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The far-field parameters of an antenna are obtained from near-Field 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 far-field 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 far-field accuracy, planar near-field measurements of a linear array were performed in an anechoid chamber at the Canadian Space Agency. A program performing Fast-Fourier 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 far-field parameters such as gain, beam pointing, beam width, sidelobes, etc.
We review the development and recent performance results of a stand-alone 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 electro-optic 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 pin-point near-field and far-field mapping of radiation patterns. The technology is readily scaleable to frequencies exceeding 20 GHz.
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 input-output (SIO) data flow paradigm. This paper describes a recently-developed Graphical User Interface (GUI) which incorporates the most mature and frequently-used APT algorithms.
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" numerically-generated 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.
The image-based near-field to far-field 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, 1-D and 2-D versions of the transform are applied to conesphere near-field measurements data and the results are compared to corresponding far-field measurements data. Transform errors obtained for these data are compared to corresponding results obtained using newly generated near-field and far-field numerical data. The image-based transform is believed to be especially applicable to the far-field correction of near-field measurements of complicated targets like aircraft or vehicles that are too large or too poorly defined to be simulated numerically.
Far-field 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 near-field measurement system from Nearfield Systems Inc. (NSI) and Hewlett Packard (HP) to improve its antenna measurement capability. The near-field system provides significant advantages over the older far-field 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 near-field system, and the results being achieved.
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 far-field antenna range.
The measurement of the characteristic antenna data by means of conventional far-field 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 mm-wave 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 far-field ranges (N/16). For mm-wave 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: MM-Wave Antenna Measurement, Compensated Compact Range, MM-Wave Transmit Module Tracking Converter
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 near-field phase measurements. We demonstrate the feasibility and convenience of the method with an example planar near-field measurement at 94GHz for a 1.1m Cassegrain reflector and we determine the relationships governing dynamic range and the requirements for sampling. Finally, two-dimensional 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.
The DTU-ESA 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 near-field to far-field (NF-FF) 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.
A waterline bistatic algorithm, based on the exact near field to far field transformation (NFFFT) and previously exercised on numerical data, is here applied to actual measured data taken at a traditional RCS range reconfigured for near field measurements. The resulting far field predictions for a lOA and 20A conesphere were initially worse than expected. Further examination of the data yielded two important observations. First, the data were found to have relative alignment errors from set to set, leading to a significant broadening of the predicted far field peaks. Second, a few data sets exhibited a constant phase offset inconsistent with the other measured data. This paper discusses the detection of the data misregistration issues highlighted above, along with their ad hoc correction. Predictions are give for the waterline bistatic NFFFT algorithm applied to the measured near field data, both before and after the corrections have been applied. The results are compared with analogous results for numerical input data.
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.
A time domain near-field far-field transformation technique based on a non redundant plane-polar sampling representation of the field is presented. The method allows to obtain the far-field with a minimum number of samples and/or a reduction of the scanning area. Various computational schemes are presented.
This paper will address the design and verification procedures for an Elevated Far Field antenna measurement facility at the Naval Explosive Ordinance Disposal Technical Center, Indian Head, MD for operation at 30 MHz – 1000 MHz.
We describe a technique which employs amplitude-only measurements of an unknown antenna combined with a synthetic reference wave to produce a hologram of a near-field antenna distribution. The hologram, which may be recorded by amplitude-only receiving equipment, is digitally processed using an enhanced theory which allows complete removal of the spurious images normally encountered with optical hologram reconstruction. The recovered near-field data are then processed using standard algorithms to calculate antenna far-fields. We present the theoretical formulation and results of measurements obtained on an 1.2 m reflector antenna.
Simulation of the antenna measurement errors caused by scattering of range clutters is presented in this paper. The Uniform Geometrical Theory of Diffraction (UTD) based NEC-Basic Scattering Code is used to simulate the measurement of antenna in a far-field range where structure scatterers present. It is known that these errors which come from various directions will impact the antenna under test differently dependent on the characteristics of the antenna under test. With the available computer codes, one can simulate and study various ranges in order to better understand the characteristics of the ranges and properly adjust, modify, and improve the facility such that better measurement results can be obtained.
Residual reflection characteristics should be evaluated for antenna radiation pattern measurements. Authors propose a method for detecting positions of reflection sources by applying the modified far-field antenna radiation pattern measurement scheme described in [1]. In this method, an “accurate” radiation pattern of antenna under test (AUT) and measurement error patterns due to residual reflected waves are separated by changing a range distance and processing Fourier transformation. Also, the positions of reflected sources can be detected from beam directions of patterns due to reflections at each distance. Experiment results confirm that this method is effective for detecting the positions of reflection sources.
The standard planar near-field to far-field transformation method requires data points on a plane-rectangular lattice. In this paper we introduce a transformation algorithm in which measurements are neither required to lie on a regular grid nor are strictly confined to a plane. Computational complexivity is O (N log N), where N is the number of data points. (Actual calculation times depend on the numerical precision specified and on the condition number of the problem.) This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable for other measurement approaches, such as plane-polar scanning, where data are collected on a non-rectangular grid.
In this paper, a near-field time domain radiation measurement is described, similar to the traditional frequency domain near-field radiation measurement. This time domain measurement approach borrows many of the principles developed in the frequency domain and is ideally suited for the measurement of broadband devices. The goal of determining the radiated far-fields of an antenna is accomplished by the transformation of near-field data collected over a planar sampling surface. The near-fields are generated with an antenna excited by a short duration transient pulse. In particular, the near-fields of an aperture antenna are collected using a digital sampling oscilloscope. The bandwidth of the excitation pulse is approximately 10 GHz.
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.