processed off-line on the SPC Image Reduction Facility. A calibration technique was developed using simple wide bandwidth horn antennas. The downrange and cross range resolution of these 1-way ISAR antenna images is half as large as with 2-way radar ISAR for the same bandwidth and angular integration interval. Image data will be shown on reflector-type antennas to illustrate the technique.
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.
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.
The calibration of gain standards for antenna measurements requires path loss measurements between three antennas if the assumption of identical antennas is not made. The equipment finds the insertion loss for pairs of antennas as if the combination of the antennas and the free space between them were a two port network. The usual setup uses a network analyzer to measure the insertion loss. The Scientific Atlanta 2020 system can be operated as a network analyzer and used for these measurements. Part of the system is a synthesized signal source which allows frequency stepping, and along with leveling, enables the repetition of both amplitude and phase of the signals. The computer control of the equipment provides for rapid stepping through the frequencies, control of the receiver, ability to read amplitude and phase, and means of data storage for off-line analysis.