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A cylindrical near-field antenna range has been designed, implemented and tested recently at the Cobb County Research Facility of Georgia Tech’s Engineering Experiment Station. While Georgia Tech has had an operational planar scanner since 1974 [1], the relocation of a portion of the Experiment Station to an off-campus site, together with the need for measurements of antennas not practical with the existing planar scanner, prompted the addition of a cylindrical near-field range. Provision was made in the range instrumentation for planar-polar and spherical near-field measurements. Computer software was written to effect the conversion from cylindrical near-field measurements to far-field patterns.
This paper describes a method for determining the radiation patterns of all possible operating modes of multi-element antennas without using the normally-required mode-forming networks. For the general case of non-symmetrical antennas, the radiation pattern amplitude and phase characteristics of each antenna element is digitally recorded with all other elements in their normal operating positions and their output ports terminated in a matched load. The patterns of all possible operating modes of the antenna are then computed by phasor summation of the element patterns using the same amplitude and phase offsets which would be imparted by a mode-forming network. This approach significantly reduces hardware requirements for testing broadband multielement antennas, particularly during early development stages. Symmetry in the antenna can be used to reduce the number of elements that need to be measured. If the antenna is rotationaly symmetric (for example, log-spirals, log-periodic monopole arrays and some cavity backed monopole arrays), only one element needs to be measured. Significant data reduction is achieved since all the orthoganal mode patterns can be derived from the field of one element. An eight-arm, constant electrical radius spiral was used as a test antenna for comparison of patterns generated by the following three methods: 1. Direct measurement with mode forming network present. 2. Computational phasor summation of all spiral arms. 3. Computational rotation, superposition and phasor summation of the output of a single arm. High correlation of results between the three methods is demonstrated. A discussion of possible error sources is included.
In principle, spherical near-field scanning measurements are performed in the same way as conventional far-field measurements except that the range length can be reduced. This provides a natural advantage to scanning in spherical coordinates over other coordinate systems due to the steady availability of equipment. However, special considerations must be given to near-field range design because of the necessity for phase measurement capability, mechanical accuracy and the need to handle large quantities of data. Based on experience with spherical near-field measurements carried out during verification testing of a spherical near-field transformation algorithm, we discuss the practical aspects of constructing a near-field range. In particular we will consider range alignment procedure, engineering of the RF signal path and times for data collection and processing.
Over the past year an extensive set of verification tests has been made on a particular test antenna in order to confirm the design and operation of a spherical near-field algorithm. The measurement checks included data taken at two frequencies at three range lengths, with two coordinate orientations and with two types of probe horns. Comparisons were made against the compact range and among the various spherical near-field tests. In this talk we show examples from this program of measurements and summarize the results which demonstrate the operation of the spherical near-field scanning technique.
Research on the near-field antenna measurement technique is now in its 15th year at Georgia Tech. Current research is supported by the Army Research office, by the Joint Services Electronic Porgram [sic], and the National Science Foundation. An overview of the current research activities will be given including a description of the Georgia Tech Planar, Cylindrical and Spherical Surface near-field ranges. A recently developed technique for analytic compensation of near-field probe positioning error will be presented.
Compact Antenna Ranges (C.R.) proved to suitable for indoor measurements of antennas of moderate size (up to about 4 feet) in the frequency ranges from 4-18 GHz. Where less accurate measurements are allowed, the upper frequency limit can be as high as 60 GHz in current C.R. design. Dimensions of such a range are approximately 4 times larger (in linear dimension) than those of the test antenna. This is due to the face that there is a considerable taper in the amplitude over the aperture of the C.R. Considerable improvements in the electrical performance may be expected for ranges in which two crossed parabolic cylindrical reflectors are used. Due to the increased focal length the uniformity of the amplitude distribution across the final aperture is increased considerably compared to conventional design. Furthermore, an asymmetrical plane-wave zone can be created which makes it possible to measure the patterns of asymmetrical antennas or devices including the direct environment (antennas on aircraft or spacecraft). A compact range which consists of a main reflector with overall dimensions of 2x2 metres has been used for experimental investigation in the 8-70 GHz frequency band. At 10 GHz the plane-wave zone has a slightly elliptical shape (100x90cm). The amplitude variations are in this case less than 0.3dB; the corresponding phase errors are less than 4 degrees. It has been shown that the reflectivity level can be kept below –60dB. Only a minor degradation in performance was found at 70 GHz. In conclusion, the performance of this new compact range is as good as, or better than that of most outdoor ranges. The upper frequency limit is about 100 GHz for ranges of moderate size (up to 3 metres). Summarizing, the main advantages compared to other compact ranges are: -Larger test zone area (up to 2x) for the same C.R. reflector size -better crosspolar performance -considerably higher upper frequency limit The last-named is due to the cylindrical reflector surfaces, which are easier and cheaper to manufacture than double-curved surfaces.
The mechanical alignment of a reflector antenna involves both the reflector shape and also the relative orientation of the feed and subreflector. The requirements for alignment are derived from the system requirements for antenna functional performance, including pointing. A typical alignment plan includes the following alignment operations: • Component inspection of reflector, subreflector and feed. • Antenna assembly, including a final baseline measurement. • Alignment to a positioner for antenna range tests. • Alignment checks before and after environmental exposures. • Installation on spacecraft, including receiving inspection, adjustment to a specific orientation, and structural distortion checks • Alignment checks on spacecraft. Six tooling balls on the back of the reflector are commonly used as a reference for both pointing and structural distortion. Additional references may be provided by mirrored surfaces, auxiliary tooling balls, machined edges, scribe lines and mounting surfaces. Special fixtures for holding the antenna throughout its test sequence have proved useful. These fixtures are designed to provide a rigid support with a minimum of mounting stresses. They also have provisions for fine angular adjustments on antenna positioners. Analytic aids include: • Calculations of the Best-Fit-Paraboloid to the measured points on the reflector surface. • Use of beam deviation factors to calculate the predicted electrical beam from mechanical measurements. • Transformation of coordinates from one system to another. The measurement methods and analytic techniques that are used for a typical set of alignment operations are described.
This paper summarizes the results of work performed for the Naval Air Development Center (NADC) on a new full-sized aircraft antenna range located in Warminster, PA. Because of the ever-increasing sophistication of aircraft systems, a facility capable of testing full scale mock-ups has become necessary to fully characterize the system in its operating environment. There are, however, several unique problems associated with such a range. Many systems of interest have a wing-tip to wing-tip baseline, which requires that the incident illumination be “uniform” over a significant aperture (approximately 40x15 feet for tactical aircraft). Differential path loss between wing-tip ends, as the aircraft is rotated, can be a source of large error, as can the parallax created by off-center rotation. Also, since today’s military aircraft carry a wide variety of systems, the range is required to be a “general use” range, operational over a wide frequency spectrum from 30 MHz to 40 GHz. A thorough examination of design trade-offs was performed relating the critical parameters of source beamwidth, specular reflection, path loss, phase error, and receive aperture size in order to choose the proper source antenna type, source height, and separation distance between source and test antennas for each frequency band of interest. Other factors in the range design were a maximum possible source height of 40 feet (approximately the height of the pedestal), and a desire to keep the separation distance fixed over the entire frequency range. Results are presented with indicate excellent performance over an 18 x 18 foot aperture for various polarizations. It was found that the range operates effectively as a ground reflection range from 30 MHz to 3 GHz, and as an elevated range at higher frequencies. Peak-to-peak amplitude ripples over the test aperture of 1.0 dB (corresponding to a reflection level of –25 dB) were acheived over a significant portion of the frequency spectrum.
The US Army Electronic Proving Ground is in Southeastern Arizona with outlying facilities located throughout Southern Arizona. The Proving Ground is an independent test and evaluation activity under the command of the US Army Test and Evaluation Command. It was established in 1954. EPG’s role in the material acquisition cycle is to conduct development (DT I & II), initial production (first article), and such other engineering (laboratory-type) tests and associated analytical studies of electronic materiel as directed. The results (reports) of these efforts are used by the developer to correct faults, and by Army and DOD decision-makers in determining the suitability of these materiels/systems for adoption and issue. Customer tests to satisfy specific customer requirements and foreign materiel exploitations are also done. EPG is assigned test responsibility for Army ground and airborne (aircraft-mounted) equipment/systems which utilize the electromagnetic spectrum to include: tactical communications; COMSEC (TEMPEST testing included); combat surveillance, and vision equipment (optical, electro-optical, radar, unattended sensors); intelligence acquisition; electronic warfare; radiac; imaging and image interpretation (camera, film, lens, electro-optical); camouflage; avionics; navigation and position location; remotely piloted vehicle; physical security; meteorological; electronic power generation, and tactical computers and associated software. Facilities and capabilities to perform this mission include: laboratories and electronic measurement equipment; antenna pattern measurement’ both free-space and ground-influenced; unattended and physical security sensors; ground and airborne radar target resolution and MTI; precision instrumentation radars in a range configuration for position and track of aerial and ground vehicles; climatic and structural environmental chambers/equipment; calibrated nuclear radiation sources; electromagnetic compatibility, interference and vulnerability measurement and analysis; and other specialized facilities and equipment. The Proving Ground, working in conjunction with a DOD Area Frequency Coordinator, can create a limited realistic electronic battlefield environment. This capability is undergoing significant development and enhancement as a part of a program to develop and acquire the capability to test Army Battlefield Automation Systems, variously called C3I, C4, and/or CCS2 systems. The three principal elements of this capability which are all automated include: Systems Control Facility (SCF), Test Item Stimulator (TIS), and Realistic Battlefield Environment, Electronic (REBEEL). In addition to various instrumentation computers/processors, EPG currently utilizes a DEC Cyber 172, a DEC VAX 11-780, a DEC System 10, and has access to both a CDC 6500 and a 6600. Under the Army Development and Acquisition of Threat Simulators (ADATS) program, EPG is responsible for all non-air defense simulators. The availability of massive real estate in Southern Arizona, which includes more than 70,000 acres on Fort Huachuca, 23,000 acres at Willcox Dry Lake, and 1.5 million acres near Gila Bend, is a major factor in successful satisfaction of our test mission. Fort Huachuca itself is in the foothills of the Huachuca Mountains at an elevation of approximately 5,000 feet and has an average annual rainfall of less than 15 inches. Flying missions are practical almost every day of the year. The Proving Ground is ideally situated between two national ranges and provides overlapping, compatible instrumentation facilities for all types of in-flight test programs. The clear electromagnetic environment, the excellent climatic conditions, and the freedom from aircraft congestion make this an unusually fine area for electronic testing. The Proving Ground consists of a multitude of sophisticated resources, many of them unique in the United States, which are an integral part of the USAEPG test facility and have resulted from an active local research and development effort over a 28-year period.
A review will be made of advances in anechoic chamber technology from the precursors of World War II to the huge complex chamber of today. A glimpse of the technology of the 80’s will be offered.
A limited number of power gain measurements for some broadband airborne antennas were analyzed by comparing the measured values to the predictions from hypothetical models for the antennas. The difference between the predicted gain and measured gain is defined as the measurement uncertainty. The measurement uncertainties were statistically analyzed to determine the accuracy of the gain measurements. The results indicated that 79 percent of measurement uncertainties were written 1.5dB.
This data acquisition and pattern analysis system uses a standard set of Scientific Atlanta antenna-pattern-taking equipment as the basic operational gear. A Tektronix 4051 or 4052 Graphic System is used as a controller to operate the S/A gear and to obtain and store output data in digital format. The TEK 4051 does this by use of the IEEE General Purpose Interface Bus (GPIB), to which three interface boxes are connected. These three: • HP-59306A Relay Actuator • Model 4883 ICS Instrument Coupler • HP-3455A Digital Voltmeter message or digitize the S/A data and put it on the GPIB lines.
Antennas are an integral part of the communications, navigation, EMC systems installed on aircraft. Aircraft, such as the Douglas DC-9, C-9A, C-9B, DC-10, KC-10A, A-3 and A-4, use approximately 20 antennas. These antennas operate from VLF to approximately 20 GHz. The radiation patterns of these antennas are affected by aircraft structure such as wings, vertical stabilizer, engines, and landing gear. Douglas Aircraft Company measures the radiation patterns of these antennas using scale model aircraft (and/or aircraft sections) to predict the performance of the associated system. This paper describes some of the scale model measurement techniques used by Douglas Aircraft Company to obtain scale model radiation pattern data.
In the area of antenna measuring, there are many components offered that are useful in testing antennas. Placing the proper components together in a system that performs one’s desired results can be difficult. The results may leave no room for upgrading to a more sophisticated system. Scientific-Atlanta has introduced a new line of antenna measurement systems. This paper describes these manual amplitude and phase/amplitude systems and now they were put together to meet specific needs and allow for future expansion to semi-automatic systems. A survey of automatic systems is included.
The changeover of personnel in some laboratories has historically resulted in high costs for software maintenance. These high costs can be traced to poor documentation of the analysis and design process during the software development. This paper illustrates the structured analysis and design methodology used to analyze, design, and implement software to automatically test performance of an Air Force advanced development communications system. The requirements definition and preliminary design are accomplished using activity models to represent the functions performed during the test. The development of the activity models is the vehicle used to do a thorough requirements definition, while the resulting functional architecture represents an understandable preliminary design. The detailed design is formed using structure charts which better reveal system characteristics that illustrate design quality. The structure charts also facilitate the coding of the software to be implemented. The combination of activity models and structure charts provide the detailed documentation of the software analysis and design phases that are required to ensure ease of maintenance, broadening of understanding, and most importantly, a complete development package that can be passed on to a new user. These features ultimately result in a significant reduction in long term maintenance costs.
At a time of increased interest in computer-controlled antenna test ranges, it is worthwhile to consider the advantages, problems, and consequences inherent in the practical application of automated measurement techniques. This paper describes some of TRW’s experiences with the utilization of an automated far-field antenna test range to measure the characteristics of a Ku-band monopulse cassegrain spacecraft antenna. It also includes several conclusions drawn from those experiences.
Near-field antenna measurement techniques offer an alternative to conventional far-field antenna measurement techniques. Of the various coordinate systems used for near-field measurements, the spherical coordinate system provides the most natural extension from the conventional far-field characterization of an antenna to a more general characterization for arbitrary range lengths. This paper describes the Scientific-Atlanta Model 2022, a user-oriented implementation of a spherical near-field antenna measurement system. An example of typical system usage is provided. System capabilities and performance are described. Key concepts required to understand and use the spherical near-field method are discussed. The advantages and disadvantages of near-field antenna testing in relation to conventional far-field testing are considered. The particular merits of spherical near-field testing as compared to other forms of near-field testing are discussed. Antenna testing situations which provide the most likely candidates for the spherical near-field measurement technique are described.
The Naval Air Engineering Center has been assigned the task of developing Near Field Measurement Techniques and Equipment for testing Navy Aircraft-mounted antennas. These efforts will be applied to Nose-mounted and Wing-mounted antennas. The ultimate objective is the development of a portable near-field test system for the Navy’s ‘O’ level. The test system will produce far field pattern predictions of installed airborne antennas by measuring and processing near field data. NAEC would, also, like the test system to determine if an installed antenna is mission capable or degraded; and in the event of a failed antenna, the test system will isolate the fault of that antenna. This paper will describe NAEC’s progress in this task by descriptions of the following: I. Electrical Hardware i.e. transmitter, receiver, interfaces, controllers II. Mechanical Hardware i.e. translator, probe carriage III. Mathematical approaches Also, recent laboratory results will be described.
The paper describes a simple but unique antenna test facility suitable for aerospace antenna developments. The total idea can be easily adopted by organizations who wish to carry out antenna measurements with minimum required instrumentation. The facility majorly caters for omni and wide beam antenna measurements, has been set up at ISRO Satellite Centre, Bangalore, India. It has been extensively used for omnidirectional antenna developments in VHF, UHF, L, S, and X-bands for India’s various space programs. Radiation pattern, gain, polarization and impedance measurements can be carried out both in near free space conditions as well as the ground reflection modes. The main feature of the facility is the use of large fiber-glass mounting structures for avoiding reflections and perturbations in radiation patterns due to impressed surface currents, specially in VHF ranges. Field probing is done by the use of a fiber-glass X-Y probe positioner. The facility used Scientific Atlanta 1752 Receiver and 1540 Recorder. Suitable software has been added to the facility for contour plotting of radiation levels, calculation of efficiency isotropy, and polarization properties.
Currently the Fleet lacks a quantitative description of their electromagnetic (EM) capabilities and vulnerabilities. A ship’s mission can be seriously degraded because of unsatisfactory performance of EM systems due to various factors found in the operational environment. In addition, a ship can become vulnerable to a potential enemy by inadequate emission control (EMCON) of EM energy. The EMPASS measurement platform is capable of collecting and analyzing three-dimensional emission data in the operational environment. The word EMPASS is the acronym of Electromagnetic Performance of Aircraft and Ship Systems. EMPASS consists of a calibrated, special equipped, measurement platform situated on an EP-3A aircraft with complementary ground based data reduction and analysis facilities. The products of the EMPASS program are the effectiveness evaluation of operational EM systems, development of the criteria for the most effective tactical use of EM systems, and the providing of the capability to conduct RDT&E in the operational EM environment. This paper presents a description of the EMPASS capabilities and the results of measurements of some representative EM systems used in the fleet.