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The radiation patterns of a low (40dB) sidelobe antenna have been measured on a variety of antenna test ranges including Near Field, Far Field and Compact versions. Originally intended to validate new Near Field Ranges, some of the early results will be presented and the variations examined. The need for some form of range validation is shown. There is also some explanation of the fundamental effects that various ranges have on results.
Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.
The measurement of cryogenic antennas poses unique logistical problems since the antenna under test must be embedding in the cooling chamber. In this paper, a method of measuring the performance o cryogenic microstrip antennas using the closed cycle gas-cooled refrigerator in a far field range is described. Antenna patterns showing the performance of gold and superconducting Ka-band microstrip antennas at various temperatures are presented.
Antenna amplitude and phase pattern measurements on combat ships and other large ships have typically relied on traditional methods which include circling a fixed buoy in the far field, tracking a shore-based transmitter with an optical device, or circling the subject ship with a smaller boat outfitted with a transmitter. These techniques required the measurement of many independent variables using less than precise methods to compute antenna patterns relative to the ship’s structure. Using the global positioning system to precisely locate the ship relative to the transmitter site location and combining this with the ship’s heading, antenna measurements can be accurately and quickly obtained. This paper will describe the traditional fixed buoy and optical follower techniques and contrast these against the more accurate and faster GPS antenna measurement technique.
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
Compact range systems have been widely used for antenna measurements. However, the amplitude taper can lead to significant measurement errors especially as the dimension of antenna is larger than quiet zone area. An amplitude taper removing technique by software implement is presented for compact range system. A 12 feet by 1.0 feet S-band rectangular slot array antenna is measured in SA5751 compact range system, which provides a quiet zone area with a 4 feet diameter. Results of corrected far-field patterns from compact range are compared with that taken by planar near-field range.
The RAFAEL general purpose radome measurement range has been modernized and refurbished, maintaining its capability to accommodate all range of radome sizes up to 1.2 meters in diameter. It is based on a 3-axis positioner placed in an open anechoic chamber with a null seeker placed 20 meters away and about 10 meters above the ground. All the positioner’s axes are controlled by an automatic positioner controller. The receiver and source are based on a HP-8510B system. The X-Y null seeker serves for boresight error measurements. It has a 0.7m x 0.7m total motion span, which is about 2º. It is controlled by a dual-motor controller, so that the scanning antenna can be moved in any kind of motion. Instrumentation control and data acquisition and analysis is performed using a HP-330 UNIX controller. Present software handles monopulse antennas with or without a comparator, and can implement the comparator in software. There are two major measurement modes: One for BSE measurements and the other for radiation patterns.
The prototype of the March Microwave Single-Plane Collimating Range (SPCR) has been in operation at Arizona State University’s ElectroMagnetic Anechoic Chamber (EMAC) facility for approximately three years. The unique SPCR produces a cylindrical-wave test region by bouncing spherical wavefronts off a parabolic cylindrical reflector. Consequently, a simplified algorithm can be applied to determine antenna far-field patterns. Both computation and acquisition times can be reduced considerably when compared to classical NF/FF cylindrical scanning techniques. To date, this is the only SPCR in operation. Some of the fundamental quantities which characterize an antenna/RCS measurement range are the size and quality of the “quiet zone”, usually expressed in terms of ripple and taper of the illuminating fields relative to an ideal planar wavefront. Direct one-way probing of the quiet zone fields in the vertical and horizontal planes has been recently completed at ASU. An overview of the range geometry, the field probing methodology, and the data processing will be presented. The results of the quiet zone scan will be presented as amplitude ripple, amplitude taper, and phase ripple versus frequency from 4 GHz to 18 GHz in four bands. The vertical-scan phase deviations are relative to an ideal planar wavefront, while those of the horizontal scan are relative to an ideal cylindrical wavefront.
The experience with near-field scanning at Scientific-Atlanta began with a system based upon a analog computer for computing the two-dimensional Fourier transform of the main polarization component. When coupled with a phase/amplitude receiver and a modest planar near-field scanner this system could produce far-field patterns from near-field scanning measurements. In the 1970’s it came to be recognized that the same advances, which made the more sophisticated probe-corrected planar near field measurements possible, would enable conventional far-field range hardware to be used on near-field ranges employing spherical coordinates. In 1980 Scientific-Atlanta first introduced a spherical near-field scanning system based upon a minicomputer already used to automate data acquisition and display. In 1990, to meet the need of measuring complex multistate phased-array antennas, Scientific Atlanta began planning a system to support the high volume data requirement and high speed measurement need represented by this challenge. Today Scientific-Atlanta is again pursuing planar near-field scanning as the method of choice for this test problem.
Probe correction is necessary in near-field measurements to compensate for non-ideal probes. Probe compensation requires that the probe’s far-field pattern be known. In many cases direct far-field measurements are undesirable, wither because they require dismantling the probe from te near-field range set-up or because a far-field range is not available. This paper presents a unique methos of deriving probe-correction coefficients by measuring a probe on a near-field range with an “identical” probe and taking the square root of the transformed far field. This technique, known as the “robe-square-root” method can be thought of as self-compensation. Far-field comparisons are given to show that this technique is accurate.
A recently developed analytic technique that can correct for probe position errors in planar near-field measurements to arbitrary accuracy [1] is shown to be also applicable to spherical near-field data after appropriate modifications. The method has been used to successfully remove errors in the near-field, hence leading to more accurate far-field patterns, even if the maximum error in the probe’s position is as large as 0.2?. Only the error-contaminated near-field measurements and an accurate probe position error function are needed to be able to implement the correction technique. It is assumed that the probe position error function is a characteristic of the near-field range, and that it has been obtained using state-of-the-art laser positioning and precision optical systems. The method also requires the ability to obtain derivatives of the error contaminated near-field defined on an error-free regular grid with respect to the coordinates. In planar geometry the derivatives are obtained using FFTs [1], and, in spherical geometry, one needs to compute derivatives of Hankel functions for radical errors, and derivatives of the spherical electric and magnetic vector basis functions for errors in the ? and Ø coordinates. The error-correction technique has been shown to work well for errors in and of the spherical coordinates r, ? or Ø. Efficient computer codes have been developed to demonstrate the technique using computer simulations.
Far-field pattern prediction of a mm wave reflector antenna from a scan of the near-field modulus is reported. The phase retrieval algorithm utilises minimisation and the generalized error reduction algorithm to retrieve both aperture amplitude and phase from a single planar intensity scan. The far-field pattern is calculated from the retrieved complex aperture. Experimental results from measurement of a 1.12m diameter reflector at 32 GHz are presented to illustrate the practicality of the algorithm for millimeter and submillimeter applications.
The Antenna Metrology Group of the National Institute of Standards and Technology (NIST) has recently developed and implemented measurement procedures to diagnose faults on a flat phased-array antenna. First, the antenna was measured on the NISTplanar near-field (PNF) range, taking measurements on a plane where the multiple reflections between the probe and the antenna under test are minimized. This is important since the PNF method does not directly allow for these reflections. Then, the NIST PNF software which incorporates the fast Fourier transform (FFT) was used to determine the antenna’s gain and pattern and to evaluate the antenna’s performance. Next, the inverse FFT was used to calculate the fields at the aperture lane. By using this technique, errors in the aperture fields due to multiple reflections can be avoided. By analyzing this aperture plane data through the use of detailed amplitude and phase contour plots, faults in the antenna were located and corrected. The PNF theory and utilization of the inverse FFT will briefly be discussed and results shown.
In near field antenna measurements, knowledge of the the [sic] probe antenna’s pattern, polarization and gain are of vital interest. To calibrate a probe for near field measurements is a delicate task, especially if the probe is small, i.e. low gain. The near field probe and the parameters general to a probe calibration are presented. The delicate task of obtaining an accurate gain for small aperture antennas as well as the problem of transfering [sic] the calibration from the facility where the probe is calibrated to the facility where it is to be used are focussed [sic] upon For a small aperture, the pattern is that of the radiating aperture. The unwanted scattering may be removed by filtering in the spherical mode domain thus obtaining the true aperture radiation. The gain derived from this may however be of little use in reality since the aperture always needs some form of mounting. Such a mounting may be covered with absorber which may reflect and diffract and thus affect the gain value.
In this paper, an antenna measurement technique based on modified cylindrical NF/FF transformation will be presented. In conventional cylindrical near-field scanning techniques, the near fields are probed on a cylindrical surface surrounding the test antenna. This required extensive data acquisition and processing time which can be reduced substantially if the antenna under test is illuminated by a cylindrical wave. In this hybrid approach, cylindrical wave illumination is generated using a single parabolic reflector in combination with a (point) source. The far-field pattern is then computed by a powerful one-dimensional NF/FF algorithm. It is concluded that this alternative approach combines the attributes of the compact-range technique and the classical NF/FF transformation.
The Concept of Compact Test Range has been recently much used for antenna testing facilities, its main characteristic of having far-field conditions in a small and closed place, for a very large frequency band, makes it very attractive. Antenna manufacturers are building them up when the millimetric waves and the spacecraft flight model antennas become part of their activities. The change of the point of view of the antenna characteristics – now, parameters like Gain and Radiation Patterns are replaced by EIRP, Flux Density or Coverage- modifies the classical test philosophy. It makes different the Test Procedures which, in addition, have to take into account the cleanliness and the quality control required for handling flight models, as well. The Compact Payload Test Range (CPTR) in ESTEC shows up a PWZ of 7 x 5 x 5 metres for a frequency range from 1.5 to 40 GHz.; it has been created for testing whole Spacecraft Payloads in space required cleanliness area. The particular properties of the CPTR as such as shielded room, feed scanning, multiaxis test positioner, etc. are used to improve its test possibilities.
Problems exist with the measurement of large aperture antennas due to the far field requirement. This paper discussed a new method to measure a phased array at about 1/10 the normal far field. The basic idea involves focusing the test array at probe antenna a distance R away from the aperture. In the described measurement technique the probe antenna is placed on an arm that rotates 100º on the focal arc given by Rcos(?). This arc minimizes defocusing due to phase aberrations. To minimize the amplitude errors, the pattern of the probe antenna is carefully matched in order to compensate for the 1/R variation induced amplitude error. The application of this technique will enable arrays to be measured in anechoic chambers, allowing convenient classified testing, while avoiding the effects of weather, and will reduce the risks inherent in the high power testing on transmit. The results of a computer simulation is presented that characterizes the validity and limitations of the technique.
Beamspace techniques are usually employed to synthesize phased array antenna patterns of arbitrary shape. In this paper a beamspace method is used to calibrate the pattern of a 32-element linear array with a conventional array taper. By measuring the antenna pattern in specific directions the beamspace technique permits the actually applied excitation function to be determined with little mathematical effort. Iterative corrections can then be made to the excitation function to maintain low sidelobe performance, or to compensate for element failures. Since local corrections to the array pattern result in global changes to the excitation function, explicit knowledge of where an element failure has occurred is not required. The beamspace analysis was carried out using antenna patterns obtained by electronically scanning the array past a far-field source. Such pattern measurements offer the possibility of maintaining phased array performance in an operational environment.
When attempting to make accurate Radar Cross Section (RCS) measurements, it is vital to understand the background levels of both the range and the target support fixture. Typically these support fixtures are either foam columns or metal pylons. Determining the RCS levels of the metal pylons requires the installation of a termination device to hide the rotator which has a significantly lower RCS than the pylon being measured. Quite often this is an impossible task, especially at lower frequencies. An algorithm that accurately determines the pylon background levels independent of the RCS contribution of the pylon terminator is presented. This algorithm requires translating the terminator linearly and isolating the background from the resulting interference pattern. Data is included that validates the implementing computer code.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.