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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.
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.
The direct far field pattern measurement of an aperture antenna becomes more difficult as the size of the aperture increases. Recent measurements on reflector antennas with 2D2/? =1500’ at The Ohio State University ElectroScience Laboratory have demonstrated the usefulness of the compact range in obtaining the complete far field pattern of antennas with large far field distances.
Techniques developed for the design of shaped, off-set reflector antennas have been applied to the design of compact ranges. Shaped optics which map an axially symmetric feed pattern into an elliptical aperture distribution have been designed. Some of the major design considerations for this type of system are examined in this paper. The design has been verified both analytically and experimentally.
When performing antenna pattern measurements on far-field antenna test ranges or in anechoic chambers, one of the main problems concerning the pattern accuracy is range reflections. Previous works dealing with this have been limited to the one-dimensional case.
An antenna radiation pattern measurement technique which allows near real time pattern capture is presented. This technique uses relatively simple field probles and detectors to cover a reasonably broad operating band. The captured pattern data is digitized with a resolution of 1.0 degree and has an angular range of 150 degrees. Many captured patterns or snap-shots could be recorded during a given time interval and later viewed for diagnostic evaluations where rapid changes in the pattern are expected.
This paper describes a novel technique for acquisition of far-field antenna patterns from a very low side-lobe antenna. The low side-lobe requirement imposes stringent multipath restrictions on the measurement range and to accommodate this requirement a vertical range configuration is employed rather than the more conventional range which is parallel to the earth's surface. To assure accurate measurement of side-lobe levels, multipath levels were specified at minus seventy dB (-70 dB) relative to the direct-path, peak-of-the-beam level. In this novel range configuration, an Antenna Under Test (AUT) is oriented to face skyward and operated in a receive mode with E-Field illumination provided from an airborne source. An optical tracker provides data of airborne source location and time-division multiplexing of both frequency and antenna beam position enable optimization of data acquisition efficiency. Post-acquisition processing provides de-interleaving of the desired beam(s)/frequency(s). This paper will present a discussion of the problems encountered and the techniques employed to overcome them in the design of this range. A description of the range will also be presented.
The Georgia Tech Research Institute is designing a large outdoor compact range for the U. S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. This range will be used to measure patterns of antennas installed on aircraft and vehicles. The goal of full hemispherical coverage with vehicles weighing up to 140,000 pounds has resulted in a unique positioner design, described in this paper. The 5-foot diameter quiet zone is centered 42.5 feet above the ground. The positioner's azimuth over elevation geometry keeps even large systems inside the quiet zone through the full range of positioner motion. The turntable is driven in continuous azimuth rotation by a hydraulic motor. The tilt table is driven through its -1 degree to +91 degree elevation range by two hydraulic cylinders. The tower is designed to carry a 140,000 pound vehicle in a 100 MPH survival wind. The structure consists of two steel frames, joined at the top. Both are enclosed in sheet metal shells to minimize scattering into the quiet zone.
It is well known that the compact range can be and has been used very successfully for scattering measurements. Recently, the compact range at The Ohio State University ElectroScience Laboratory was used to measure the patterns of two 8-foot diameter reflector antennas and their microwave horn feeds. Very good measurements have been achieved. In the paper, the results of the horn antenna measurements are presented while the results of the reflector pattern measurements are discussed in another paper. [1].
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.
Microwave holographic metrology is considered to be a key technique for achieving improved performance from large reflector antennas, especially at the shorter wavelengths. An important benefit of microwave holography is that the mathematically transformed data yields precise information on panel alignments on a local scale [1-5]. Since the usage of the holographic technique requires both the amplitude and phase data of the measured far-field patterns, one must carefully assess the impact of systematic and random errors that could corrupt the data due to a variety of measurement error sources.
The holographic antenna measurement system developed for the COMSAT Labs far-field range was tested with various antennas including axis-symmetric reflector antennas, offset single and dual reflector antennas, and phased-array antennas. Numerous examples which demonstrate the value of holographic measurement as an antenna diagnostic tool are presented. Microwave holography utilizes the Fourier transform relation between the antenna radiation pattern and the antenna aperture electromagnetic field distribution. Complex far-field date are collected at sample points and a Fourier transform is performed to give amplitude and phase contours in the antenna aperture plane. These contours facilitate reflector antenna diagnosis. The feed illumination and blockage pattern are provided by the amplitude distribution. The aperture phase distribution allows simple determination of deviations in the reflector surface and feed focusing. For phased-array antennas, the contours provide a measure of the complex element excitation. Measurement system parameters including pointing accuracy, phase stability, and measurement dynamic range were studied and refinements implemented to increase speed, accuracy, and resolution of the contour plots. To prevent aliasing errors, sampling criteria were explored to determine the optimum parameter ranges. For most antenna positioners, the antenna center is displaced from the rotation center. The importance of properly accounting for this displacement is discussed in the final section.
The techniques of near-field antenna pattern measurement can be extended to near-field RCS measurement. The motivation for doing so is precisely the same as that for near-field antenna measurements; i.e., the convenience of an indoor antenna range, and an improvement in accuracy. Although the near-field measurement problem is solvable in principle in a manner analogous to the near-field antenna problem, it requires a significantly larger amount of time to take the necessary data, and to subsequently process the data to obtain useful quantities. BDM is currently involved in an on-going program to evaluate the feasibility of near-field bistatic RCS measurements. At the time of this writing, a complete set of mathematics has been formulated to handle the probe correction and data processing. The hardware has been built, software development is near completion, and the analysis of canonical scattering objects has been completed. Experimental data soon to be taken for these objects will be presented. It is hoped that the technique will prove to be a practical approach to RCS measurements.
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.
A 15-meter diameter self-deployable antenna has been developed which utilizes the hoop-column structural concept with a gold-plated molybdenum mesh reflector. This antenna was developed to determine if a system could be designed and built with the dimensional tolerances necessary for in-space operational performance and for use as a test article in a ground based technology development program. One feature of the design is the provision for reflector surface shape control by cable adjustment. The antenna was deployed and tested at the Martin Marietta Denver Aerospace Near-Field Test Laboratory to measure its surface shape and its electromagnetic performance. RF test results show very good agreement between predicted and measured radiation patterns. The antenna is currently undergoing modifications which will allow automated surface adjustments and adaptive feeds to be utilized for further improvement in the electromagnetic performance. Controls, structural, and simulated thermal deformation tests will be integrated with future electromagnetic tests.
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.
The Boeing V-22 Osprey tilt rotor aircraft is a candidate platform for use as an airborne surveillance radar system. The impact of radar RF energy scattering from the aircraft's large propellers is a concern due to the potential for interference with an airborne pulse doppler radar where frequency changes are used to discriminate moving targets from ground clutter. In order to ascertain the effects of the scattering, a unique measurement system was devised for recording the time modulated antenna pattern of an array antenna.
This paper will discuss the disadvantages of a conventional Earth Station Antenna (ESA) pattern measurement technique compared with an alternative, time-proven technique. These measurement techniques are used to verify that a particular ESA has been properly constructed, focussed correctly and meets or exceeds a manufacturer's pattern envelope and gain specifications. These tests are performed on-site through a satellite link.
Many experimental and analytic studies on travelling waves have been performed in relation to their radiation properties for antenna applications. One common structure that has supported a fast travelling wave is a slotted waveguide. Such structures can also support travelling waves from a scattering viewpoint. This aspect was verified by incorporating a trough in an almond test body to observe its scattering characteristics using aspect angle patterns, frequency spectra and transient signatures from compact range measurements at the ElectroScience Laboratory, OSU. The travelling wave behavior is also correlated to the calculated travelling wave propagation constant for this structure with good agreement.