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Simulation of the antenna measurement errors caused by scattering of range clutters is presented in this paper. The Uniform Geometrical Theory of Diffraction (UTD) based NEC-Basic Scattering Code is used to simulate the measurement of antenna in a far-field range where structure scatterers present. It is known that these errors which come from various directions will impact the antenna under test differently dependent on the characteristics of the antenna under test. With the available computer codes, one can simulate and study various ranges in order to better understand the characteristics of the ranges and properly adjust, modify, and improve the facility such that better measurement results can be obtained.
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges and anechoic chambers contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The fabrication of a feasibility demonstration model Antimultipath System (AMPS) is complete. This AMPS uses a 10 MHz wide phase-shift-keyed spread spectrum modulated signal to illuminate the rotating AUT to tag each multipath by its delay. The novel receive section of the AMPS sorts out each multipath component by its delay and adaptively synthesizes a composite cancellation waveform (using delay, amplitude, and phase estimates of the scattered components) which is subtracted from the total signal received by the AUT. After subtraction the resultant is the desired direct path signal which produced the free space pattern of the AUT. Laboratory and antenna range test results are presented and show the promise of measuring sidelobe levels 60 dB below the main beam.
Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.
Residual reflection characteristics should be evaluated for antenna radiation pattern measurements. Authors propose a method for detecting positions of reflection sources by applying the modified far-field antenna radiation pattern measurement scheme described in [1]. In this method, an “accurate” radiation pattern of antenna under test (AUT) and measurement error patterns due to residual reflected waves are separated by changing a range distance and processing Fourier transformation. Also, the positions of reflected sources can be detected from beam directions of patterns due to reflections at each distance. Experiment results confirm that this method is effective for detecting the positions of reflection sources.
The antenna repair shop of McClellan Air Force Base near Sacramento, California has been involved in the repair of military high frequency antennas for many years. With the acquisition of precision microwave measurement equipment, in the last five years, the antenna shop has developed innovative methods of gauging its antenna material repair processes. This paper will focus on the work done on a LS-band phased-array, satellite ground station antenna. Specific processed examined will be radome-point selection, phased-array receive element cleaning, and radome bonding. The history of the problems that required the repairs will be discussed. Several antenna/radome repair processes and RF test methods will be described. Manufacturer specifications will be examined where available. Included in this paper will be RF test data and data analysis.
Planar near-field antenna measurements have developed into a mature science. Nonetheless, unique difficulties arise when measuring some modern antennas, such as high gain satellite antenna systems. In a typical planar near-field measurement, the antenna under test (AUT) has a collimated beam in the near-field which is perpendicular to the scan plane (i.e. the AUT boresight is parallel to the normal of the scan plane). On the other hand, the scan plane is positioned close to the AUT to maximize the valid angular range in the far-zone patterns. Unfortunately, it is not always possible to place the AUT very close to the scan plane and keep the near-field beam perpendicular to the scan plane. An investigation of the benefits and pitfalls of a planar near-field measurement where the AUT beam is not perpendicular to the scan plane is presented. The measurements of antennas tilted 45 degrees with respect to the scan plane normal are used as examples. With this atypical arrangement, some of the usual errors in a near-field measurement are emphasized. Procedures to identify and reduce these errors will be presented.
The standard planar near-field to far-field transformation method requires data points on a plane-rectangular lattice. In this paper we introduce a transformation algorithm in which measurements are neither required to lie on a regular grid nor are strictly confined to a plane. Computational complexivity is O (N log N), where N is the number of data points. (Actual calculation times depend on the numerical precision specified and on the condition number of the problem.) This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable for other measurement approaches, such as plane-polar scanning, where data are collected on a non-rectangular grid.
This paper reports on the results of computer simulations of planar near-field test-zone-fields. Techniques for the improvement of the quality of the test fields are presented and demonstrated. These techniques include the use of larger scan areas and the use of window functions applied to the measured near-field data. Test-zone-field quality is measured by the angular spectrum of the error of the test-zone-field as compared to an ideal plane wave test-zone-field. This investigation sought the minimum scan length, L, for a given critical angle, ?c and separation, S. It is shown that significant improvements in test-zone-field quality can be realized if the test zone is extended from the standard length, Ls=D+2S(tan(?c)) by an amount 20?/cos(?c). This scan length is approximately 30? larger, for a critical angle of 50 degree and 60? larger, for a critical angle of 70 degrees, than the standard length. A raised cosine amplitude/quadratic phase window applied to the measured near-field data can significantly reduce scan length requirement while maintaining the increased accuracy of the extended scan length. The recommended scan length with window is given by Lw=D+2S(tan(?c))+2W, where W is the length of the window applied to each end of the scan measurements. The window description and required length are presented.
This paper summarizes the state of the art for using one-dimensional and two-dimensional arrays of modulated scattering elements to rapidly measure the near-field electromagnetic fields 1) radiated by antennas with or without radomes and 2) scattered by targets located in free-space or buried in lossy dielectric media. The application of rapid near-field scanning via measurement arrays based on the Modulated Scattering Technique (MST) in both France and the U.S. is discussed in this paper.
In this paper, a near-field time domain radiation measurement is described, similar to the traditional frequency domain near-field radiation measurement. This time domain measurement approach borrows many of the principles developed in the frequency domain and is ideally suited for the measurement of broadband devices. The goal of determining the radiated far-fields of an antenna is accomplished by the transformation of near-field data collected over a planar sampling surface. The near-fields are generated with an antenna excited by a short duration transient pulse. In particular, the near-fields of an aperture antenna are collected using a digital sampling oscilloscope. The bandwidth of the excitation pulse is approximately 10 GHz.
The tapered chamber was originally developed about 30 years ago to provide better quiet zone fields by eliminating the reflected fields from the side walls. This concept works well if the feed antenna is mounted at or near the vertex of the tapered section. Unfortunately, there has not been a feed specifically developed for this application; as a result, range operators have been forced to use sub-optimal feed antennas. This paper describes a new tapered chamber feed that is specifically designed to optimize the total system so that the originally intended performance can be achieved. This feed has been designed, built and tested. It covers the frequency band from 100 MHz to 2 GHz and has been optimized to provide the largest quiet zone possible. The description and capability of this new feed is presented in this paper.
State-of-the-art range design requires that the feed antenna possesses key features including ultra wide band operation, stable beamwidth, stable phase center and versatile polarization capability. Traditional ultra wide band antennas such as the dual-polarized quadridged horn has versatiles polarization ability; however, the radiation beamwidth, which is dictated by the ridge structure, is not constant. Current development of the R-card version of the Slotline Bowtie Hybrid (Rcard-SBH) antenna possesses all the required features except that it is limited to linear polarization. A novel dual-polarized antenna which can meet all these requirements is presented. The feeding structure is constructed using two pairs of coaxial lines with their outer conductors commonly grounded. Each pair is connected to an ultra wide band hybrid circuit (1-18 GHz) and forms a balun structure. The guiding structure is made of numerous radial wires that form an orthogonal bow-tie geometry. Note that with these wire structures, for each polarization, only two guiding plates are visible; while, the other side plates having wires orthogonal to the E-polarization are nearly invisible. By integrating the rolled edge concept into the guiding structure, for each polarization, this new dual-polarized antenna has similar performance as the conducting rolled edge SBH antenna developed earlier.
This paper reports on a ground penetrating radar (GPR) antenna with an electronically steered beam, currently being developed at South Dakota Tech. The increased power and directivity that result from beam-steered operation have potential utility in deep/lossy GPR environments. The antenna is a transmitting array of up to eight bow-tie dipoles, each driven by a narrow pulse generator connected directly to the dipole. The beam is steered in real time by controlling the timing of the individual element transmitters using digitally-programmed pulse delay units. Reception is through a conventional GPR receiver using a single bow-tie antenna. Modeling the air-ground interface as a lossy half-space, numerical results indicate that, under certain conditions, time-domain beam-forming is possible in such an environment. Antenna patterns and standard antenna measurement parameters, such as beamwidth and directivity, are presented in support of this finding.
A slot spiral antenna and it associated feed are presented for conformal mounting on a variety of land, air, and sea vehicle. The inherent broadband behavior and good pattern coverage of the spiral antenna is exploited for the integration of multiple frequencies, and thus multiple transmitting and receiving apertures, into one compact, planar antenna. The feasibility of the broadband slot spiral antenna relies on the use of an equally broadband, balances, planar, and non-intrusive feed structure. The design of the slot spiral, its feeding structure, and the reflecting cavity are discussed with emphasis on the experimental validation and construction of the antenna.
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.
New results on the complete scattering matrix measurements during the interference between a sphere and a second object are presented. The objects involved are strings of two sized, a rod, and a dihedral. In cases for the strings or the rod, in-phase oscillations in HH and VV are observed. For the dihedral, the HH and VV responses are exactly out-of-phase. We find that the results are in excellent agreement with the characteristics of the scatterer types. Use of targets other than the sphere for cross-polarized calibrations is discussed.
The designer of a linear-FM homodyne RCS measurement system must consider the nonlinearity present in the chirp waveform. Two basic methods employed in obtaining the chirp waveform are to apply either a digital ramp or an analog ramp to a YIG oscillator source. Nonlinearity can occur as the result of the characteristics of the YIG oscillator and the applied ramp waveform. The point spread functions useful in characterizing the performance of both the digital and analog ramp excited YIG oscillator systems are given. Both range resolution and dynamic range of the measurement system are dependent on the target range and can be adversely effected by the nonlinearity. Theory shows that the point spread function of a digital ramp is suitable for short range RCS measurements. However the analog ramp system has improved performance at extended range. By using the analog ramp, we have been able to improve performance of RCS measurements over the digital ramp. Experimental data from both the digital and analog ramp systems are provided.
Absorbing material can suppress unwanted reflections in a compact range chamber to some extent. The simultaneous use of time-domain filtering (gating) to extract only the desired signal from measurements, serves to improve measurement results. There are two types of time-domain gating, namely software and hardware gating. This paper discusses time-domain filtering performed using hardware gating. The concept of a test zone, as created by the hardware gates in a down-range sense, is introduced. This test zone is explored through measurement as well as through computer simulation of the hardaware gating process.
So far, Azimuth-over-Elevation rotators on RCS pylon tips were of large size (typically 10” for 500 lb. load, over 2’ for a 6000 lb. load). Therefore, RCS measurements of small but heavy targets were very difficult if not impossible to perform. The new design supports loads of 5,000 lb. with an Azimuth turntable diameter of only 136 mm, close to the pylon’s maximum width. The Azimuth and Elevation axes mechanisms are hidden inside the pylon body. The Azimmuth rotator is mounted on the top surface of the elevation main plate. The Elevation plate is attached to the pylon tip on one side and on the other side to the actuator, which is attached to the base of the tip. The actuator drives the Elevation plate to the required rotation angle. Even with its small size, the new design does not compromise on performance. The Azimuth axis moves a full 360° continuous motion at 22 deg/min with 0.03° accuracy, 0.03° backlash and 0.01° repeatability. The Elevation axis moves in a 0°-40° sector at 1.5 deg/min with 0.05° accuracy, 0.05° backlash and 0.01° repeatability.
We use a genetic algorithm to optimize a linear array of aluminum cubes in order to reduce the relative Radar Cross Section at a specific angle. A genetic algorithm models biological reproduction and natural selection on a computer in order to optimize the output of some function or experiment. We also examine the initial parameter tradeoffs required to achieve an optimal design with a minimal number of unique measurements. We actually discovered two very different designs, each providing more than 44 dB reduction at five degrees off broadside.