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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.
A monostatic C.W. radar cross-section facility in the X-band at the Radar and Communication Centre, Indian Institute of Technology, Kharagpur, India is described. This set up is capable of automatically measuring the c.w. monostatic radar cross-section over the range of aspect angle 0 to ±180o for both TE and TM polarizations. The transmitting/receiving antenna and the rotating target is housed on the roof-top of the building and the microwave circuit with recording arrangement is in the air-conditioned laboratory. It is capable of handling a target of arbitrary shape of maximum size equal to 70 cm and uses a two-stage background (without target) cancellation technique employing Magic-T. A typical value of effective isolation between the transmitted and received signals is of the order of 70 dB and a dynamic range of 35 dB. Measurements made in this set up with different types of targets show a fair agreement with the results obtained by analytical investigations. The same set-up with necessary modifications for measuring the phase of the scattered field along with the amplitude data is expected to provide the amplitude and phase information for target identification and classification problems.
The loop cell is fabricated using two intersecting metal sheets joined at the intersection and forming a 36 deg angle. A section of a loop is mounted between two coaxial panel jacks, one on each sheet at a distance equal to the loop radius from the intersection. A known current through this section of electrically small loop produced calculable E and H fields between the sheets in the plane of the loop. These known fields may be used to determine the antenna factor of small E and H antennas placed in the field if the mutual impedance due to the antenna images in the sheets is negligible and the antenna is not close to the open edges of the cell. Measured and calculated antenna factors agree within ±2 dB between 0.25 MHz and 1000 MHz.
Far field antenna radiation patterns of vehicle mounted antennas are often recorded on the antenna range by rotating the entire vehicle/antenna system with a multiple axis vehicle positioner. Antenna patterns, obtained in this manner, consider the antenna and vehicle as a system and include the effects of the vehicle structure. These patterns are more representative of the operational antenna patterns than the “free space” patterns of the antenna itself. When the antenna is arbitrarily directed on the vehicle, standard antenna pattern cut trajectories, recorded in the coordinate system of the vehicle, become skewed when referenced to the coordinate system of the antenna. With proper adjustment of the fixed angles of the vehicle positioner however, selected standard antenna pattern cut trajectories, referenced to the antenna, may be obtained. The required fixed vehicle positioner angles are obtained from solutions to systems of equations representing the coordinate transformations for the positioner/vehicle/antenna system. In this paper, two general methods of obtaining the coordinate transformation equations are reviewed. These equations are then solved to obtain expressions for the positioner angles necessary for specific cut trajectories. A practical example of a six axis transformation associated with measurements of a three axis gimballed aircraft mounted radar antenna and a three axis vehicle positioner is used to illustrate the techniques (This example was taken from a recent RADC/Newport measurement program.
A strong emphasis is now being placed on techniques for reduction of radar cross-section. A missile or aircraft which is invisible to radar has an important strategic advantage. With this fact in mind, the user of a weapons system may place an upper limit on the radar cross-section that he will permit his missile or aircraft to have. The designer must then make use of “stealth technology” to reduce the cross-section to an acceptable level. In order to verify the design, radar cross-section measurements must be made. Thus the current emphasis on cross-section reduction leads to an important need for accurate and reliable methods of measuring radar cross-section.
Scientific-Atlanta, Inc. was selected by the United States Air Force to design and install a complete turn-key test facility for depot maintenance support of the F-16 fighter aircraft. These facilities have been installed at Hill Air Force Base, Utah. Four complete facilities have been supplied, each consisting of a Series 2020 Antenna Analyzer and a Series 5750 Compact Antenna Range. Two facilities are configured for antenna testing and two for radome testing. This paper describes the equipment furnished for this program. The hardware is discussed as well as the special software designed to perform specific radome and antenna tests.
An instrument has been built which allows the electromagnetic measurement of the surface accuracy of a large millimeter-wavelength antenna. The University of Texas 4.9 m radio telescope has been measured with this technique at 86.1 GHz to an accuracy of 4 µm at the surface. Our technique is an interferometric one which is fast, accurate, and able to measure the whole antenna surface at once. While the technique is illustrated by its use on a large antenna, it could be used in a near field measurement of a smaller antenna. Several antenna surface maps are presented. A comparison of run-to-run repeatability was made. The technique itself was tested by deforming the antenna surface in a known way and subsequently detecting the deformation. In addition, important factors which influence the overall error budget have been identified. These include errors in setting the antenna angular position and fluctuation noise in the atmosphere and electronics. An instrument has been built which allows the electromagnetic measurement of the surface accuracy of a large millimeter-wavelength antenna. The University of Texas 4.9 m radio telescope has been measured with this technique at 86.1 GHz to an accuracy of 4 µm at the surface. Our technique is an interferometric one which is fast, accurate, and able to measure the whole antenna surface at once. While the technique is illustrated by its use on a large antenna, it could be used in a near field measurement of a smaller antenna. Several antenna surface maps are presented. A comparison of run-to-run repeatability was made. The technique itself was tested by deforming the antenna surface in a known way and subsequently detecting the deformation. In addition, important factors which influence the overall error budget have been identified. These include errors in setting the antenna angular position and fluctuation noise in the atmosphere and electronics.
This paper explored the details of the mechanical and electrical design of a multipurpose scanner. Planar, cylindrical and spherical scans as well as separation scanning (for extrapolation gain method) are accomplished by allowing any two of the five axes to be selected for program control. Special laser interferometers are available for the X-Y planar scanning. However, all axes are fitted with two-speed synchros. The method of driving and counter-weighing the X-Y probe carriage reduced the moving mass significantly which helps in the areas of start-stop agility, resonances, bearing wear and structual bending.
The Naval Underwater Systems Center has under construction an antenna pattern arch for measuring the radiation pattern of submarine antennas protruding above the sea water surface. The 70-foot radius tripodal arch is constructed of laminated wood members located over a 66-foot by 93-foot concrete pool which will contain a six inch depth of sea water. A well is located off-center in the pool for mounting the antenna under test. Pattern measurements will be made from 20 MHz to 2 GHz and at antenna heights of up to 15-feet above the sea water. Heretofore this over-sea water pattern information has been unobtainable. The important criteria for far-field antenna measurements are mentioned. The Numerical Electromagnetic Code (NEC) was used to model typical submarine antennas at various frequencies in order to predict the accuracy of the arch range. NEC uses moment methods to determine the arch patterns and the far-field patterns.
This paper reviews established design principles and techniques for control of electromagnetic characteristics of an antenna test environment. In particular, the discussions here relate to test facilities which are intended for use in direct measurement of the free-space far-zone performance of antennas. The bulk of the material presented here was excerpted by permission from reference 1.