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Recently, we proposed a new method for the testing of antennas or the measurement of RCS in submillimeter wave region. A specific feature of this technique resides in that investigated object or its scaled model is mounted inside a quasi-optical waveguide in the form of a circular hollow dielectric waveguide (HDW) so as to determine the scattering parameter of the waveguide dominant HE11 mode which is certainly related to the wanted RCS of the object under study. In this paper, we intend to theoretically substantiate the proposed method for measuring RCS inside a circular HDW by using geometrical optical ray representation of guided modes and "virtual" waveguide concept. Then, a correspondence between RCS of an object inside a HDW and in a free space is established. Also, RCS of reference objects such as a perfectly conducting square flat plate inside a circular HDW are measured and compared with predicted returns in free space.
ORBIT/FR has recently installed and qualified a combined microwave (2-18 GHz) and millimeter wave (92.5-95.5 GHz) RCS system in an existing compact range based chamber. The facility is used for scale model reflectivity measurements on a wide variety of targets. The system features a unique, high power hardware gating system at the millimeter wave band that contains an integrated compact range feed assembly specifically designed to optimize RCS performance. Changeover between the microwave and millimeter wave bands is possible by placement of the appropriate compact range feed assembly on the feed stand, with locating pins being utilized to assure repeatable performance of the feeds in the compact range system. The system utilizes the FR959 RCS Measurement Workstation and HP 8530/85330 "turbo" based receiver system. Appropriate upconversion and downconversion hardware is integrated into the millimeter wave gating system to allow a common set of HP 8360 series sources and the HP 8530 IF receiver to be utilized for operation in both bands. The system is capable of producing high quality ISAR images at the millimeter wave frequencies, as well as in the microwave band.
This paper discusses the design, fabrication, installation, and testing of a Scientific-Atlanta Model 5702 Compact Range used for radar system testing. The unique feature of this compact range is that it provides a plane wave target source for automated closed loop radar system testing. Techniques employed for meeting and verifying stringent specifications such as phase and amplitude gradients over the quiet zone are discussed. Results from closed loop testing of radar systems in the compact range are also presented.
ORBIT/FR has recently delivered an integrated facility capable of being used for Antenna, Radar Cross Section (RCS), and EMI measurements to the Naval Underwater Warfare Center in Newport, RI. The facility includes a shielded anechoic chamber, a compact range system capable of producing a 6 foot diameter quiet zone, multi-axis positioning equipment, and a complete complement of Antenna, RCS, and EMI measurement instrumentation and data collection hardware/software. The facility is capable of operation over a frequency range of 100 MHz to 50 GHz, with compact range operation feasible above 2 GHz. The facility can be reconfigured to go between antenna and RCS measurements in any band using both frequency band and antenna/RCS mode switching. In addition, automatic positioning of the appropriate compact range feed to the reflector focal point is available. EMI measurements require minimal relocation of absorber in an isolated area of the chamber floor. Performance of the system is optimized by location of critical RF equipment on the compact range feed carousel or on the positioning system rail carriage. This system offers a unique combination of performance and convenience for making all three types of measurements.
This paper discusses Radar Cross Section (RCS) measurement capability at Very High Frequencies (VHF) in the Boeing 9-77 Range in Seattle, Washington. This indoor facility provides a unique asset to the RCS measurement community. Initially operational in 1989, the 9-77 Range was upgraded in 1995 to include a VHF measurement capability. This was achieved using a 56 foot square array of 256 elements, for RCS measurements at frequencies from approximately 140 to 220 MHz, with a 40 foot quiet zone. In this paper, we discuss results from the characterization process used to verify the initial capability and ongoing operation of the RCS measurement system at VHF. We include data demonstrating the sensitivity, stability and dynamic range of the system. We also present samples of recent field probes, and background subtraction and stability measurements. A comparison is made between calculated and measured canonical target signatures.
Range characterization is becoming a very important topic for the operators of RCS measurement ranges. Techniques for characterization can be expensive and time consuming. We present a top down approach that recognizes that the range construction and optimization is the responsibility of the range operators. Once the range is operating satisfactorily from the point of view of the range operator, then characterization of t he range performance as achieved can be done. Measurements are proposed that perform this characterization rapidly and inexpensively.
Recently, several deployable, ground-to-ground col lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
The Ohio State University ElectroScience Laboratory (OSU/ESL) has built a series of radars that transmit UWB random noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. When the response of the system is taken as a function of the delay time, the result is proportional to the impulse response of the system. After background subtraction and calibration, the impulse response of the target results. We will present a description of the variable delay line system and show an example ISAR image made from measurements taken in the OSU compact range.
The ground reflection range is popular for outdoor measurements at low frequencies. When operating in ground reflection mode, it is necessary to change the height of the source antenna for different test frequencies. This requirement limits our ability to time multiplex test frequencies and ultimately reduces range efficiency and increases cost. This paper describes an investigation into the use of a simple two antenna array as a feed for the ground reflection range. Computer simulations are used to assess the overall performance of a typical range with the two element source array. The array weighting is optimized using a search algorithm to provide uniform fields on the range over a three to one frequency band. Techniques for accomplishing frequency changes electronically, without the need for mechanical repositioning of the source antenna are described.
Multipath on far-field ranges causes distortion of pattern measurements. The multipath components can be removed by illuminating the antenna under test with short-duration pulses and applying a time domain gate. Equivalently, the measurements can be made in the frequency domain and transformed to the time domain with the Fourier transform. After gating, the time-domain data are transformed back to the frequency domain, yielding improved CW patterns at discrete frequencies. Virginia Tech has recently added time-domain gating capability to its far-field antenna range. The data acquisition and processing software is implemented using the LabVIEW language, which makes the data acquisition and time-domain processing very easy to control. Practical guidelines for selecting a gate are given. Results are presented for an open-ended waveguide and conical dipole. With wideband antennas, gated patterns show significantly improved symmetry and null depth.
Concurrent engineering concepts that have been in use in other industries for many years can be used successfully in millimetre-wave antenna production for space applications. This paper demonstrates the key role played by the proper consideration of RF testing as part of this overall production process.
This paper examines antenna measurement errors attributable to instrumentation, and their effect on measurement uncertainty.
The site attenuation of a practical open area test site differs from that of an ideal (infinite ground plane) test site. For example when transmit and receive antennas are vertically polarised there is a significant ripple in the site attenuation as a function of frequency. This causes an uncertainty in the measured antenna factor. This paper describes a theoretical model that accurately predicts the site attenuation for any test site geometry. It is shown that it is insufficient to consider the conducting ground plane in isolation: the region outside the ground plane must be taken into account. The work is illustrated with measurements made on the National Physical Laboratory's (NPL) 30 m by 60 m test site. .
Wideband channel measurements have been used extensively to determine path loss and time dispersion characteristics of radio channels (e.g., [1], [2], [7]). The principles used to temporally resolve individual received signal components for wideband propagation measu rements can be applied to antenna pattern measu rements to achieve more accurate results. Multipath, a propagation phenomenon which occurs when reflecting or scattering objects exist in an environ ment, causes inaccuracies in measured patterns when narrowband signals (e.g. continuous wave) are used to perform far-field antenna measu rements. Using the wideband technique described in this paper, the effects of multipath can be completely eliminated from pattern measurements. The method described here is especially useful when antenna range dimensions are limited in space or when multipath signal components caused by distant reflectors are irreducible.
The G/T measurement of large antennas using radio source techniques is well established. What is less well known is that this technique is also useful in characterizing relatively small antennas . The application of radio source techniques to smaller antennas is discussed. An estimate of the minimum size antenna that can be measured by this technique is provided. The measurement technique is reviewed along with techniques to measure the total system temperature, which is required when the antenna gain must be established.
The Wright Laboratory at WPAFB, OH, operates an advanced compact range facility (ACRF) for RCS measurements. The ACRF employs a dual chamber compact range system to generate a plane wave in the target zone. The main reflector, which is a blended rolled edge paraboloid, is housed in the main chamber; whereas, the feed assembly and the subreflector, which is a serrated edge ellipsoid, is housed in the sub chamber. The two chambers are electromagnetically coupled through a small opening near the focal point of the main reflector. The compact range system was originally designed to perform RCS measurements at frequencies above 1 GHz. Recently, there has been some interest in us ing the ACRF to perform RCS measurements at lower frequencies, from 100-1000 MHz. In fact, the ACRF facility has been successfully used to measure small targets at these lower frequencies, but one would like the target zone to be as large as possible. In order to accommodate a larger target zone, the first step was to evaluate the performance of the ACRF at lower frequencies. The performance evaluation revealed that the subreflector edge diffraction was leaking through the coupling aperture into the target zone. Some feed spillover was also observed in the target zone. To control these stray signals in the target zone, an absorber fence was designed for the ACRF. The absorber fence sits near the focal point of the main reflector. A prototype absorber fence has been built and installed in the ACRF. The performance of this absorber fence is discussed in terms of the improvement in the target zone fields.
Recently, we designed two doubly periodic curved pyramidal absorbers using Rantec absorber material. One of the pyramidal absorbers is 4011 high and is designed to operate at frequencies as low as 300 MHz; whereas the second pyramidal absorber is 6011 high and is designed to operate at frequencies as low as 200 MHz. The design goal was to achieve at least 45 dB attenuation for normal incidence. Based on our design, Rantec built the new pyramidal absorbers. The back-scattered fields of the new pyramidal ab sorbers were measured in the Wright Laboratories' (WL) advanced compact range facility (ACRF) us ing a 12' x 12' panel. In this paper, the measured data is presented and compared with the theoretical predictions. For reference, the scattered fields of a 72" pyramidal absorber are also included. The 72" pyramidal absorber was built by Ray Proof.
Sanders, A Lockheed Martin Company, measures radar cross section (RCS) and antenna performance from 2 to 18 GHz at the Com pany's Compact Range. Twelve feed horns are used to maintain a constant beam width and stationary phase centers, with proper gain. However, calibration with each movement of the feed tower is required and the feed tower is a source of range clutter. To Improve data quality and quantity, Sanders and The Ohio State University ElectroScience Laboratory designed, fabricated, and tested a new wide band feed. The design requirement for the feed was to maintain a constant beam width and phase taper across the 2 - 18 GHz band. The approach taken was to modify the design of the Ohio State University's wide band feed [1]. This feed provides a much cleaner range which reduces the dependence on subtraction and other data manipulation techniques. The new feed allows for wide band images with increased resolution and a six fold increase in range productivity (or reduction in range costs). This paper discusses this new feed and design details with the unique fabrication techniques developed by Ohio State and its suppliers. Analysis and patterns measured from the feed characterization are presented as well. This paper closes with a discussion of options for further improvements in the feed.
A slot spiral antenna and its associated feed are presented for conformal mounting on a variety of land, air, and sea vehicles. By exploiting the inherent broadband behavior good pattern coverage and polarization diversity of the spiral antenna, a conformal antenna which can be concurrently used for cellular, digital personal communications (PCS), global positioning (GPS) and intelligent vehicle highway systems (IVHS) as well as wireless LAN networks has been developed. A key requirement for achiev ing such broadband behavior (800-3000MHz) is the avail ability of a broadband planar feed and balun. Such a feed was proposed last year by the authors. However, addi tional design improvements were found to be necessary to achieve satisfactory pattern and gain performance. Among them were a broadband termination for the spiral arms and the suppression of cavity and waveguide modes. Both of these improvements played a critical role in achieving acceptable performance over the 800-3000 MHz bandwidth. After a general description of the slot spiral antenna and the above modifications, this paper presents a comparison of the performance before and after the modifications.