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Probe arrays based on the Advanced Modulated Scattering Technique (A-MST) permit rapid measurements of electromagnetic fields at microwave and millimeter wave frequencies. Applications of A MST probe arrays to diagnostic testing of a) large anechoic chamber environments and b) microwave antennas are summarized in this paper.
Because the incident wave on an anisotropic material is likely to be depolarized, a complete characterization of such a media requires to measure its whole scattering matrix, which afterwards complicates the calibration process. A suitable technic is the Wiesbeck calibration method [1]. In this paper, we apply this method to two configurations, the reflection configuration and the transmission configuration, and obtain very good agreements between theoretical and experimental results.
A sampling strategy for array diagnosis is discussed. The proposed strategy is able to obtain a minimum (i.e. equal to the number of array elements), and optimum (i.e. working as well as a uniform l/2 sampling rate) number of measurements in a given region of a plane in front of the array.
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.
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.
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 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.
Bowtie dipole antennas have been widely used for surface-based ground penetrating radar ( GPR) applications. This type of GPR antennas share common problems such as low directivity, antenna ringing, unstable characteristic impedance, RFI and large size. Special treatments have been used to improve their performance. Resistive terminations have been used to reduce the antenna ringing at t he price of efficiency. Some use reflectors to increase directivity at the price of bandwidth and the risk of cavity ringing excitation. Absorbing material is also used to shield RFI with increased size and weight. Some people use horn antennas because of bet ter gain. However, they are limited to high frequency applications where their size are still reasonable to handle. This means they can only do shallow target measurements. Horn antenna approach also faces the strong reflection arising at the air-ground interface. A new type of GPR antenna design presented in this paper has been developed to overcome the above difficulties.
In a recent article in the October 1996 Antennas and Propagation Society Magazine, Milligan discussed the sampling that is required to achieve a desired antenna pattern coverage using planar near-field scanning. To ensure that this region of coverage is not corrupted we must also consider the effects of aliasing. Aliasing will occur if the near-field sampling does not contain at least two samples per period for the fastest near field variation. As a result, the periodically continued patterns begin to overlap, and the measured pattern will be the complex sum of the overlapping patterns. We show that the relation between the near-field sampling and the maximum angle of coverage is more restrictive when we also require that the effect of aliasing be negligible. We give some examples to show the consequences of not following the more restrictive requirements.
A comparison of three near-field position error correction techniques has been performed on simulated near-field data. The purpose of this study was to evaluate the allowable positional tolerances required for planar near-field scanners. Simple k-correction, extended k-correction, and Taylor series correction were applied to computed near-field data contaminated with various kinds of errors, including position errors in one and three dimensions, and random electrical noise. Ideal and error contaminated near-field data were computed for small-size, mid-size, and large-size arrays. Probe position errors up to one-quarter wavelength in each axis and one wavelength in a single axis were used. Probe position error correction was performed using all three methods, and the results were evaluated
Coherent processing using measurements on two probe scan planes with different antenna under test (AUT)-to-probe separations reduces the effects of coupling between the AUT and the probe or, alternatively, reduces the effects of room scatter. The results of these doublet scans can be coherently combined to mitigate one or the other (but not both) of these error terms. For either case, the extraneous signals cancel when the far field patterns from the two planes are coherently combined. The new "quadrille" scan technique coherently combines four separate scan planes which will cancel in one set of pattern measurements both the AUT-probe coupling error and the room scatter error. If either the coupling or the room scatter is much larger than the other, the error reduction attained by the quadrille may not merit the additional measurement time; however if the two terms are comparable the quadrille may be needed to attain precise measurements.
Measuring an antenna in a planar near field range has become a common method for characterizing an antenna's radiation properties. Planar near-field measurements are best suited for narrow beam antennas, such as large parabolic reflectors. However, it is often necessary to also measure a standard gain horn (SGH) to obtain an accurate gain level reference and the measurement of an SGH in a planar near-field range is difficult because the SGH has a broad beam. The most significant error that typically occurs when measuring an SGH is the scan plane truncation effect. In this paper, a scan data window function is de veloped for reducing the scan plane truncation effect that occurs when measuring an SGH. Applying a window function to the scan data from a near-field measurement is not a new idea, but the particular development of the window function in this paper provides the necessary physical insight to easily choose the proper window function for any given SGH and measurement configuration. A summary of the theory behind this new scan data window function is presented along with various measurement examples.
The adoption of planar near-field scanning techniques by many industrial organisations to meet their measurement requirements for large, directive antennas has led to a significant demand for calibrated probes. To compensate for the effects of the probe used in near-field scanning measurements one requires an accurate knowledge of the gain, axial ratio, tilt and pattern. While NPL has been measuring the gain of microwave antenna standards for over seventeen years, it is only in the last two years that facilities and techniques have been developed to measure the polarisation parameters and pattern of probes. For the gain and polarisation, three antenna techniques are employed and both linearly and circularly polarised probes can be calibrated. Since calibration data is required at each frequency at which the planar scanner is to be operated, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, cost and the need for interpolation between measurement points.
A technique to reduce the scan length in near field antenna measurement is presented. In the technique, the original scan length is selected for a critical angle of 30° 35°. The measured near field probe data is then extrapolated beyond the available probed region. The extended near field probe data is next used to predict the far field pattern of the AUT. The extrapolation is carried out by estimating the aperture distribution from the measured probe data. The aperture size, the separation between the AUT and the probed plane and the orientation of the probed plane with respect to the AUT are selected such that the aperture distribution leads to the minimum error between the measured near field probe data and the near field due to the aperture distribution.
Andrew Corporation, founded in 1937 and headquartered in Orland Park, Illinois, has evolved into a worldwide supplier of communication products and systems. To develop a superior, high performance line of base station products for a very competitive marketplace, several new antenna measurement systems and upgrades to existing facilities were implemented. This engineering project developed an indoor test range facility incorporating design tool advantages from among Andrew Corporation's other antenna test facilities. This paper presents a 22-foot vertical by 5-foot diameter cylindrical near-field measurement system designed by Nearfield Systems Incorporated of Carson, California. This system is capable of measuring frequencies ranging from 800 MHz to 4 GHz, omnidirectional and panel type base station antennas up to twelve feet tall having horizontal, vertical or slant (+/- 45 degree) polarizations. Far-field patterns, near-field data and even individual element amplitude and phases are graphically displayed.