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Generally, the radiating properties of passive antennas can be measured with CW test signals in either transmit or receive mode with identical results. For a variety of practical reasons, outdoor antenna ranges have traditionally been configured to receive on the antenna under test. A growing class of active antennas, however, are non-reciprocal as systems and must be tested independently in both transmit mode and in receive mode. Often, broadband (non-CW) test signals must be utilized in the testing of these systems. In this paper, antenna range configurations are compared and practical instrumentation techniques for measurement of broadband signals on the antenna range are discussed. A Rome Laboratory pulse antenna measurement receiver, designed to obtain complex time domain profiles of transmitted waveforms as a function of angle, will also be described.
In an airborne radar, the aircraft motion causes the returns from stationary ground clutter to be spread over a significant part of the Doppler space. Without flight testing, it is difficult to develop or verify the clutter suppression techniques required by future airborne radars. In this paper a technique is presented for emulating the angle and Doppler characteristics of airborne radar clutter from a fixed site, for the purpose of ground-based testing. An inverse displaced phase center antenna (IDPCA) is used to simulate the aircraft motion. The inverse displaced phase center antenna described is an 18-element linear UHF array whose phase center can be electronically shifted by means of a switching matrix. The motion emulation capability is demonstrated through the use of this antenna as an auxiliary array in conjunction with a stationary UHF surveillance radar. Examples of the clutter returns received by this system are given.
One antenna characteristic that is difficult to predict accurately is the antenna temperature. There are two basic reasons this is true. First, the effect of the full volumetric radiation pattern of the antenna must be taken into account. Secondly, the antenna temperature calculation requires knowledge of the noise power incident on the antenna, from the environment in which it is operating. This paper describes a measurement program which was undertaken to establish the accuracy of a model which is being used to predict antenna temperature for earth based reflector antennas. The measurements were conducted at 11 GHz, using an 8-foot diameter Cassegrain reflector antenna in an outdoor environment. The measurements are compared to predictions generated by The Ohio State University Reflector Antenna Code. Use of the reflector code allows the full volumetric pattern of the antenna, including all sidelobes, backlobes and cross-polarized response, to be included in the calculation. Additionally, the contribution to the antenna temperature from the various regions of the pattern can be calculated separately and analyzed.
This paper describes a calibration technique for reducing the errors due to mismatch between the measurement receiver and the antenna in microwave antenna relative gain measurements. In addition, this technique provides an accurate method for measuring the input return loss of the antenna under test. In this technique, a microwave reflectometer is mounted between the measurement receiver and the antenna test port. The reflectometer is calibrated and used to measure the return loss of both the test and calibration antennas. Using this information in conjunction with the HP 8530A antenna gain calibration, the corrected gain of the antenna under test is computed. Compact range antenna measurements verifying the calibration model and error analysis are presented. Practical implementation considerations are discussed.
Antenna testing is generally predicated upon using a Standard Gain Antenna co-located with the Antenna Under Test (AUT). At HF/VHF/UHF frequencies Standard Gain Antennas (Horns) are too large for co-location on the Antenna Under Test's (AUT's) rotating platform. Co-location is desirable for maintaining equal range lengths and equality in the environment; a prime source of multipath effects. In the HF/VHF/UHF frequencies bands the Log-Periodic is quite often employed as the "Source Antenna" but not necessarily the "Reference Antenna". Dipoles, monopoles above a large ground plane and horn antennas are often chosen as the Reference Antenna. The Log-Periodic Antenna, although also large, has pattern characteristics similar to the Standard Gain Horn's, has a superior and flatter bandwidth and is considerably lighter in weight. This paper will discuss a technique for using a Log-Periodic Antenna as a Standard Gain Antenna when co-location with the Antenna Under Test is not feasible.
A full RCS calibration technique using a dihedral corner reflector is presented in this paper. This scheme is valid for monostatic configuration and characterized by three aspects: (1) the frequency responses of four measurement channels can be mutually independent and thus, no special care has to be taken for signal paths; (2) only scattering matrix measurements of the dihedral at two orientations about the line-of-sight direction are needed since the transmitter and receiver are related through the reciprocity theorem; and (3) simple and useful expressions are used to solve for the calibration parameters. This technique is verified by several 2-18 GHz wideband RCS measurements performed in the OSU/ESL compact range.
Mathematical techniques (calibration, background subtraction, software range gating, imaging, etc.) have become integral to the process of generating precision radar cross section measurements. The "reference target method" is a powerful RCS correction algorithm which yields plane wave illumination results from data acquired under an arbitrary but known illumination. This method is analogous to a two dimensional RCS calibration. Measurements of long bars (at X- and Ku-bands) and of a scale model aircraft (at C-band) were performed under the cylindrical wave illumination produced by March Microwave's Single-Plane Collimating Range (SPCR) at Arizona State University. The targets were also measured under the quasi-plane wave illumination produced by a March Microwave dual parabolic-cylinder CATR. The SPCR measurements were corrected using the reference target method. The corrected SPCR measurements are in good agreement with the CATR measurements.
The paper discusses the results from a series of experiments to measure the dynamic radar cross section (RCS) for high-velocity targets at millimeter wave (60GHz). The low observable nature and detectability of the threats at millimeter wave are addressed. Date processing will provide calibrated dynamic RCS time series, from which RCS scintillation analysis and detection modeling can proceed. The data collection, reduction, analysis and target Doppler signatures are addressed.
This paper presents precision measurements of the RCS of a simplified aircraft geometry called the "generic aircraft". The RCS is measured over a frequency range of 2 to 18 GHz, and for incidence angle from "nose-on" through "broadside" to "tail on". This data is presented in the form of RCS contours as a function of frequency and incidence angle, and is compared with the computed RCS using wire-grid modeling. The contours show distinct patterns due to airframe resonance and due to the interference of the scattered field from the nose and from the tail of the aircraft.
This paper will address the issue of estimating and measuring the RCS of simple objects on a finite sized ground plane. RCS measurements of a one inch diameter hemisphere on a ground plane were collected at X-band and are shown to compare favorably with two different models of a hemisphere on a finite pc ground plane; a simple Geometric Optics (GO) model, and a EM Body of Revolution (BOR) model. The beauty of the GO model is borne out due to the insight which is gained in understanding the scattering mechanisms taking place. With the addition of a Physical Optics traveling wave component for the ground plane, the two models can be brought into good agreement with the measured data. Measurements were also conducted for a cylinder, cone and bicone whose results are also presented.
Vibrational motion imparted on targets during RCS measurements will demonstrate a distortion phenomena equivalent to Phase Modulation (PM). Vibrational PM distortion has been witnessed outdoors resulting from wind vibrating a foam column and indoors from vibration in the target rotation mechanism. The vibrational frequency and maximum downrange scatterer movement determine the location and magnitude of effective PM sidebands in the image domain. The impact of this modulation ranges from minor distortions in the image domain to a complete invalidation of the data. This will paper (sic) provide examples and describe how conventional communication theory can be used to describe this distortion phenomena.
RCS measurements with frequency and angle diversity offer the benefit of spatial resolution obtained by synthesizing the equivalent of short pulses and large apertures. Recent research in several specialized fields has been directed to spectral estimation techniques which seek to maximize the achievable resolution beyond limits imposed by traditional Fourier methods. These techniques, known as: autoregressive modeling, linear predictive modeling, super-resolution, or maximum entropy, offer the possibility of enhanced resolution and band-limited signal extrapolation. The methods apply to situations in which an estimate of a required feature is derived from an incomplete set of measured data linked to the required feature by a known relation. In RCS applications, spatial distributions of reflectivity are linked to measured band-limited frequency responses by a Fourier transform. Maximum entropy methods, therefore, apply directly to the objectives of increasing spatial resolution or extrapolating band-limited frequency responses.
MUSIC and ESPIRIT are two popular eigen analysis based super-resolution estimation techniques. The use of these techniques in scattering analysis is of interest in this paper. A comparative performance evaluation of these techniques is reported based on the time domain study of the RSC of various targets.
A method has been developed by which the fair-field RCS of a target can be evaluated from its RCS measured in the near field. The method can compensate for the nonuniformity of the antenna pattern which can be a function of the angle, the frequency, and the target distance. A correction transform is evaluated which depends on the antenna pattern, the frequency, the target distance and the target size. The correction transform is independent of the target geometry. The RCS of a target is measured in the near field, in a band of frequencies around the frequency at which the far field RCS of the target is desired. The method can practically handle directional scattering elements, shading of the scattering elements by each other, and interactions among the scattering elements. The reconstructed RCS evaluated by this method shows excellent agreement with the actual far-field RCS.
The applications of conventional reflector type compact antenna test ranges (CATR), becomes increasingly difficult above 100 GHz. The main problems are the tight surface accuracy requirements for the reflector, and therefore the high manufacturing costs. These problems can be overcome by the use of a new hologram type of compact range, in which a planar hologram structure is used as a collimating element. This new idea is described, and its performance is studied with theoretical analyses and measurements at 110 GHz.
This paper addresses measurement requirements for space communication antennas and identifies antenna parameters most influenced by indoor compact range quiet zone quality. These parameters include sidelobe level, beam pointing, and gain. The compact range mechanisms limiting measurement accuracy are identified and discussed. Proven methods for characterizing quiet zone performance are described and demonstrated through illustration and example. Analysis is presented which related quiet zone quality characteristics to antenna measurement accuracy. The paper summarizes typical measurement results and error levels achievable for modern compact range systems. Methods for improving compact range performance for satellite antenna testing are also presented.
A method to estimate the rms error in the compact range reflector surface is presented. The method uses the target zone field of the reflector and is based on the fact that the random errors in the reflector surface cause energy to subtract from the main beam resulting in reduction of the axial gain. The reduction in the axial gain can be used to estimate the rms error. It is shown that if the target zone fields of the reflector are probed at high frequencies such that the irregularities in the reflector surface are the main source of error in the target zone fields, then the proposed technique gives a good estimate of the rms error in the reflector surface.
Implementation of a two-ring array for feeing a compact range reflector is investigated. The array is designed to produce a shaped beam with a null at the angle corresponding to the rim of a circular-aperture offset paraboloid. Therefore fields diffracted from the reflector rim are reduced and no reflector edge treatment is necessary. The advantages and disadvantages associated with various feed systems are discussed. A dielectric-filled radial transmission line is proposed as a simple, cost effective implementation of the array beam-forming network. Curves for determining the required dielectric constant for null placement at a desired angle are presented. System bandwidth is examined. Methods for impedance matching and suppression of higher order modes in the beam-forming network are proposed.
This paper presents a study of (Broadband) feeds for a large compact range. Single feeds would be used for an octave or more, in place of 1/2 octave feeds. The study indicates improvement from mounting broadband feeds closer to the subreflector. The McDonnell Douglas Technologies Inc. (MDTI) large compact range uses a Harris 1630 system. The Harris system employs 1/2 octave feedbands. This creates limits for certain measurements. Requirements of the collimator system include fairly constant, relatively high gain feeds (Narrow beamwidth over a broad frequency range.) MDTI made initial studies of various broadband feeds. This study used an AN10F, borrowed from the vendor (GTE Government Systems). The AN10F approximates the required characteristics at its upper frequency range, (upper X - Ku Band). Field probe data taken with the feed installed near the focus of the sub reflector of the Harris collimator confirmed excessive amplitude taper below Ku Band. Further study illustrates the possibility of improved performance with the feed positioned nearer the Sub reflector. (Defocussed)
Increasing use is being made of millimeter-wave systems and there is a need for improved antenna measurement facilities operating at these higher frequencies. Although the practical implementation of compact range and near-field/far-field techniques becomes increasingly difficult, by using a hybrid approach, the attributes of these existing schemes can be exploited and their limitations overcome. The technique uses a linear near-field probe to carry out an instantaneous integration of the field in the date acquisition requirement, together with a quasi-real-time prediction capability. This contribution reviews a number of implementation schemes for the semi-compact antenna test range (SCATR) approach which have been investigated over the past decade and presents the latest results. An implementation of the SCATR with amplitude-only data is presented as an economical and viable method for millimeter-wave frequencies.