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To perform antenna measurements, it is necessary that the entire antenna structure is illuminated by a uniform plane wave. Since almost all sources radiate spherical waves, plane wave field configurations can be achieved locally only at very large distances from the source. The proliferation of compact range designs have reduced the distance required to achieve nearly plane wave field configurations to distances which can be satisfied by indoor facilities. While most compact ranges have been designed to create a nearly plane wave field configuration, at Arizona State University an operational compact range exists which creates a nearly cylindrical wave field structure. The pattern measured under cylindrical wave illumination is transformed, using analytical and numerical methods, to obtain the plane wave response of the antenna system. Measurements have been performed, using the cylindrical wave compact range, of a 15 GHz axial waveguide antenna on a 1/10 scale Advanced Attack Helicopter model. The measurements were then transformed and compared with those made of the same antenna system in a plane wave compact range facility.
Implementing an antenna test range has traditionally been viewed as a major and costly undertaking, requiring significant long term facility planning, computer hardware interfacing, and software development. This paper describes a complete low cost, yet high accuracy portable near-field measurement system that was privately built for less than $2,000 and interfaced to a PC compatible computer. The design and operation of this system, including the scanner, microwave hardware, and computer system will be described. This system has since been extended into a commercial product capable of providing rapid and accurate measurements of small to medium size feeds and antennas within a small office or lab space at significantly lower cost than standard antenna test techniques. The system has demonstrated an equivalent sidelobe noise level of less than -50 dB, includes a probe corrected far-field transform and holographic back projections, and can output pattern cuts, contour plots, 3D plots, and grey scale images of antenna performance.
In the past, models suspended indoors for radar cross-section measurements have weighed up to several hundred pounds, suspended on the order of 20' or less from the ground, and measured statically or rotated for great circle cuts. Under these circumstances it has been acceptable to choose the best string configuration from a signature point of view and simply wait for the model to reach a visually calm state before testing. However, indoor ranges are now requiring suspension of models weighing several thousand pounds 40' or more above the floor. In addition, the demand for imaging data during model conics requires both precise dynamic control and model stability. This work discusses techniques developed at Boeing's 9-77 Range in Seattle, to achieve model stability during suspension and manipulation. In addition, techniques to determine spring and damping constants of suspension systems for individual models are addressed.
Dielectric straps can support very heavy targets and have a low radar cross section (RCS), especially at low frequencies (below 8 GHz). In this paper, the scattered fields of dielectric straps surrounding a perfectly conducting structure are presented, and the computed results are compared with experimental data. Empirical formulas for the strap scattered fields are also given. These formulas are good for general convex structures and are expected to give a reasonable estimate of the true RCS of the dielectric straps when used as a target support structure.
A stressed-skin inflatable target support provides an improvement over a foam column for radar cross section (RCS) measurements in an anechoic chamber. Theoretical analysis indicates that backscatter from the support is minimized because its mass is reduced below that of a foam column and is distributed to favor incoherent scattering. Compared with a foam column, a pressurized thin shell has superior mechanical stability under both axial and transverse loads. Experimental observations using Mylar -- a low dielectric constant, high tensile strength film -- confirm these results. Spurious reflections from rotational machinery located below an inflatable column are reduced by a layer of absorber within the base of the inflatable support.
Many targets today exhibit radar cross sections sensitive to the angular orientation of the target. While some of these targets have prominent scattering centers which can be exploited to obtain a relative positional reference, many targets unfortunately do not. In addition, many complex targets have a highly directional scattering behavior requiring careful alignment to the incident planar field. This need for accurate positioning has prompted the development of laser alignment techniques for the compact range. One such system has been under development at the ElectroScience Laboratory, and the designs and results of the first prototype are presented here. Performance goals and design criteria are discussed, and future improvements are considered. In addition, similar systems for feed and pedestal location reference systems are presented.
The National Institute of Standards and Technology (NIST) has developed a multi-axis controller and software data acquisition system that has improved probe position accuracies in near-field scanning. This extends the usefulness of the NIST planar near-field scanner to higher frequencies. This system integrates programmable power supplies into an existing planar measurement system with new software that controls the power supplies and the data acquisition. It provides the higher positioning accuracy required for millimeter wave measurements at a reasonable cost. This system uses the NIST planar near-field scanner's existing DC motors, computer and laser. The programmable power supplies are connected to the motors, with a separate power supply for each motor'a armature and a common power supply for each of the motor's field windings. This allows for concurrent movement in each axis and eliminates delays in switching between axes. Directional control, motor protection, and special software features are implemented by logic control.
The Georgia Tech Research Institute has designed and installed a large outdoor compact range for the U.S. Army Electronic Proving Ground at Ft. Huachuca, Arizona. This range will primarily be used to obtain performance data for antennas installed on full-size tanks, aircraft and other vehicles to characterize antenna/vehicle interactions. This paper describes the vehicle positioner that is being used with the compact range. Design considerations have resulted in a challenging positioner design. Some of the features of the positioner include: * positioning of large vehicles weighing up to 70 tons approximately 42.5 feet above ground * using a hydraulic servo system to drive the positioner * minimizing RF reflections by using ogive shaped shells on the positioner legs and tilting the legs forward
The third generation of Power Processing Units (P.P.U.), for antenna positioners, incorporates the following state-of-the-art technology: - Power MOSFET with high voltage handling capability including fast integral diode; - Isolated current sensors with wide bandwidth and dc level measuring capability; - Complex integrated circuits such as PAL and PMW controllers. This paper introduces one member of the third generation P.P.U.'s family - a six-axes P.P.U. (switching one at a time), capable of driving dc motor up to 1 1/3 h.p. This P.P.U. has the following exclusive characteristics: a. Three electrically separated grounds for: Speed control output circuitry P.W.M. control Output power stage b. Dynamic brakes c. Current + Voltage Control d. A switching frequency of 20 KHz e. A high filtered output - pure dc. The two main aspects of this paper are: design considerations and method of utilizing the new component to achieve an efficient and reliable power unit.
There is often a need for a laboratory to make quick, inexpensive, and accurate measurements on individual absorber samples. Different types and sizes of absorber need to be quickly analyzed at several frequencies to determine which type best maintains or improves the facility's RF characteristics. The National Institute of Standards and Technology has devised an improved version of the Doppler shift method to measure the scattering levels of different sizes and types of microwave absorber. This technique is useful as an inexpensive and simple method for measuring individual absorber pieces with good accuracy and sensitivity. The system does not require a large anechoic facility nor a sophisticated measurement system for gating out background scattering. Reflectivity levels on the order of -80 dB can be measured and relative changes of 1 dB can be detected. Sample results for absorber with and without fire retardant salts and different sizes are presented.
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
This paper describes techniques for coherently suppressing multipath and other error sources in planar near-field measurements. Of special interest is a simple, yet effective technique of suppressing axial multipath and mutual coupling between the nearfield probe and an antenna. This is of particular value in the testing of low sidelobe antennas. Traditionally, self comparison tests with different separations between the probe and the antenna under test are used to identify the magnitude of multipath errors. What is not generally realized is that these tests can be used to produce a coherent estimate of the induced error, which can often be suppressed. A series of tests was performed with a small X-band phased array antenna, resulting in a reduction of the sidelobe noise background from a 25 dB level to better than 50 dB.
Obtaining the tangential electric fields on a planar near field surface has many important applications. The need for this information may not, however, justify the expense of a planar near field test range. A technique has been developed to obtain planar near field data from spherical near field measurements. Spherical near field measurements can be performed on practically any range that employs a roll over azimuth positioner and a phase/amplitude receiver.
Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.
Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period off time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.
Due to the limited size of a compact range, an antenna with low sidelobes, broad bandwidth, broad beam, small physical signature, low scattering level and reasonably high power handling are required. Historically, slot line antennas are circuit board type antennas noted for their thin cross-section, low cost of fabrication, scalability and high package density in array applications. A broadband version, fed by a microstrip line (and therefore easily connected to microstrip transceiver circuits etched on the same circuit board) is described in this paper. Test models with different shapes and using different dielectric materials were built and tested. The measured VSWR, radiation and scattering patterns of the various antenna designs are presented.
An argon laser source in conjunction with an interferometric fringe generation technique allowed projection of high contrast fringes on to the surface of an antenna over a height of 20 to 60 feet. The projected beam, located at the base of the antenna, made an angle of ~ 15 degrees with the surface. The viewer was placed near the central axis of the antenna ~ 80 feet away where the illuminating antenna surface was imaged on a Ronchi grating. A low light level video camera viewed the moire contours through the Ronchi grating. The spacing between two contours represented a surface height variation of ~ .050". Panel edge misalignments of .005" were readily discernible. Applications of this technique are illustrated with photographs.
The effects of metallic tapes which are used to cover gaps in a compact range reflector are studied in this paper. To study these effects, the normalized tape scattered fields are computed in the target zone. A method of moments technique is used to compute the tape scattered fields. It is shown that the tape scattered fields are directly proportional to the thickness and width of the tape and are inversely proportional to the square root of the distance from the tape. Using the computed results, an empirical formula for the tape scattered field is developed. One can use the empirical formula to compute the highest frequency for which a given size tape can be used.
After having delivered a model 1630 and a model 1640 compact range plus a number of smaller 1606 and 1603 ranges, Harris has improved their product to meet the demanding needs for operating frequencies of 35 GHz and higher. In characterizing the two large ranges, it was discovered that the surface accuracy as originally optimized would not support the highest operating frequency. Achieving the required surface accuracy required additional surface measurement data in combination with RF contour plots and was very time consuming. From those lessons learned, several features have been incorporated into the next generation of compact ranges that make more accurate reflector surfaces easily achievable. The features include optimally located adjustment mechanisms, additional targets on each panel, software for best fitting the panel surface to minimize steps, techniques for eliminating panel steps in place, and gravity bias setting of panels.