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AMTA Paper Archive

Fast electrical functional testing of the ERS-1 synthetic aperture radar antenna
R. Halm (ESTEC),A. Lagerstedt (Ericsson Radio Systems), K.V. Klooster (ESTEC), R. Peterson (Ericsson Radio Systems), November 1989

The Synthetic Aperture Radar Antenna for the European Remote Sensing Satellite ERS-1 is a 10 by 1 metre deployable slotted waveguide array operating at 5.3 GHz. Electrical performance of the complete antenna is derived at the end of the environmental test programme from near field measurements on a planar NF scanner. In order to obtain very early information on electrical integrity of the flight model antenna, suitable for pre- and post-environmental comparison, a fast electrical functional test was implemented in the total test sequence. It basically consists of a 2D slot probing of a well distributed number of slots in combination with complex input impedance measurements. The paper describes the method and presents results of different test steps. The data of pre-/post-environmental measurements are compared.

A Quasi-far-field measurement systems: hardware, software and experimental results
R.E. Shields (CSIRO Division of Radiophysics, Australia),G.M. Simms (CSIRO Division of Radiophysics, Australia), November 1989

The hardware and software developments undertaken to upgrade two far-field measurement facilities - a 12-m anechoic chamber and a 35-m outside range - are described. A method (termed quasi-far-field, QFF) for deriving antenna far-field patterns from a single plane scan at a distance less than the traditional distance of 2D2/? is described. The QFF technique involves pattern sample and subsequent pattern transform and reconstruction, from the easement distance to the far-field distance. A discussion of the limitations inherent in the QFF transform, including range length, is given. Experimental results for measurements made on circular-aperture antennas with both symmetric and asymmetric illumination, and on antennas with elliptical apertures, are described.

Methods of transforming antenna Fresnel region fields to far region fields
K. Wu (Electrospace Systems, Inc.),S. Parekh (Electrospace Systems, Inc.), November 1989

For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.

Application of nonuniform sampling techniques for antenna pattern measurements
Y. Rahmat-Samii (University of California Los Angeles), November 1989

The nonuniform sampling technique utilizes measured (or simulated) amplitude and phase far-field data at nonuniformly sampled data points and constructs the pattern from these limited number of measured data. The technique relies on the fact that the antenna far-field pattern is proportional to the Fourier transform of a function which is related to the induced current on the antenna. The application of nonuniform sampling technique becomes important in the situation for which it will be difficult (or impossible) to measure the far field at regular intervals. In this paper, the application of the nonuniform sampling technique is demonstrated for antenna pattern measurements. The foundation of the technique is first reviewed and the required mathematical steps for the implementation of the technique is summarized. Both one dimensional and two dimensional cases are reviewed with attention given to the applicability of closed form expressions for the determination of the sampling coefficients. Numerical results are presented and comparison to measurements are shown. In particular, the application of this technique to a recently proposed space-station based antenna experiment is presented.

Antenna far-field pattern accuracies at millimeter wave frequencies using the planar near-field technique
M.H. Francis (National Institute of Standards and Technology), November 1989

In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.

Aramis - a flexible near-field antenna test facility
O. Silvy (Electronique Serge Dassault), November 1989

A flexible near-field antenna test-facility is presented. This system gathers all that is necessary to design, to debug and to validate the high performance antennas which are made by ESD. ARAMIS has been operational since January 1988. Its applications are: - Near-field measurements (for diagrams): * planar, * cylindrical. - High speed field mapping (for default analysis): * planar radiating surface, * cylindrical radiating surface. - Generation of element excitation (active phased array testing): * planar antennas, * cylindrical antennas. - Direct far-field measurements (probes, small antennas), - Circuit measurement (S parameter). The facility features a specially designed scanner. Thanks to its six degrees of freedom, this positionner allows the differents types of measurements to be made. The instrumentation is based upon the HP 8510 B network analyzer. A single computer performs the measurements, transforms the data and presents the graphics (linear diagrams, color maps, three-dimensional colored projections). In order to grant a high scan speed, the system uses the FAST CW mode of the HP 8510 B. An external trigger is provided during the motion process of the probe. A rate of 500 measurements/sec. has been proved. This on-the-fly process is clearly depicted. Experimental results are presented which include: - Low sidelobe (-38 dB) antenna diagrams. - Default analysis through: * Amplitude mapping (leakage short-circuit in a microstrip antenna). * Phase mapping (out-of band comparison between two radiating element technologies). * Measurement of excitation laws. * 3-D transformation. - Simultaneous on-the-fly acquisition of up to three antenna outputs.

Cylindrical wave helicopter antenna pattern measurements, corrections, and comparisons
C.A. Balanis (Arizona State University),C.R. Birtcher (Arizona State University), D.G. Shively (NASA ), G.C. Barber (NASA ), M. Gilreath (NASA Langley Research Center), V.J. Vokurka (Eindhoven University), November 1989

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.

A Low cost portable near-field antenna measurement system
D. Slater (Nearfield Systems Incorporated),G. Hindman (Nearfield Systems Incorporated), November 1989

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.

String support in compact ranges
J.B. Elbert (Boeing Advanced Systems), November 1989

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.

Scattering from thin dielectric straps surrounding a perfectly conducting structure
Z. Al-Hekail (The Ohio State University ElectroScience Laboratory),I.J. Gupta (The Ohio State University ElectroScience Laboratory), W.D. Burnside (The Ohio State University ElectroScience Laboratory), November 1989

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.

Inflatable target support for RCS measurements
D.G. Watters (SRI International),R.J. Vidmar (SRI International), November 1989

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.

Target alignment techniques for the compact range
H. Shamansky (The Ohio State University),A. Dominek (The Ohio State University), M. Poirier (The Ohio State University), November 1989

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.

Automated multi-axis motor controller and data acquisition system for near-field scanners
J. Guerrieri (National Institute of Standards and Technology),D. Kremer (National Institute of Standards and Technology), November 1989

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 Positioner for the U.S. Army EPG compact range
T.L. Wilkey (Georgia Institute of Technology),E.H. Atkinson (Georgia Institute of Technology), H.P. Cotten (Georgia Institute of Technology), J.F. Kirksey (Georgia Institute of Technology), J.M. Hudgens (Georgia Institute of Technology), O.D. Asbell (Georgia Institute of Technology), November 1989

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

Third generation of power processing units
B.J. Friman (Orbit Advanced Technologies), November 1989

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.

Advanced RF chamber absorber using resistive gradient techniques
R.N. Hubbard (Brunswick Corporation, Defense Division), November 1989

A new method for producing chamber absorber (patent applied for) is described. The absorber makes use of a resistive gradient through the bulk foam in contrast to standard pyramidal and wedge absorbers that use a geometric shaping of bulk uniform resistive material such as foam loaded with carbon. The absorber is a lightweight open cell reticulated foam, with a metallic material coated onto the fibers of the foam, with varying thickness and varying material properties used to produce the electrical gradient.

Absorber characterization
D. Kremer (National Institute of Standards and Technology),A. Newell (National Institute of Standards and Technology), D.A. Agee (National Institute of Standards and Technology), November 1989

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.

Characterizing the bistatic performance of anechoic absorbers
S. Brumley (Denmar, Inc.),R.G. Immell (Motorola Govt. Elect. Group), November 1989

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.

A Synthetic aperture imaging method for evaluating anechoic chamber performance
R.G. Immell (Motorola Govt. Elect. Group),S. Brumley (Denmar, Inc.), November 1989

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.

Error suppression techniques for near-field antenna measurements
G. Hindman (Nearfield Systems Incorporated),D. Slater (Nearfield Systems Incorporated), November 1989

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.







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