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Spherical near-field testing of antennas requires the acquisition of a great volume of data. In general, to compute the far-field of the antenna under test in any direction requires the acquisition of data at sample intervals related to the size of the antenna under test over a spherical sampling surface completely enclosing the antenna under test. This data must also be sampled as a function of probe orientation. Even for the simplest possible case, two probe orientations (or two probes) must be used.
A set of near-field measurements has been performed by combining the methods of non-probe-corrected spherical near-field scanning and gain standard substitution. In this paper we describe the technique used and report on the results obtained for a particular 24 inch 13 GHz paraboloidal dish. We demonstrate that the gain comparison measurement used with spherical near-field scanning give results in excellent agreement with gain comparison used with compact range measurement. Lastly we demonstrate a novel utilization of near-field scanning which permits a gain comparison measurement with a single spherical scan.
This paper reports on research being conducted at Georgia Tech on the spherical surface near-field measurement technique. The popularity of the spherical surface near-field measurement technique is indicated in the list of near-field ranges as shown in Table I. This popularity is, in large part, due to the availability of the scientific Atlanta Spherical Near-Field Antenna Analyzer. Specifically, the paper reports on the status of (1) the Georgia Tech spherical surface near-field range, (2) comparison of non-probe compensated spherical surface near-field to far-field transformation techniques, (3) a probe position error compensation technique for spherical surface measurements, and (4) alternative spherical surface near-field to far-field transformations which include probe compensation.
Conventional antenna test techniques – both far field “slant ranges” and near field – pose limitations for radiative RF testing of satellite antennas and payload systems, of increasing complexity in terms of size, operating frequencies, configurations and technology, particularly when such systems need to be evaluated in their “in-situ” locations on typical satellite platforms, in their flight configurations. Often, combination of tests and simulation has been the only recourse for evaluating system performance. In this paper, a methodology is proposed to achieve these test objectives via the use of a suitable configures, wideband, large (Quiet zone 7m x 5m x 5m), compact range for evaluation od system parameters like E.I.R.P., G/T, C/I, BER, and RF sensing performances. The test plan and evaluation schemes appropriate for these tests are elaborated to demonstrate the validity and usefulness of the approach. For some specific parameters like C/I (for a multibeam payload system) and the radar parameters (for a satellite borne radar system), it turns out that the proposed test methodologies offer the only realistic and complete tool for evaluating such system at satellite level.
Spherical near-field testing and other specialized antenna measurements require precise range and positioner alignment. This paper presents a method based on optical techniques to conveniently measure and monitor both range alignment and the positioner axis orthogonality and intersection. The hardware requirements consist of a theodolite and a unique target mirror assembly viewable from either side.
The development of a high resolution instrumentation radar is described. This radar constructed at X-band uses a chirp waveform to achieve a 4.9” range resolution capability. A key feature of this development is the use of cos2 x amplitude weighting to control the range sidelobes. An example of a high resolution radar response is described.
The operation and application of the Scientific-Atlanta Model 1784/ 1785 Millimeter-Wave to Microwave Converter is discussed. The converter allows single channel or multi-channel coherent measurements to be made with excellent sensitivity at Millimeter-wave frequencies. The converter improves the dynamic range of Millimeter-wave measurements by up to 30 dB over conventional measurements made with broad-band microwave receivers operating at high mixer harmonic numbers.
Test results will be presented for a four foot S-Band reflector antenna together with the compact range modification and test verification at 2.2 GHz. Similarly compact range test results will be presented for an eight foot K-band reflector antenna at 23 GHz.
A General Extrapolation Technique which corrects for the effects of ground reflections in absolute gain measurements is described. It utilizes the Extrapolation Method developed at NBS which, in its present form, utilizes only amplitude versus distance data. However, for broadbeam antennas such as those encountered below 1 GHz, ground reflections may produce unwanted oscillations in the amplitude versus distance data. However, for broadbeam antennas such as those encountered below 1 GHz, ground reflections may produce unwanted oscillations in the amplitude versus distance data. Hence the data are not amenable to the curve fitting procedure of the Extrapolation Method. This problem can be overcome by including phase versus distance information to negate the effects of ground reflections.
Just as with far-field or compact ranges, it is important to evaluate spherical near-field ranges with electromagnetic field-probe measurements. Recall that the fundamental motion for utilizing the spherical near-field measurement technique is to permit antenna measurements to be made at short range lengths, relieved from the constraint of the far-field criterion. Just as the illumination function in the test zone of an ideal far-field range is a uniform planar wavefront, the ideal illumination function for a near-field range is a spherical wavefront from an elemental dipole. The field probe measurements provide a quantitative and qualitative assessment of the deviation of either a near-field or far-field range from ideal conditions.
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.
Large scale electromagnetic simulation programs such as NEC (Numerical Electromagnetic Code) which employ method of moments and/or geometrical theory of diffraction are available. These codes are effective design and analysis tools for both the antenna designer and the antenna metrologist. This paper illustrates the ability of these codes to model actual antennas and antenna ranges. Several comparison examples are provided of electromagnetic models and the physical devices.
The concept of an Orbiting Standards Package (OSP) has been discussed as a means of making direct measurements of fields, patterns, and polarization states of signals radiated from large earth station antennas. It would also have the capability of producing test field of known intensities and arbitrary but well-defined polarization states, thereby enabling the determination of such parameters as G/T and Effective Receiving Area of earth stations. Recent developments in microwave six-port networks and in standard antennas would permit the all-electronic generation and detection of these signals. Moreover, it appears possible to recalibrate the satellite standards package to laboratory state-of-the-art accuracy following launch.
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.
The experimental techniques presented here can be used to obtain the approximate time domain transfer function and pattern of underground radar antennas. These techniques provide an easy approach to obtaining relative antennas performance. The experimental setup which is used to perform these experiments consists of slanted hollow plastic pipes bored in the ground, the receiver unit, transmitter unit, controller and processor units etc. A buried antennas is used to transmit to a test antenna on the ground surface. The data obtained from two separate test antennas are presented and compared.
The USAF Rome Air Development Center has recently constructed a laboratory building which has recently constructed a laboratory building which has been designed to implement the measurement of microwave antennas and electromagnetic systems. The new facility consists of dual elevated open-ended chambers with retractable doors, a 2700 foot outdoor range, a variable short range and a 40 x 20 x 18 foot anechoic chamber. Wide frequency band instrumentation is installed to provide efficient high speed data collection and analysis required to support the center’s technology development mission in C3I. A presentation of the facility’s capability and design will be given as well as a brief historic overview of significant antenna measurements of the past.
An S-band telemetry antenna system was designed and fabricated using a 30-inch diameter lightweight Luneberg lens as the aperture. It is equipped with four feeds in the azimuth plane to achieve single beam patterns or multiple beam patterns. Initial measurements with the lens without a radome were made with various feeds and feed combinations in the compact range of the Georgia Tech Engineering Experiment Station. The final design also done by Georgia Tech to Kentron Specifications, uses a custom designed quad ridged circular feed with orthogonal linear polarization outputs which are converted to left- and right-hand circular polarization using 90o hybrid couplers. A control panel permits the operator to manually select a single beam coverage of 11o x 11o, twobeams combined for 22o x 11o sector coverage, or four beams combined for 44o azimuth x 11o elevation sector coverage. A automatic mode permits the full gain of a single beam, about 22 dB, to be attained and switched automatically to the RF feed containing the greatest signal power as sensed by eight total power radiometer receivers; one for each orthogonal polarization for each of the four antenna feeds. Selectable integration time constants are 0.1, 1, 10, and 100 milliseconds. Dependable switching is obtained for signals of -99 dBm or greater. The RF switching is achieved by PIN-diode switches in 10 nanoseconds. The system employs eight state-of-the-art gain and phase matches GaAs FET low-noise preamps which have a noise figure of 1.1 dB and gain of 51 dB. External limiters at the input of each LNA protect the devices from accidental RF inputs up to six watts average power. The system was designed as a removable package to be flown aboard the U.S. Army’s C-7A Caribou aircraft with an opened rear cargo ramp to collect terminal TM data from missile reentry vehicles (RV’s) impacting near the Kwajalein Missile Range. Flight testing of the system against target of opportunity missions began the third week of June 1982. The system is expected to be declared an operational system in support of ballistic missile testing by December 1982.
Automated antenna testing has become economical with the Scientific-Atlanta Series 2080 Antenna Analyzer. Since its introduction last year, new computer hardware and software additions have enhanced the system performance. This paper will provide a brief overview of the system and its enhancements. It is recognized that testing requirements differ and an automated system must be capable of adapting to a specific test. The Series 2080 has a flexible data base and display programs which permit special antenna testing. A discussion of meeting special test requirements and the cost benefits of automated testing will be made.
The Automated Digital Antenna Measurement (ADAM) System developed by Flam & Russell, Inc. (FR) relieves the antenna engineer and technician from the constraint of designing a test plan/procedure dictated by the architecture of his automated test system. By contrast, ADAM’s flexibility allows the user to design a test configuration and interface with that instrumentation which is optimum for the performance evaluation of the antenna system under test in terms of data rate and accuracy. Further, as the test needs and configurations change, ADAM changes with them. For instance, if the engineer is testing antennas for a phase/amplitude interferometer, the test set-up might include a Systron-Donner frequency synthesizer and a Scientific-Atlanta receiver – thus sacrificing speed for accuracy. The same facility could be used later in the day for production testing where frequency accuracy is less critical and high data rates are the objective. In this case the signal source might be a voltage controllable Wiltron sweeper and the receiver an HP network analyzer. ADAM accommodates this change by merely identifying the test equipment through a menu.
This paper presents the results of research conducted to compensate near field measurements for known errors in near field probe position. The complete solution for probe position error compensation and associated computer algorithm developed by Corey as a Ph.D. dissertation resulted in a large computer memory and computation time requirements. Corey’s results showed, however, that the prime effect of probe positioning error was a change in the near field measurement phase in the direction of main beam propagation. It was also shown that the sinusoidal components of the probe position error produced spurious sideband propagation directions in the calculated far field patterns. This information has been used to develop a simplified probe position error compensation technique which requires negligible computer storage and computation time. An early version of this technique has recently been implemented at RCA for the Aegis near field measurement facility. The technique and sample results are presented for a small probe position errors and for a low sidelobe level antenna measurement.