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Proper operation of a planar NFR (near field range) includes probe correction as part of the processing of the measured data to result in accurate far field angle patterns, particularly for low cross polarized patterns. The far field transform of the near field data produces the angular spectrum which is the product of the plane wave transmission coefficient pattern of the AUT (antenna under test) with the plane wave receiving coefficient pattern of the probe. Probe correction consists of dividing the angular spectrum by the complex probe angle pattern resulting in the pure far field pattern of the AUT [1]. For best accuracy of co and cross polarized AUT patterns one needs to use accurately measured probe complex co and cross polarized patterns in probe correction for each NFR test frequency. The most accurate probe measurements are usually obtained from specialized test laboratories. However, if the number of frequencies is large, this may create problems due to cost or schedule. Because of this it is typical to procure probe calibration at only a few frequencies spanning the test band for each AUT even though pattern measurements are needed at several additional frequencies falling between the calibration frequencies. A typical strategy at any given test frequency is to perform probe correction using the nearest-neighbor-frequency probe calibration data. This strategy produces some unknown error in the processed probe corrected far field patterns of the AUT at each non-calibrated frequency. Inthis paper we will show a method for estimating the non-calibrated frequency probe correction error for co and cross polarized patterns with examples.
A bias error in planar near-field measurement data comes from receiver crosstalk or leakage effects [1, 2, 3]. The bias error is a complex constant added to every near-field data sample. After transformation from the near-field to the far-field, the bias error becomes an easily identifiable spike located at the center of k-space. If one is measuring a horn, then the bias error produces a small bump or spike at the center of the far-zone pattern (i.e. at the center of k-space). If one is measuring a highgain antenna with the antenna beam pointed away from the center of k-space, then the bias error causes an erroneous sidelobe spike at the center of k-space. The bias constant is difficult to estimate be cause it may be more than 60 dB below the peak near-field level. Nevertheless, if the effect of the bias error can be seen in the far zone pattern of the test antenna, then it can be estimated and removed from the measured data. An algorithm is presented that is used to estimate the bias constant directly from the near-field data, then the bias constant is simply subtracted from the data. Examples using measured data are used to illustrate how the algorithm works and to show its effectiveness.
The NIST 18 term error budget is used to estimate the magnitude of each individual source of error and then combine them to the total uncertainty for the planar nearlield range designed for antenna characterization of the ERIEYE Airborne Early Warning System. The ERIEYE AEW System consists of two large phased array antennas, one at each side of the Dorsal Unit which is located on the top of the airplane fuselage. T/R-modules are connected to the antenna waveguides to control the beamsteering and the very low sidelobe level. The sidelobe level is supervised by a calibration during operation, using a table of calibration data. The table of calibration data is produced by iterative computer runs of programs performing the two transformations Near-field-to-Far-field and Far-field-to-Waveguide Excitation - the characterization. Characterization to very low sidelobe level in the calculated farfield is possible when using for instance planar nearfield technique to measure an active antenna. The errors at the planar nearfield range are misleadingly compensated for by the characterization. Therefore a minimization together with a continuous control of the noise level is necessary.
Traditional measurement methods assume that very accurate antenna to range alignment of the antenna under test (AUT) is convenient or possible. It has recently been shown that the use of non-rectilinear co-ordinate systems are of particular use for the purpose of correcting antenna to range misalignment. Additionally, this misalignment correction can be used to construct an extended composite measurement plane from a series of mis-aligned scans that themselves can be considered as constituting a polyhedral measurement surface. This paper describes the additional processing that is required to yield corrected near and far field data from an acquisition of a mis-aligned AUT. This technique is then illustrated with example results. The agreement of the corrected results is determined via the application of image classification techniques which correlate antenna patterns in a reduced vector pattern space in terms of their overall global features.
Compact range collimating reflectors provide far-field conditions for radar signature measurements. Traditionally, the quiet zone is presented uniformly about the collimator boresight and depends upon both the size of the reflector and the beamwidth of the illuminating antenna, with a maximum determined by the reflector dimensions. Targets are placed in the center of the quiet zone and rotated about the center of gravity (cg) during measurement. Limitations on target size are defined by the quiet zone bounds. For large targets with a non-central cg location, a portion of the target may extend beyond the quiet zone boundary. A technique for synthesizing a larger quiet zone uses displacement of the collimator beam by means of feed point offset to allow far-field measurement of an asymmetrically-mounted extended target. Simultaneous measurements for each offset are then combined to produce the complete measurement. This technique was implemented for measurements of an ARIES ballistic missile target.
A near-field antenna range will often utilize a flexible microwave cable assembly as a means to transport the sampled signal from the moving sample antenna to a receiver as part of the measurement system. The performance of that cable directly impacts the quality of the final far-field pattern. It has been observed that the cable had been exhibiting a flex life much shorter than anticipated. Analysis of a failed cable revealed that the problem was the result of non-uniformities in the extruded jacket, which produced sites of high stress. These sites ultimately caused the cable conductors to work harden and fracture. A cable which utiized a woven expanded Polytetrafluoroethylene (ePTFE) fiber as an outer jacket was substituted, resulting in a threefold improvement in flex life to date, with the cable still in operation at this writing.
SATIMO has recently installed a spherical near-field antenna measurement system for ALLGON MOBILE COMMUNICATIONS, the market leader in the field of antennas for mobile telephones. This spherical near-field system, as shown in Figure 1, allows for real-time measurements of antennas and will among other be used for the measurements of the radiation characteristics of mobile telephones and satellite terminals in the presence of the human operator. The system consists of a circular of 4m diameter containing 64 dual polarized measurement which are electronically scanned giving a real-time near-field pattern cut over 310° in elevation. A full sphere measurement including near-field to far-field transformation is available in seconds with a single +/- 90° azimuth rotation. The paper will present the measurement system and the results of the final acceptance tests. The acceptance tests are based on both range inter comparisons and also by measurement of key terms in the overall error budget.
The development and implementation of novel measurement systems for rapid electromagnetic (EM) field testing by using linear arrays of modulated scattering elements is presented and discussed. The measurement systems employ the Advanced Modulated Scattering Technique (A-MST) to accomplish rapid sampling of the incident electromagnetic field along the length of the linear probe array at rates that are faster than conventional mechanical scanning of a single probe by a factor of 10 to 1000 or more. The A-MST probe array may be located in the nea r-field (NF) or far-field (FF) of the EM sources.
Characterization of radiated EMI by means of near zone measurements is examined by computer simulations. Electric field radiated by a test structure is expanded in spherical wave modes. The influence of the number of spherical wave modes on the accuracy to predict the maximum far-field magnitude and the total radiated power is examined. The examinations of this paper support the electric field measurements of small equipment at small measurement distances in the standard radiated EMI frequency range 30 - 1000 MHz. Results are presented as a function kr0, where k is the wave number and r 0 is the radius of the minimum sphere which fully encloses the EUT. Results of this paper give valuable guidelines for choosing an appropriate number of measurement locations for predicting the far field by means of a near-zone measurement.
Characterization of radiated EMI by means of near zone measurements is examined by computer simulations. Electric field radiated by a test structure is expanded in spherical wave modes. The influence of the number of spherical wave modes on the accuracy to predict the maximum far-field magnitude and the total radiated power is examined. The examinations of this paper support the electric field measurements of small equipment at small measurement distances in the standard radiated EMI frequency range 30 - 1000 MHz. Results are presented as a function kr0, where k is the wave number and r 0 is the radius of the minimum sphere which fully encloses the EUT. Results of this paper give valuable guidelines for choosing an appropriate number of measurement locations for predicting the far field by means of a near-zone measurement.
Saab Ericsson Space and Ericsson Microwave Systems have recently completed the installation of a new efficient test facility. The facility is a fully automated test range designed for high th roughput of measurements. The facility is mainly used for tests of antennas for satellites and for mobile com munication. It is used as a far-field range for small antennas or as a spherical near field range for directive antennas. The frequency range covered is 0.8 - 40 GHz. A design driver for the facility was the logistics of measurements, short test time and easy access to the AUT during measurements. To achieve this, high speed positioners and easy access to the AUT via a drawbridge in the anechoic chamber were introduced. The computer controlled RF system allows the use of automatic mode switching to test the AUT in either receive or transmit mode and to change frequencies and mixers without operator intervention.
Design optimization and measurement of the Luneburg lens antennas are the focus of this paper. One of the important design aspects of an optimal Luneburg lens antenna is to construct a high performance lens with as low number of spherical shells as possible. In a uniform Luneburg lens, the gain is decreased and unwanted grating lobes are generated by reducing the number of shells. This deficiency in the radiation performance of the uniform lens may be overcomed by designing a non uniform lens antenna. The optimized non-uniform spherical lens antenna is designed utilizing the dyadic Green's function of the multi-layered dielectric sphere integrated with a Genetic Algorithm (GA)/Adaptive Cost Function optimizer. Additionally, a novel 2-shell lens antenna is studied and its performance is compared to the Luneburg lens. Finally, measured results for far field patterns and holographic images are shown for the Luneburg lens antenna using the UCLA's bi-polar near field facility.
Airborne Doppler Velocity Sensors require precise boresight information in determining a Doppler solution. Far-field ranges have been extensively used to provide this boresighting capability. This paper discusses an empirical investigation to determine the feasibility of using near-field techniques to fulfill the boresighting requirement.
A microwave holographic technique based on equivalent magnetic sources reconstruction is presented. This technique, initially used as a main plane near-field to far-field (NF-FF) transformation, can also be used to detect defective elements in arrays as well as to detect irregularities in the surface of reflector antennas.
Electromagnetic field measurement systems based on the Advanced Modulated Scattering Technique (A MST) permit fast and accurate diagnostic testing to be performed in the near-field (NF) or the far-field (FF) of antennas and scattering objects. A-MST probe arrays are particularly effective for rapid diagnostic testing applications where it is desired to obtain overall measurement duration reductions of 80% to 98% compared to conventional single-probe measurement times.
This paper describes an antenna measurement system which combines three types of measurements into one integrated range operating from 45 MHz through 18 GHz. A vehicle can be measured at short ranges inside a protective enclosure at various elevation angles for both low frequency Far Field (FF) measurements and higher frequency Near Field (NF) measurements. The vehicle can also be measured on an extended FF 120m range by radiating through the transparent enclosure. The vehicle enters a 12m radius radome and is mounted on a 6m diameter turntable which enables continuous rotation of the vehicle at a maximum speed of 3 rpm. An elevation positioner moves a lOm arm equipped with linear and roll axes at the top, which provide the NF probe movement. Azimuth rotation of the vehicle and elevation movement of the arm provide a complete hemispherical scan. During FF measurements from outside of the radome, the arm is stored below ground level and is covered.
In this paper, a near-field time domain scattering measurement technique is described. Near-field measurements are typically performed for radiation applications but not scattering applications. This time domain measurement approach borrows from many of the principles developed in the frequency domain and is ideally suited for broadband scattering characterization. The goal of determining the scattered far-fields of a structure is accomplished by the transformation of near-field data collected over a planar sampling surface. The scattered near-fields were generated with a probe excited by a fast rise time step. In particular, the near-fields were sampled with a second probe and digitized using a digital sampling oscilloscope. The bandwidth of the excitation pulse was approximately 15 GHz. The overall accuracy of this approach is examined through a comparison of the transformed far-field pattern to a numerical calculation.
Over the past six years the Navy has developed a portable measurement capability. As part of the validation of this tool a comparison test was developed to understand the issues involving testing complex targets in a near-field cluttered environment. The test was designed to evaluate not only the effects of near field curvature, but how clutter from ceiling and walls have an effect on the accuracy of the measurement. The test measured all test objects in the far-field as a baseline, then repeated the same measurements at five different near-field configurations. The results of the test will be shown on a simple 15 ft. pole target, along with the metrics for evaluation of the results.
The scattered field from an arbitrary target may include a variety of scattering mechanisms such as specular and diffraction terms, creeping waves and resonant phenomena. In addition, buried within such data are target-mount interactions and clutter terms associated with the test environment. This research presents a method for decomposing a broadband complex signal into its constituent mechanisms. The method makes use of basis functions (words) which best describe the physics of the scattered fields. The MUSIC algorithm is used to estimate the time delay of each word. A constrained optimization refines the estimate and determines the energy for each. The method is tested using two far-field radar cross section (RCS) measurements. The first example identifies targetmount interactions for a common calibration sphere. The second example applies the method to a low observable (LO) ogive target.
DATE is a portable, rapid assembled, planar near field measurement system for ERIEYE Airborne Early Warning System. DATE shall be used both as a production range at Ericsson Microwave Systems (EMW) and as a maintenance equipment delivered with the ERIEYE AEW System. Up to now ERIEYE has been measured and phase aligned at EMW's large nearfield range. The active antenna is interfaced through a Beam Steering Computer (BSC) and hardware interface. The disadvantages with this approach is a slow communication speed and reduced Built In Test. Since the large nearfield range is designed to meet the requirements from many different antenna types the transport, mounting, alignment and range error analysis are very time and personnel consuming. The DATE-scope is to provide a portable planar near field test system that's custom-made for ERIEYE. The time from stored system to completed measurement shall be very short and performed by a "non antenna test engineer". This is done by: • Incorporate the BSC as a radar-mode. • Use the radar receiver and transmitter for RF measurement. • Reduce alignment time and complexity by a common alignment system for antenna and scanner. Scanner alignment for very high position accuracy. • Automatic Advanced Data Processing: Transformation from near field to far field to excitation to new T/R-module setting-up-table in one step.