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In the last few years, the interest in millimeter wave systems, like radars, seekers and radiometers has increased rapidly. Though the size of narrow-beamwidth antennas in the 60-200 GHz range is limited to some 20 inches, an accurate far-field antenna test range would need to be very long. The achievement of precision antenna pattern measurements with a 70' or even longer transmission length requires the use of some power that is hardly available and expensive. A cost-effective and more accurate solution is to use a lab-sized compact range that presents several advantages over the classical so-called far-field anechoic chamber: - Small anechoic enclosure (2.5 x 1.2 x 1.2 meters) meaning low cost structure and very low investissement in absorbing material. No special air-conditioning is needed. This enclosure can be installed in the antenna laboratory or office. Due to the small size of the test range and antennas under test, installation, handling and operation are very easy. For spaceborne applications, where clean environment is requested, a small chamber is easier to keep free of dust than a large one. - The compact range is of the single, front fed, paraboloid reflector type, with serrated edges. The size and shape of the reflector and serrations have been determined by scaling a large compact range of ESD design, with several units of different size in operation. The focal length of 0.8 meter only accounts in the transmission path losses and the standard very low power millimeterwave signal generators are usable to perform precision measurements. The largest dimension of the reflector is 1 meter and this small size allows the use of an accurate machining process, leading to a very high surface accuracy at a reasonable cost. The aluminum alloy foundry used for the reflector is highly temperature stable. - Feeds are standard products, available from several millimeter wave components manufacturers. They are corrugated horns, with low sidelobes, constant and broad beamwidth over the full waveguide band and symmetrical patterns in E and H planes. - The compact range reflector, feeds and test positioner are installed on a single granite slab for mechanical and thermal stability, to avoid defocusing of the compact range. - A micro-positioner or a precision X Y phase probe can be installed at the center of the quiet zone. Due to their small size, these devices can be very accurate and stable. Due to the compactness of this test range, all the test instrumentation can be installed under the rigid floor of the enclosure and the length of the lossy RF (waveguide) connections never exceeds 1 meter.
This paper discusses the configuration and performance of millimeter-wave measurement systems comprised of standard Harris Shaped Compact Ranges, Hewlett Packard (HP) 8510B Network Analyzer, and Millitech frequency extenders designed for use with the network analyzer. Millimeter-wave capabilities have been integrated into the Harris automated measurement system to allow computer controlled millimeter-wave compact range characterizations. This system offers a new measurement alternative for antenna and Radar Cross Section (RCS) measurements. Measured 35 GHz data from the Harris Model 1606 compact range, and 95 GHz data from the Model 1603 compact range are included.
Comparisons of predicted and measured phase and amplitude data have been made for the Model 1640 compact range. The predicted data is generated using either a physical optics model for shaped offset reflectors with a feed model which takes the corrugation depth and spacing into account or a similar geometric optics model. Excellent agreement has been obtained including accurate prediction of the effects of axial feed movement.
This paper discusses the results of a recent study on the UHF performance of a Harris Shaped Compact Range. The design process for the dual polarized, 70% bandwidth UHF feedhorn is summarized. Measured data is presented for primary feedhorn patterns and for one-way CW field probe measurements with open-ended waveguide. The measured data is overlaid with computer predictions to validate the modelling tools and the measurement procedures. The automated quiet zone characterization procedure for amplitude and phase is also discussed.
This paper summarizes the performance of the new Harris 1612 Compact Range Standard Product. The Harris 1612 is a dual shaped reflector collimator system. Measured data, both amplitude and phase from the 12-foot diameter quiet zone is presented. The quiet zone was characterized using an automated two-way HP8510B based measurement system. The inherent system benefits for both antenna and RCS measurements are also discussed.
In an earlier paper the virtual vertex compact range reflector was introduced and data from a specific design was reported. This paper describes the extension of the vertical vertex serrated edge concept to other reflectors that serve a wider range of application. Two new 12 ft focal length reflectors have been built that possess 3 ft and 6 ft diameter symmetric test zones. We describe the electromagnetic considerations and the mechanical design approach that has been used for these reflectors. We demonstrate the performance with field probe data showing the excellent surface accuracy of these units.
Measured component RCS results are frequently dominated by the test body and target mounting structures. This paper will present a measurement technique that will improve measurement accuracy using a less complex and expensive test body. The design of the test body and measurement geometry allows isolation in both range and cross range from the static return of the room and mounting structure. This is accomplished by first creating an ISAR image of the target and test body, gating the image in two dimensions, then transforming back into the frequency and spatial angle domains to determine the scattering levels of the target by itself. Details of this technique, covering both its advantages and limitations, will be discussed. Data will be presented to verify the approach and illustrate the level of performance attainable using this technique.
This paper describes methodology for performing high resolution target radar cross section (RCS) diagnostic measurements with a new type of portable multifrequency radar. The Model 200 radar system is capable of operating at extremely short ranges, and does not require an anechoic chamber for performing highly sensitive radar cross section measurements. Measurements can be made in conventional low range resolution polar plot modes, in high-range-resolution (HRR) mode, in Inverse Synthetic Aperture (ISAR) mode, and in Synthetic Aperture (SAR) mode. The radar is described and the implications for present and future measurement technology are discussed.
In the past, most radar-cross-section imaging has been done after data has been taken. At best, this off-line processing generates images that are returned to a customer the next day. Many projects can benefit significantly by having concurrent imaging and data acquisition. This allows for real-time cause and effect type diagnostics without rescheduling range time. As RCS range time becomes increasingly more expensive and difficult to schedule, real-time imaging provides the project engineer with a valuable tool to optimally use his range time. A technique has been developed to render real-time radar cross section images while acquiring data. All image processing is performed to achieve a fully enhanced image. Focussing, interpolation, and windowing are all used to give a detailed image. The system uses a Hewlett Packard 8510B for data taking and Hewlett Packard computers for data acquisition and image processing.
An image processing method which uses the bistatic scattered fields of a target obtained in a compact range is presented in this paper. The transmitting and receiving antennas can be either two compact ranges or one compact range and a horn antenna. The compact range reflector can be either focussed or defocussed so that a near field situation can be simulated. The bistatic scattered fields are collected as a function of frequency and the angle of rotation of the target. Then they are processed coherently to determine the cross-range and down-range scattering centers of the target. Experimental results are presented to validate this image processing technique.
The scattering characteristics of a moving object are different than those of a stationary one. A common property of a moving object is the doppler frequency shift. For objects with rotating propeller blades, another property is the frequency shift from the periodic rotation of the propeller blades. This frequency shift is due to an amplitude modulation of the carrier frequency. Two techniques are presented involving the use of amplitude modulation. The first technique is for the observation of such modulation due to slowly rotating structures in a compact range. These results can be scaled to obtain results for this structure rotating at any speed. This is a great advantage since the model does not have to be designed to rotate propellers at high RPM's. The second technique utilizes the power spectrum density of the dynamic scattering signature of the rotating propellers to reduce undesirable clutter in the measurement.
RCS instrumentation systems capable of combining wide-band and ISAR techniques to obtain two-dimensional images are widely used to perform RCS diagnostic and measurement functions. Objects involving rotating structures, such as blades of propulsion systems complicate the diagnostic task. The paper address the utilization of diagnostic RCS systems to meaningfully determine the radar signatures of objects with rotating components and presents results obtained from a generic data set, typically available from wide-band RCS instrumentation systems. The results provide valuable insight to the signature of objects with rotating components.
Signal processing techniques may be used in radar signature analysis to obtain radar target impulse response. In general there is a one to one relationship between specific scattering mechanisms and the time such mechanisms appear in the impulse response. One of the difficulties of this type of analysis is that complex targets often have multiple interactions. Many of these multiple interaction mechanisms can be identified as such by the application of the bispectrum to the radar scattering data. Also, the bispectrum forms a basis for discriminating between targets. Classification of unknown radar targets based on their bispectral response is performed in this study.
Performance verification of an adaptive array requires direct, real-time sampling of the antenna pattern. For a space-qualified array, measurements on a far-field range are impractical. A compact range offers a protected environment, but lacks a sufficiently wide field of view. Conventional near-field measurements can provide antenna patterns only indirectly. This paper shows how far-field antenna patterns can be obtained in a relatively small anechoic chamber by focusing a phased array in the near-field. The focusing technique is based on matching the nulls of far-field and near-field antenna patterns, and is applicable to conformal or nonuniform phased arrays containing active radiating elements with independent amplitude and phase control. The focusing technique was experimentally verified using a 32-element, linear, L-band array. Conventionally measured far-field and near-field patterns were compared with focused near-field patterns. Very good agreement in sidelobe levels and beamwidths was achieved.
A special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.
This paper describes the techniques applied to a fully automatic computer controlled, HP8510 based, range gated digital data acquisition system used to provide scale modeled large aperture synthesis, evaluation of aircraft blockage effects, array patterns, element cancellation ratios, as well as providing a large accurate data base for radar simulation exercises.
Purcell's Image method is useful for measuring the absolute gain of small antennas. The method is simple to use, and utilizes only one antenna with a reflecting plane to provide an image for the receiving antenna. However, the method yields accurate results only if the antenna is matched to its waveguide. This paper describes an image method for absolute gain measurement under mismatched conditions. A gain formula was derived based on the waveguide junction analysis with the antenna terminal treated as a mismatched junction. This method does not require an accurate measurement of the radiated power, and therefore, appears most suitable for measuring the gain of small microstrip patch antennas. Experimentally, this method has been demonstrated to produce accurate results for a single rectangular patch and a two-layer parasitic patch array.
Scatter from the nearby obstacles on a pattern test range has been removed by synthesizing a short pulse with 16 CW measurements. With suitable weighting, a low time-sidelobe pulse, is synthesized to remove scatter to close as 25' for a 32' low sidelobe UHF array. In addition to the pulse results, equivalent CW data is extracted with an FFT from frequency to time, truncation, and in inverse FFT back to individual frequencies. The time samples provide considerable insight into the source of reflections at the range. The procedure gives good agreement with CW patterns taken in another manner. It does so with a minimum of range modification, and operates at very low sidelobe levels.
The gain-to-temperature ratio (G/T) is a fundamental parameter used in describing the performance of a receiving antenna. This single parameter is sufficient to describe the antenna's contribution to the sensitivity of the receiving system because it includes the antenna gain as well as noise producing factors such as sidelobes and ohmic losses. The G/T of a phased array antenna is determined by the size of the aperture and the aperture distribution as well as the ohmic losses, electronic gains and noise figures of the components in the array assembly. Measurement of phased array G/T is an important means of verifying that the integrated assembly is performing to the level equivalent to the sum of the subassemblies. One method of measuring antenna G/T uses the sun as a source of radio frequency noise. This method is equivalent to the hot/cold source method used to measure the noise figure of a two port device. The sun is used as the hot source and the clear sky is used as the cold source. The difference in the noise output from the antenna with it pointed at the hot sun and the cold sky is used along with the known solar flux density and appropriate correction factors to calculate the G/T. The measured G/T is corrected for the antenna beamwidth and losses in the post antenna measurement system. This paper describes the solar method used to measure the G/T of a large S-Band airborne phased array designed and built by the Georgia Tech Research Institute (GTRI). The measurement results are compared to the G/T values calculated knowing the performance of the components that make up the array assembly.
As in any real world measurement application, there are performance trade offs to be made between hardware and software gating in RCS measurements. Hardware gating adds significant complexity and cost to the measurement system and, incorrectly applied, can result in degraded measurement performance. A measurement system based on software gating is the simplest and most cost effective to implement but may have performance limitations on some types of ranges. The objective of this papers to provide useful guidelines to use in choosing between an HP 8510B RCS measurement system based on software gating, hardware gating, or a combination of the two approaches.