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The Georgia Tech Research Institute has designed and built a field probe for the U.S. Army Electronic Proving Ground Compact Range. The field probe is an R-0 scanner covering a 59-foot diameter area. It includes a laser-referenced Z-axis correction servomechanism, a polarization positioner, and a cable handling system for one-way data acquisition.
Data collection is increasingly becoming the limiting factor in overall antenna and RCS measurement time. An equation for data collection time for multiple parameter measurements is presented along with and ordering function for determining the optimum nesting order for parameters. An example is used to demonstrate measurement speed enhancement techniques, reducing data collection time by 65 percent. Changing from stepped to linear near-field scanning reduced collection time by 75 percent.
Many current, and near future, antenna and Radar cross section measurement requirements dictate improvement in microwave power amplifier performance and capabilities. Maturing of Traveling Wave Tube (TWT) technology and breakthroughs in modulator and power supply design now enable exploitation of the maximum possible RF performance from TWTs.
The three cable method for removing the amplitude and phase variations of microwave cables due to temperature change and movement can offer a substantial improvement in antenna measurement accuracy. Implementation details of the method are provided for a planar near-field range. Items specifically addressed are range configuration, hardware requirements, data collection methodology, identification and assessment of error sources, and data reduction requirements.
The measurement of modern low sidelobe antennas has brought a greater need for accurate site characterisation in order to quantify the effects of site scatterers. A multi-frequency Hankel function out-propagation technique is used to locate and identify site scatterers whose effects may degrade the patterns of antennas measured on the site.
The new antenna measurement facility in C.A.S.A. Space Division is described. The system, designed and installed by Grupo de Radiación of the Polytechnic University of Madrid , provides antenna measurement set-up for Far Field and both Planar and Spherical Near Field.
Calibrated measured results are presented that characterize the performance of a rhombic shaped TEM parallel plate horn antenna to transmit and receive ultra-wide bandwidth (UWB) waveforms, the standard narrow band antenna parameters, such as gain, are inadequate in characterizing the antenna. In this paper, the antenna is viewed as a transducer in which the transmitting and receiving antenna can be fully described by complex transfer functions. These functions provide a more natural means of characterizing an antenna for UWB applications. The time domain transmit and receive transfer functions of our test antenna are presented in a contour map as a function of angle for the two principal planes, and the responses are correlated to physical attributes. In addition, the waveform dispersion and the total received energy for a bandwidth limited impulse excitation are used to characterize its use for UWB synthetic aperture radar (SAR) applications.
The effects of measurement distance on the sidelobe sum and difference patterns are examined. Highly efficient and robust aperture distributions, the Taylor ñ and the Bayliss ñ, are used to generate date representative of all such distributions. Patterns are obtained through numerical integration of the near-field inegral with exact phase term. Taylor ñ patterns are computed for sidelobe levels to -60 db (published in 1984), and Bayliss patterns for sidelobe levels down to -50 db (new results). For both sum and difference patterns, the change in first sidelobe height, in db, is linear with the log of the measurement distance normalized by 2D(squared)/(lambda). In both cases the lines for different sidelobe levels have the same slope. These results, and typical patterns showing sidelobe changes, will be presented.
Using superconducting Yttrium Barium Copper Oxide (YBCO) film for the RF power distribution network of large microwave array antennas can reduce Ohmic losses considerably and thereby increase antenna efficiency. To achieve these benefits the array must be cooled down to liquid Nitrogen temperatures. Typically, in the laboratory, the cooling of the array is achieved by means of a cryostat. Performing transmission measurements with the array antenna encapsulated inside the cryostat housing however, becomes a very challenging measurement procedure. In this paper we will describe this measurement technique and demonstrate the measured performance of a sixteen-element superconducting proximity coupled array antenna.
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.
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.