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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.
A unique and novel concept is presented for the visualization of Radio Frequencies. The concept will allow the visualization of radio waves, independent of frequency, which can not only provide detection of their presence but which can also describe their spatial properties, e.g. intensity distributions and footprints.
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.
A sophisticated software package FARANA (FAR-field ANAiysis) is presented for transforming planar near-field test data to far-field antenna patterns, including enhanced analysis of far-field results. FARANA is coded in MATLAB version 5.0. MATLAB (MATrix LABoratory) is an interactive mathematical modelling tool based on matrix solutions without dimensioning. Using MATLAB, numerical engineering problems can be solved in a fraction of time of time required by programs coded in FORTRAN or C. FARANA operates with a state-of-the-art graphical user's-interface, is intuitive to use and features high speed and accuracy. This paper addresses an assessment of the program, discusses its use and enhanced far-field analysis capabilities.
We describe a technique which uses field intensity patterns formed by the interference of an unknown test antenna and a known referenceantenna - holograms in the classical optical sense - for determining the far-field pattern of the unknown antenna. The field intensity is measured by acquiring an infrared picture of the tem perature distribution on a resistive screen heated by incident microwave energy. The output of the camera is processed to yield the electric field intensity on the surface of the resistive screen. Required measurements are the field patterns of the unknown antenna and two holograms taken with relative phase differences between the reference and unknown antennas of 0° and 90°. In addition, the amplitude and phase of the reference field at the measurement plane are needed. These can be obtained from a separate measurement of the reference using standard near-field techniques. The algorithm gives the complex near field of the antenna under test which can then be processed to obtain the far-field pattern of the antenna under test. We present results showing far-field patterns which acceptably reproduce the main beam and near sidelobes. Such techniques will allow rapid testing of certain antenna types.
The mechanical rotator must be correctly aligned and the probe placed in the proper location when performing spherical near-field measurements. This alignment is usually accomplished using optical instruments such as theodolites and autocollimators and ideally should be done with the antenna under test mounted on the rotator. In some cases it may be impractical to place the alignment mirrors on the AUT or optical instruments may not be available. In these and other cases, it is desirable to check alignment with electrical measurements on the actual AUT and probe. Such tests have recently been developed and verified. Appropriate comparison and analysis of two near-field measurements that should be identical or have a known difference yields precise measures of some rotator and probe alignment errors. While these tests are independent of the AUT pattern, judicious choice or placement of the antenna can increase the sensitivity of the test. Typical measurements will be presented using analysis recently included in NSI software.
The objective of this study is to evaluate the measurement errors of a near-field range at in order to develop some techniques to minimize them. Measurements were performed on a standard gain horn as references. The methodology presented demonstrates that it is feasible to calculate the far-field radiation from near-field measurement with one deconvolution that will include all the errors introduced by the instrumentation
When a planar near-field measurement is done, errors are introduced due to imperfections in the mechanical and electrical parts of the measurement equipment. In order to identify the characteristics of different types of errors, a MatLab program that simulates the near-field from an antenna has been developed. The near-field is transformed to far-field and the errors are evaluated. This paper looks into four different error types: 1) Truncation errors (if the measurement surface is to small the near-field will be truncated before it reaches adequately low levels), 2) Probe-AUT distance errors (fluctuations in the probe AUT distance over the measurement surface), 3) Zigzag errors (due to data being acquired during both travel directions of the probe), 4) I,Q amplification errors (different amplification for the I and Q channels in the receiver). The results are presented in plots which illustrate where in space the largest antenna pattern errors occur.
A series of measurements to validate the performance of a Vertical Planar Near-field Antenna Test Range located at the Hughes Space and Communications Company (HSC) was performed. These measurements were made as part of a task to provide validation of this particular range for detailed Production Antenna Testing. This validation was required in preparation for measuring a particular flight antenna. The range validation consisted of a series of self comparison tests and far-field range pattern comparison tests using an offset reflector antenna as the validation antenna. This antenna had been previously measured on a far-field antenna range which is in constant use to test flight antennas. This paper describes the range validation tests and presents some of the results. Comparisons of some far-field patterns measured on the validation antenna at both the far-field and near-field ranges is presented.
NSI recently delivered a Turnkey Near-field Antenna Measurement System (TNAMS) to the Naval Surface Warfare Center - Crane Division (NSWC-CD) in Crane Indiana. The system supports characterization and calibration of the Navy's active array antennas. TNAMS includes a precision 12' x 9' vertical planar near-field robotic scanner with laser optical position measurement system, dual source microwave instrumentation for multiple frequency acquisition, and a wide PRF range pulse mode capability. TNAMS is part of the Active Array Measurement Test Bed (AAMTB) which supports testing of high power active arrays including synchronization with the Navy's Active Array Measurement Test Vehicle (AAMTV), now under development. The paper summarizes the hardware configuration and unique features of the pulse mode capability for high power phased array testing and the TNAMS interface to the AAMTV and AAMTB computers. In addition, range test data comparing antenna patterns with various pulse characteristics is presented.
A new edge treatment approach using resistive cards (R-cards) for compact range reflectors is introduced in this paper. This new treatment has proven to be successful in achieving the design goal of reducing the diffracted fields from the reflector edges in the quiet zone. The design key of this new treatment relies on the proper choice of both the resistance distribution and placement of the R-cards in front of the reflector edge. Preliminary analysis and design studies show the potential of this new treatment in reducing the ripple level in the target zone over a wide range of frequencies. The simplicity, flexibility and the low cost of this new approach provide a viable alternative approach to the other edge treatments, such as serrated or blended rolled edges.
A development work of a 2.4 m x 2.0 m hologram for testing the 1.1 m offset reflector of the Odin satellite at 119 GHz is reported. The analysis of the hologram is based on physical optics (PO) and finite difference time domain method (FDTD). The hologram is fabricated with an etching process. A comparison between the theoretical and measured quiet-zone fields of the hologram type of compact antenna test range (CATR) is made.
Previous researchers have demonstrated the ability to probe the range field illuminating an antenna under test (AUT) on the surface of a sphere and to calculate the spherical wave coefficients from these measurements. This paper, describes a technique for obtaining a plane wave model of the range using these spherical wave coefficients. An algorithm for creating this plane wave model is presented and a plane wave model for a measured range field is shown. A plane wave spectrum of the range field is calculated and the directions of the largest peaks of this plane wave spectrum are used as initial guesses for the direction of plane waves. The directions of incidence of these plane waves are optimized such that the least square error between the range spherical wave coefficients and plane wave model spherical wave coefficients is minimized. An example of a measured range is presented which consists of a horn range antenna and a smaller horn as an extraneous source located approximately 20 degrees off the range axis. The plane wave model is used in a range compensation algorithm described in a companion paper.
Leader in the field of space environment simulation (vibrations, thermal vacuum, acoustics, EMC), INTESPACE company has built a new compact range for antenna measurement called MISTRAL with a view to providing an overall satellite test service. The purpose of this new full-scale test facility is to determine the radioelectric characteristics of integrated satellite antennas covering : - classic antenna tests such as radiation pattern and gain measurement, - payload-specific end-to-end tests such as EIRP, SFD, GIT, Gain/Frequency, etc. The aim of this paper is : - first, to present the main and extra features of the MISTRAL compact range, - second, to show the major improvements and system optimization achieved through the study and development phases of MISTRAL, - third, to present the results of the intensive acceptance tests (quiet zones probing and antennas measurements) confirming the high quality of the test facility.