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Far Field
Technique to reduce the scan length in nearfield antenna measurements, A
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.
Cylindrical nearfield measurement of Lband antennas
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 22foot vertical by 5foot diameter cylindrical nearfield 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. Farfield patterns, nearfield data and even individual element amplitude and phases are graphically displayed.
Nearfield data processing using MATLAB version 5.0
A sophisticated software package FARANA (FARfield ANAiysis) is presented for transforming planar nearfield test data to farfield antenna patterns, including enhanced analysis of farfield 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 stateoftheart graphical user'sinterface, is intuitive to use and features high speed and accuracy.
This paper addresses an assessment of the program, discusses its use and enhanced farfield analysis capabilities.
Microwave antenna farfield patterns determined from infrared holograms
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 farfield 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 nearfield techniques. The algorithm gives the complex near field of the antenna under test which can then be processed to obtain the farfield pattern of the antenna under test. We present results showing farfield patterns which acceptably reproduce the main beam and near sidelobes. Such techniques will allow rapid testing of certain antenna types.
Simulation of planar nearfield errors
When a planar nearfield 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 nearfield from an antenna has been developed. The nearfield is transformed to farfield and the errors are evaluated. This paper looks into four different error types: 1) Truncation errors (if the measurement surface is to small the nearfield will be truncated before it reaches adequately low levels), 2) ProbeAUT 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.
Range validation testing of a planar nearfield range facility at Hughes Space and Communications Co.
A series of measurements to validate the performance of a Vertical Planar Nearfield 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 farfield range pattern comparison tests using an offset reflector antenna as the validation antenna. This antenna had been previously measured on a farfield 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 farfield patterns measured on the validation antenna at both the farfield and nearfield ranges is presented.
Plane wave, pattern subtraction, range compensation for spherical surface antenna pattern measurements
This paper presents a new technique for performing range compensation of full sphere antenna patterns measured on fixed lineofsight antenna ranges where pattern measurements are made over a spherical surface. Such ranges include farfield, compact, and spherical nearfield ranges. A plane wave model of the range field illuminating the antenna under test (AUT) is determined as described in another paper. This plane wave model consists of a small, selectable number of plane waves. Equations are given describing the transformation of range coordinates to AUT coordinates. This allows the response of an AUT to a plane wave from an arbitrary direction to be defined using only the farfield pattern of the AUT. The error pattern added to the pattern measurement by the extraneous plane waves is then estimated using the plane wave model and the measured pattern. This error pattern is subtracted from the antenna pattern measurement to obtain a compensated pattern. The compensated pattern and error pattern are improved iteratively. This paper demonstrates the technique using simulated data. The rotation of the spherical AUT grid with respect to the range grid during the measurement requires an interpolation of the measured fields to estimate the error pattern. Investigations of interpolation error are presented. The computational complexity of the compensation algorithm, excluding the plane wave model, is on the order of the number of measurement points on the spherical measurement grid.
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Novel cellular/PCS basestation antenna measurement system, A
Cellular and PCS basestation antennas are basically arrays with highly directive elevation patterns and broad azimuth patterns. This causes measurement problems because they are large but not directive in both principal planes. As a result, the pattern measurements of these antennas that have been performed outside have been unreliable in many cases because they are very receptive to interference and range clutter. Thus, one wants to move inside but the antenna size can significantly impact the overall range cost. This paper describes a very practical solution to this problem.
Since basestation antennas are long and narrow, one can use a near field scanner approach to deal with the length. In fact by using a sectorial horn probe, the narrow dimension of the antennaundertest is illuminated by a cylindrical wave. Thus, the scanner need only probe the field along the antenna length. This linear scan data can then be transformed to generate the desired far field elevation pattern. The details of this novel design will be described as well as the results, to illustrate the system capability and accuracy.
Nearfield measurement deconvolution
A technique was developed to recover the nearfield function on a larger data set than the one that is measured. It requires the preliminary determination of functions containing the information relating the two data sets. The simplest way of obtaining such a function is to measure the nearfield function on the larger and the smaller data set. This seems to be a drawback to the technique. However. after making one such pair of measurement it is therefore non necessary to do so again and the field of the antenna can be obtained, from the smaller data set measurement, with comparable accuracy. The technique is somewhat different when compensating for a sampling rate reduction. However, in both cases an analytical extension is required to fill the desired domain of definition, followed by a division. In the case of the sampling area the division of the spectral functions f2 by f1 is made in the spectral domain while in the case of the sampling rate the division of the nearfield functions E' by E is made in the nearfield domain.
An experiment was performed to demonstrate the applicability of the above technique. A full nearfield measurement of a linear array antenna was performed and processed, then after displacement of the antenna, measurements were done, in one case, on a truncated smaller scan area, and in another case with a larger sampling interval. The technique was applied to recover the complete farfield characteristics of the antenna from the smaller data set. The farfield characteristics of the antenna obtained by this technique were shown to be very similar to the results obtained from a more complete near field measurement.
Efficient uniform geometrical theory of diffraction based far field transformation of spherical near field antenna measurement data, An
A method is presented for computing far field antenna patterns from spherical near field antenna measurement data. The new method utilizes a novel Uniform Geometrical Theory of Diffraction (UTD) based transformation of spherically scanned antenna tangential electric (or magnetic) near field measured values to more efficiently obtain the antenna far field. Examples illustrating the accuracy and speed of UTD based spherical near to far field transformations for large to moderately large antennas are presented.
Farfield accuracy vs sampling parameters of a linear array
The farfield parameters of an antenna are obtained from nearField measurement with an accuracy that is limited by the sampling area and the sampling rate used to collect the measurement data. It is therefore important to know the relation between the farfield parameters and the sampling parameters. A parametric study of the far field parameters accuracy versus the sampling parameters was made. In order to determine the optimal choice of the sampling parameters to achieve the desired farfield accuracy, planar nearfield measurements of a linear array were performed in an anechoid chamber at the Canadian Space Agency. A program performing FastFourier Transform was used to process the data and to obtain spectral domain and reconstruct the far field patterns. A methodology developed in [1] was used to compare different spectral and far field patterns obtained from different sampling conditions. Parametric curves were developed for the farfield parameters such as gain, beam pointing, beam width, sidelobes, etc.
Minimally perturbing photonic broadband EM field sensor system with environmental compensation
We review the development and recent performance results of a standalone fiber optic based EM field sensor system. The sensor heads are miniature (lcm), electrically passive, and are directly coupled to optical fibers at the remote sensing site. Sensor conversion of EM fields to optical intensity is carried out by mounting small antenna structures directly onto high speed lithium niobate electrooptic modulator chips. Optical power to the sensor head is derived from a stabilized laser which is located within a system chassis at a control room location. Sensor and fiber temperature drift effects are compensated by specialized remote bias control electronics. Recent broad spectrum tests have demonstrated a system bandwidth of about 20 GHz, and a minimum detectable field in the lO's of mV/m. Ultra wideband pulse measurements have demonstrated real time pulse signals of about 2 Vpp for 3 KV/m fields. The sensor system is slated for application in EMI effects such as EM compatibility, and for pinpoint nearfield and farfield mapping of radiation patterns. The technology is readily scaleable to frequencies exceeding 20 GHz.
Graphical user interface for the APT/IMGMANIP toolbox, A
Ell has been extensively involved in the development of advanced processing techniques (APT) to improve the quality and utility of both indoor and outdoor RCS/ISAR measurements. These include algorithms for removal of clutter, RFI, and targetsupport contamination (including interactions), prediction of far field RCS from near field measurements, suppression of multipath contamination, and extraction of scattering features/components. These techniques have been implemented in a framework based on ERIM International's IMGMANIP signal/ image processing toolbox and stream inputoutput (SIO) data flow paradigm. This paper describes a recentlydeveloped Graphical User Interface (GUI) which incorporates the most mature and frequentlyused APT algorithms.
Application of a MoMbased network model NFFFT to measured conesphere data
Based on the method of moments (MoM), a network model algorithm perturbs the linearized electromagnetic interaction model (the admittance matrix) of a simulated target to match an actual measured data set in the least squares sense, resulting in a more accurate interaction model for the physical target. Since the admittance matrix is independent of source location this technique is amenable for use as a near field to far field transform algorithm. In this ansatz, a MoM admittance matrix is perturbed to match a set of near field measurements, then the far zone field is predicted using the perturbed admittance matrix multiplied by the appropriate far field measurement vector.
This paper describes the application of the MoM network model technique to measured and numerical data for ten and twenty wavelength conespheres. Initially, a discussion is given of the code modifications necessary to adapt JRMBOR for network model use. A validation is then provided using "perfect" numericallygenerated near field data to perturb an admittance matrix rendered inaccurate through a deliberate undersampling of the conesphere geometry.
Finally, results are given for the MoM network model algorithm with measured near field data, with the resulting predictions compared to measured far field truth. Algorithm performance is examined as a function of frequency for monostatic near field input data.
Application of an imagebased nearfield to farfield transformation to measured data
The imagebased nearfield to farfield transformation is based on a reflectivity approximation that is commonly used in ISAR imaging. It is a limited but computationally efficient transform whose accuracy, for appropriate targets, rivals that of computationally more intense transforms.
Previous results include applications of the transform to lOA. long wire and lOA. long conesphere numerical data. Here, 1D and 2D versions of the transform are applied to conesphere nearfield measurements data and the results are compared to corresponding farfield measurements data. Transform errors obtained for these data are compared to corresponding results obtained using newly generated nearfield and farfield numerical data.
The imagebased transform is believed to be especially applicable to the farfield correction of nearfield measurements of complicated targets like aircraft or vehicles that are too large or too poorly defined to be simulated numerically.
Implementation of a spherical nearfield measurement system in mainland China
Farfield range testing has been the standard at the Southwest China Research Institute of Electronic Equipment (SWIEE) and at other facilities in mainland China. SWIEE has recently commissioned a new spherical nearfield measurement system from Nearfield Systems Inc. (NSI) and Hewlett Packard (HP) to improve its antenna measurement capability. The nearfield system provides significant advantages over the older farfield testing including elimination of weather problems with outdoor range testing, complete characterization of the antenna, and improved accuracy. This paper will discuss the antenna types at SWIEE tested with the NSl/HP nearfield system, and the results being achieved.
Practical considerations for making pulsed antenna measurements
Antenna designs continue to evolve and change as the telecommunications market expands, and current trends are towards more complex and higher performance antennas. In particular active transmit/receive (T/R) modules have enabled manufacturers to build antennas with multiple beams and significantly improved performance. These antennas present challenges for performance verification and testing not previously encountered in continuous wave (CW) antenna measurements. For example, testing in a pulsed operating mode, multiple beam state testing and testing in high power transmit and low power receive modes.
This paper examines pulsed antenna measurements and considerations for the range design. An instrumentation configuration is presented for a pulsed farfield antenna range.
Sensor measurements up to 200 GHz in the compensated compact range with broadband transmit and receive modules
The measurement of the characteristic antenna data by means of conventional farfield ranges in frequencies up to 200 GHz requires measurement distances of some kilometers. The high atmospherical attenuation and the low available transmit power limit the dynamic range of the measurements considerably. The DASA Compensated Compact Range (CCR) /1/ is a high precision test facility; which avoids these disadvantages and allow measurements with considerably higher accuracy under controlled environmental conditions. The precision reflectors have an extremely high surface accuracy of 25 µm RMS, which allow their use even in the mmwave range. For the frequency band of about 200 GHz, the relative roughness is in the order of N/60. This results in considerably lower degradation for the DASA CCR compared to the typical degradation on farfield ranges (N/16).
For mmwave application the test facility is equipped with broadband transmit and receive moduls, which covers the frequency range from 75 to 220 GHz. The basic transmit frequency is generated in a tunable Gunn oscillator, which is phaselocked to an externally supplied I 0 MHz reference signal. This optimized concept allows measurements with a dynamic range of more than 60 dB at 200 GHz. For a cost efficient solution the complete equipment for the transmit and receive moduls consists of commercial components.
Keywords: MMWave Antenna Measurement, Compensated Compact Range, MMWave Transmit Module Tracking Converter
Holographic nearfield/farfield for TeraHertz antenna testing
Gabor holography is an appropriate technique for near field measurements at THz frequencies when apertures of the order of thousands of wavelengths are involved. The method permits pattern prediction over a restricted angular range from intensity measurements, providing a direct method of recovering phase which overcomes cable, planarity and atmospheric effects; problematic to conventional nearfield phase measurements. We demonstrate the feasibility and convenience of the method with an example planar nearfield measurement at 94GHz for a 1.1m Cassegrain reflector and we determine the relationships governing dynamic range and the requirements for sampling. Finally, twodimensional numerical simulations for a lm antenna at 0.5THz, with a 10m scan distance, will be presented to demonstrate the feasibility of the method for large terahertz antennas.
Alignment errors and standard gain horn calibrations
The DTUESA Spherical Near Field Antenna Test Facility in Lyngby, Denmark, which is operated in a cooperation between the Danish Technical University (DTU) and the European Space Agency (ESA), has for an ex tensive period of time been used for calibration of Standard Gain Horns (SGHs).
A calibration of a SGH is performed as a spherical scanning of its near field with a subsequent nearfield to farfield (NFFF) transformation. Next, the peak directivity is determined and the gain is found by subtracting the loss from the directivity. The loss of the SGH is determined theoretically.
During a recent investigation of errors in the measurement setup, we discovered that the alignment of the antenna positioner can have an extreme influence on the measurement accuracy. Using a numerical model for a SGH we will in this paper investigate the influence of some mechanical and electrical errors. Some of the results are verified using measurements. An alternative mounting of the SGH on the positioner which makes the measurements less sensitive to alignment errors is discussed.

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