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Ths paper demonstrates the plane wave, pattern subtraction method for performing range compensation of full-sphere antenna patterns measured on a fixed line-of-sight far-field range. The range field is measured on the surface of a sphere and a plane wave model of the range field illuminating the antenna under test (AUT) is determined. The range compensation algorithm uses information contained in both the plane wave model and the AUT pattern measurement to estimate the error pattern that is added to the measured AUT pattern by an extraneous source. This estimated error pattern is subtracted from the antenna pattern measurement to obtain a compensated pattern. The compensated pattern and estimated error pattern are improved iteratively. This paper demonstrates the technique using measured data. The AUT is measured in a far-field anechoic chamber that contains a secondary horn antenna located 20 degrees off-axis from the range antenna, which is used as an extraneous source. The AUT is a 474 element planar array operating at a frequency of 9.33 GHz.
During the design of spacecraft antennas a well defined geometrical configuration of antenna components is supposed. Also the requirements for the accuracy of the antenna integration normally will be given. The antenna alignment processes have to ensure, that the designed configuration with the required accuracy can be met. Additionally the antenna pointing has to be determined with respect to the RF measurement facility. In this paper the concepts are treated, how to determine the actual and the designed orientation and location of the components of the space antennas during subsystem and system level integration and tests. This includes also the definition of needed references for the antenna components, the creation and application of coordinates or orientation matrices at manufacturing or integration level, the used coordinate systems and the attainable accuracy for different methods. For the evaluation of the RF pattern performance, the correlation between the spacecraft coordinate system and the facility coordinate system has to be known. Basic principles of this pointing alignment and an error analysis of the measurement accuracy will be explained. The presented concepts are based on the experience at DSS' test facilities with various antenna types and agreed with different antenna manufacturers and customers.
Diagnostic tools for the determination of the excitation coefficients of a multifeed antenna based on pattern measurements are extremely useful during a spacecraft antenna design. Due to the complexity of state of the art multifeed antennas, it is not straight forward to trace back to the location of possible error sources, if deficiencies or non-compliance's are detected during an antenna measurement campaign. Therefore a method was developed and tested at DSS which directly determines all effective excitation coefficients from pattern measurements. The method approximates the measured composite array pattern a set of computed element beam pattern, weighted by a set of unknown excitation coefficients. The resulting equation system is solved using the Method of Moments (MoM). The tool was extensively tested at DSS. The accuracy obtained for the calculations of the coefficients was in the 2% range beeing compareable to the accuracy of Beam Forming Network (BFN} measurements using a network analyser. In this paper the theoretical background of the method as well as some application cases will be described.
The effect of the platform in the radiation pattern of antennae on board satellites, aircraft or ships has to be taken into account in order to know the actual performance of antenna systems. To have an evaluation of this effect, software prediction codes are developed, providing a fast, cost efficient and comfortable solution compared to the usual measurement campaigns. Nevertheless, these codes have to be validated. Specific tests have been done in order to validate the prediction code FASANT, developed by the Universidad de Cantabria from Spain and based on the Uniform Theory of Diffraction (UTD). A description of the code is first done to follow with the measurement project that has been performed at the INTA facilities in Madrid. A mountable mock-up of the Hispasat satellite has been used to obtain different configurations. Special geometrical shapes have been added to the satellite platform to check for different scattering effects.
Most often when performing antenna and RCS measurements, integrating the results is performed with some type of computer generated simulation or model of the application scenario. In the case of Missile Engagements for Fuze Radars, there is an opportunity to engage full size targets in a near real engagement. The missile fuze antenna can be mounted on the test cart which is able to position the fuze antenna in azimuth, pitch and roll. For instrumentation the MESA Facility has available a PN coded BiPhase multi-range gate radar system. Various Full size targets are available for use in the arena. The target are positioned for a multitude of trajectories utilizing an overhead target positioning system. The Overhead Target Positioning System suspends and moves the targets using a multipoint string system that controls, Pitch, Roll, height, and azimuth positioning. The Overhead Target Positioning System (OTS) is also controlled in lateral movement. (across the range) This paper will show the verification of antenna patterns and RCS returns of full size targets using the MESA Radar system, and verification of these measurements using a hardware in loop fuze radar system simultaneously.
This paper examines antenna measurement errors attributable to instrumentation, and their effect on measurement uncertainty.
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.
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.
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.
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.
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.
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.
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.