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Far Field
Exploration of the Feasibility of Adaptive Spherical NearField Antenna Measurements
The feasibility of using adaptive acquisition techniques to reduce the overall testing time in spherical nearfield (SNF) antenna measurements is investigated. The adaptive approach is based on the premise that nearfield to farfield (NFFF) transformation time is small compared to data acquisition time, so that such computations can be done repeatedly while data is being acquired. This allows us to use the transformed FF data to continuously compute and monitor predefined decision functions (formed from the antenna specifications most important to the particular AUT) while data is being acquired. We do not proceed with a complete scan of the measurement sphere but effectively allow the probe to follow a directed path under control of an acquisition rule, so that the sampled NF datapoints constitute an acquisition map on the sphere (the geographical allusion being purposeful). SNF data acquisition can be terminated based on decision function values, allowing the smallest amount of data needed to ensure accurate determination of the AUT performance measures. We demonstrate the approach using actual NF data for several decision functions and acquisition rules.
Advanced Spherical NearFieldToFarField Software for Modern Computers
The speed of spherical nearfield scanning is increased significantly when measurements are not restricted to standard measurement locations, i.e., the locations that are equidistant in theta and in phi. Measurement positions can be chosen so that mechanical positioners perform scans with a continuous motion; this will decrease the time it takes to acquire data for nearfield measurements. The issue then becomes transforming the data acquired with nonuniform spacing. This paper describes the development of a spherical nearfield to farfield transform that can efficiently process data acquired on a nonuniform grid.
An Improved Antenna Gain Extrapolation Measurement
An improved system for antenna gain extrapolation measurements is proposed. The improved method consists of a vector network analyzer, a pair of RF optical links, and a pair of waveguide mixers. This change in hardware equates to a system with better dynamic range and a simplified reference measurement. We present a detailed description of the new extrapolation measurement setup, discuss the advantages and disadvantages, and validate the new setup by measuring the gain of an antenna previously measured with a traditional extrapolation setup. After presenting the comparison, we will discuss applications of this measurement system that extend beyond extrapolation gain measurements (e.g., spherical near and farfield pattern measurements).
Estimating the Effect of Higher Order Modes in Spherical NearField Probe Correction
The numerical analysis used for efficient processing of spherical nearfield data requires that the farfield pattern of the probe can be expressed using only azimuthal modes with indices of µ = ±1. (1) If the probe satisfies this symmetry requirement, nearfield data is only required for the two angles of probe rotation about its axis of . = 0 and 90 degrees and numerical integration in . is not required. This reduces both measurement and computation time and so it is desirable to use probes that will satisfy the µ = ±1 criteria. Circularly symmetric probes can be constructed that reduce the higher order modes to very low levels and for probes like open ended rectangular waveguides (OEWG) the effect of the higher order modes can be reduced by using a measurement radius that reduces the subtended angle of the AUT. Some analysis and simulation have been done to estimate the effect of using a probe with the higher order modes (2) – (6) and the following study is another effort to develop guidelines for the properties of the probe and the measurement radius that will reduce the effect of higher order modes to minimal levels. This study is based on the observation that since the higher order probe azimuthal modes are directly related to the probe properties for rotation about its axis, the nearfield data that should be most sensitive to these modes is a nearfield polarization measurement. This measurement is taken with the probe at a fixed (x,y,z) or (.,f,r) position and the probe is rotated about its axis by the angle .. The amplitude and phase received by the probe is measured as a function of the . rotation angle. A direct measurement using different probes would be desirable, but since the effect of the higher order modes is very small, other measurement errors would likely obscure the desired information. This study uses the planewave transmission equation (7) to calculate the received signal for an AUT/probe combination where the probe is at any specified position and orientation in the nearfield. The plane wave spectrum for both the AUT and the probe are derived from measured planar or spherical nearfield data. The plane wave spectrum for the AUT is the same for all calculations and the receiving spectrum for the probe at each . orientation is determined from the farfield pattern of the probe after it has been rotated by the angle .. The farfield pattern of the probe as derived from spherical nearfield measurements can be filtered to include or exclude the higher order spherical modes, and the nearfield polarization data can therefore be calculated to show the sensitivity to these higher order modes. This approach focuses on the effect of the higher order spherical modes and completely excludes the effect of measurement errors. The results of these calculations for different AUT/probe/measurement radius combinations will be shown.
Using Spherical Nearfield Transforms to Determine the Effects of Range Length on the Measurement of Total Radiated Power
Total radiated power (TRP) and total isotropic sensitivity (TIS) are two metrics most commonly used to characterize the performance of a wireless device. These integrated measurement parameters are not as sensitive to the measurement distance as a single point measurement such as an antenna gain measurement, but it is difficult to accurately quantify the effects of measurement distance on these two parameters. This paper presents a simple approach to quantifying the effects of measurement distance using spherical nearfield transforms. Data is taken on a typical wireless device at different range lengths and transformed to the farfield using a spherical nearfield transform. The total radiated power is then calculated for both the measured data and the transformed data. The difference in the two calculations shows the effect of a finite range length on the measurement. Measured results are presented for three different range lengths. For each of these range lengths the data is transformed to the farfield and the TRP is calculated.
Multiport RF 16Switch Controller at Air Force Research Laboratory Rome's Newport Test FacilityMultiport RF 16Switch Controller at Air Force Research Laboratory Rome's Newport Test Facility
The Air Force Research Laboratory (AFRL), Information Directorate, has served as an Air Force center for antenna measurements for over thirty years. AFRL's Newport Research Facility consists of multiple far field outdoor test ranges with 3axis positioner towers. The range supports a wide variety of test activities including measurements on simple antennas, complex active phased arrays, avionics, communications and electronic countermeasure systems. The trend towards increasingly complex antenna systems led to a requirement for a faster, more adaptable data acquisition system. To support the changing test requirements, AFRL developed a multiport 16RFswitch controller as part of our data acquisition system. In a typical antenna test at Newport, multiple aircraft antennas are multiplexed into a single RF output through a programmable matrix of solid state RF switches. The switch controller is installed inside the aircraft under test which is mounted on a 3axis positioner. We can then configure and reconfigure each aircraft for a variety of antenna tests at Newport.
Shortcomings in Simulating Formulas for the Farfield Pattern emitted by a Kband Openended Rectangular WaveguideShortcomings in Simulating Formulas for the Farfield Pattern emitted by a Kband Openended Rectangular Waveguide
Measurement of E and H plane far field patterns for an openended rectangular waveguide in the free air operating between the frequencies of 16 and 19 GHz are shown and compared with the simulated patterns derived by several authors. Although the theoretical expressions give a broader pattern for the Eplane than for the Hplane, which is observed, measurements exhibit a sharper decay in the Eplane than the one obtained by simulation. In this work, we calculate the errors associated with the use of the different models that fail to correctly approximate the Eplane. Finally, we introduce a parameter in the best model to adjust the effective aperture dimensions in order to obtain a more realistic representation of the measured far field.
Shortcomings in Simulating Formulas for the Farfield Pattern emitted by a Kband Openended Rectangular WaveguideShortcomings in Simulating Formulas for the Farfield Pattern emitted by a Kband Openended Rectangular Waveguide
Measurement of E and H plane far field patterns for an openended rectangular waveguide in the free air operating between the frequencies of 16 and 19 GHz are shown and compared with the simulated patterns derived by several authors. Although the theoretical expressions give a broader pattern for the Eplane than for the Hplane, which is observed, measurements exhibit a sharper decay in the Eplane than the one obtained by simulation. In this work, we calculate the errors associated with the use of the different models that fail to correctly approximate the Eplane. Finally, we introduce a parameter in the best model to adjust the effective aperture dimensions in order to obtain a more realistic representation of the measured far field.
A Cone Shaped Taper Chamber For Antenna Measurements Both In Near Field And Far Field In The 200 Mhz To 18 Ghz Frequency Range
Traditionally Taper chambers are constructed using a square based pyramidal shaped taper. The taper is then shaped into an octagon and finally transformed into a cylindrical launch section. This approach is related to the manufacturability of different absorber cuts. This presentation introduces a chamber where the conical shape of the launch in continued through the entire length of the taper chamber. The results are of the free space VSWR test over a 1.5m diameter quiet zone are presented at different frequencies. The conical taper appears to have a better illumination wave front and better QZ levels even at frequencies above 2GHz than the standard traditional approach. The implementation of this design was done in Singapore and the actual chamber was designed to have a secondary Near Field range for Planar and spherical scans.
Comparative Probe Parameter Error Analysis For Planar
NearField Measurements With A Novel Approach For
Reduced ProbeAut Interaction
Farfield uncertainty due to probe errors in planar nearfield measurements is analyzed for the fast irregular antenna field transformation algorithm. Results are compared with the classical technique employing two dimensional Fast Fourier Transform (2D FFT). Errors involving probe's relative pattern, alignment, transverse and longitudinal position, interaction with AUT etc. have been considered for planar measurements. The multiple reflections error originating from the interaction of the probe and the AUT tends to deteriorate the radiation pattern to a greater extent. Therefore, a novel technique which utilizes nearfield measurements on two partial planes is presented to reduce the multiple reflections between the probe and the AUT.
An Experimental Validation Of The NearField  FarField
Transformation With Spherical Spiral Scan
This work concerns the experimental validation of a probe compensated nearfield – farfield transformation technique using a spherical spiral scanning, which allows one to significantly reduce the measurement time by means of continuous and synchronized movements of the positioning systems of the probe and antenna under test. Such a technique relies on the nonredundant sampling representations of the electromagnetic fields and makes use of a twodimensional optimal sampling interpolation formula to recover the nearfield data needed to perform the classical spherical nearfield – farfield transformation. The good agreement between the so reconstructed farfield patterns and those obtained via the classical spherical nearfield – farfield transformation assesses the effectiveness of the approach.
Exact Solutions In Antenna Holography Using
Planar, Spherical, Or Cylindrical NearField Data
We present exact solutions to antenna holography problems based on planar, spherical, or cylindrical nearfield data. Full field distribution information in the source region is determined exactly, from two tangential field components over a planar, spherical, or cylindrical surface. Stated in so many words, all three components of both electric and magnetic fields in the antenna aperture are obtained exactly from twocomponent nearfield data. Conventional antenna holography relies upon back transformation for planar nearfield data, and upon optimization schemes for both spherical and cylindrical nearfield data. It is both acknowledged and accepted that the back transform is only an approximate solution due to its farfield nature, whereas optimization algorithms are vulnerable to convergence instability and, moreover, are computationally intensive. Our approach tackles holography by solving an inverse scattering problem, with exact solutions derived on the basis of three common types of nearfield data. A mapping algorithm is proposed herein which determines the field everywhere, in both interior and exterior regions, based on a singleslice nearfield data capture. It provides exact antenna holography solutions analytically, with the full electric and magnetic fields disclosed throughout the source region. The field mapping algorithm is a direct, closedform solution which is numerically straightforward and efficient. Verification is carried out and demonstrated by analytic examples and numerical simulations, as well as by hardware measurements. Nine test examples are given. Analytic examples include dipole arrays deployed across planar, spherical, and cylindrical regions, and a narrow azimuthal slot on a conducting sphere. The simulation example exposes the structure of a slotted array antenna based upon its nearfield data as generated by a commercial software package. The hardware measurements address themselves to a concrete embodiment of that same slotted array antenna, an elongated sector antenna, and to a patch antenna. Excellent agreement is found in all test cases.
Outdoor FarField Antenna Measurements System For Testing Of Large Vehicles
The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.
Outdoor FarField Antenna Measurements System For Testing Of Large Vehicles
The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.
On The Development Of 1845 Ghz Antennas For Towed Decoys And Suitability Thereof For FarField And NearField Measurements
The development of a wideband, highpower capable 1845 GHz quadridge horn antenna for a small towed decoy platform is discussed. Similarity between the systemdriven antenna specifications and typical requirements for gain and probe standards in antenna measurements (that is, mechanical rigidity, nullfree forwardhemisphere patterns, wide bandwidth, impedance match, polarization purity) is used to assess the quadridge horn as an alternative probe antenna to the typical openended rectangular waveguide probe for measurements of broadband, broadbeam antennas. Suitability for the spherical nearfield measurements is evaluated through the finite elementbased fullwave simulations and measurements using the inhouse NSI 700S30 system. Comparison with the nearfield measurements using standard rectangular waveguide probes operating in 1826.5 GHz, 26.540 GHz, and 3350 GHz ranges is used to evaluate the quality of the data obtained (both amplitude and phase) as well as the overall time and labor needed to complete the measurements. It is found that, for AUTs subtending a sufficiently small solid angle of the probe’s field of view, the discussed antenna represents an alternative to typical OEWG probes for 1845 GHz measurements.
Laboratory Tests on the Nearfield to Farfield Transformation with Spherical Spiral Scan Optimized for Long Antennas
In this communication, the experimental verification of a probe compensated nearfield  farfield (NFFF) transformation with spherical spiral scanning particularly suitable for elongated antennas is provided. It is based on a nonredundant sampling representation of the voltage measured by the probe, obtained by using the unified theory of spiral scans for nonspherical antennas and adopting a cylinder ended in two halfspheres for modelling long antennas. Its main characteristic is to allow a remarkable reduction of the measurement time due to the use of continuous and synchronized movements of the positioning systems and to the reduced number of required NF measurements. In fact, the NF data needed by the classical NFFF transformation with spherical scanning are efficiently and accurately reconstructed from those acquired along the spiral, by employing an optimal sampling interpolation formula. Some experimental results, obtained at the Antenna Characterization Lab of the University of Salerno and assessing the effectiveness of such a NFFF transformation technique, are presented.
NearField – FarField Transformation With A Planar WideMesh Scanning: Experimental Testing
This communication deals with the experimental validation of an efficient nearfield  farfield (NFFF) transformation using the planar widemesh scanning. Such a scanning technique is so named, since the sample grid is characterized by meshes wider and wider when going away from the center, and makes possible to lower the number of needed measurements, as well as the time required for the data acquisition when dealing with quasiplanar antennas. It relies on the use of the nonredundant sampling representation of electromagnetic fields based on the use of a very flexible modelling of the antenna under test, formed by two circular "bowls" with the same aperture diameter but eventually different bending radii. A twodimensional optimal sampling interpolation formula allows the reconstruction of the NF data at any point on the measurement plane and, in particular, at those required by the classical NFFF transformation with the conventional planerectangular scanning. The measurements, performed at the planar NF facility of the antenna characterization laboratories of Selex ES, have confirmed the effectiveness of this nonconventional scanning, also from the experimental viewpoint.
Antenna Diagnostic, Echo Suppression and Equivalent Sources Representation Capabilities of the Fast Irregular Antenna Field Transformation Algorithm
The Fast Irregular Antenna Field Transformation Algorithm (FIAFTA) determines the equivalent sources of an antenna under test (AUT) from arbitrarily located sampling points of the antenna field. The application of Fast Multipole Method (FMM) principles to the formulation of the forward operator shows that the influence of the measurement probe is fully corrected based on its farfield radiation pattern. For antenna diagnostic purposes, equivalent surface current densities represent the unknown equivalent AUT sources. However, the FMM gives the possibility to settle the unknowns of the inverse problem in the ^kspace domain. The expansion of the appearing plane wave spectra in spherical harmonics leads to a compact representation of the equivalent plane wave sources. The forward operator is evaluated in a multilevel fashion similar to the Multilevel Fast Multipole Method (MLFMM). This enables to incorporate a priori knowledge about the geometry of the AUT in the antenna model by placing nonempty FMM boxes where sources are assumed.
BestFit 3D PhaseCenter Determination and Adjustment
There are several applications in which knowledge of the location of the phase center of an antenna, and its twodimensional variation, is an important feature of its use. A simple example occurs when a broadbeam antenna is used as a feed for a reflector, where the center of the spherical phase fronts should always lie at the focal point of the paraboloidal surface. Here, the ability to determine the phase center of the feed from knowledge of its farfield phase/amplitude pattern is critical to the reflector's design. Previously published methods process a single cut of data at a time, yielding 2D lateral and longitudinal phasecenter offsets. Eand Hplane cuts are thus processed separately, and will, in general, yield different answers for the longitudinal offset. The technique presented here can process either one line cut at a time or a full ThetaPhi raster. In addition, multiple frequencies can be processed to determine the average 3D phasecenter offset. The technique can merely report the phasecenter location, or it can also adjust the measured phases to relocate the origin to the computed phase center. Example results from measured data on multiple antenna types are presented.
PlanoConvex Lens with Reduced Amplitude Variation
We recently introduced large, lightweight, broadband planoconvex RF lens for closerange measurement of farfield antenna radiation pattern [1]. While the lens can drastically reduce the phase variation of the field across the transverse plane at a relatively short distance from the lens, the amplitude of the field in the same plane is affected by the diffraction from the circular edges of the lens, and to some extent by the transmitted field after internal reflections inside the lens. Furthermore, while the phase variation is minimal (within ±10°) and almost independent of the distance of the transverse plane from the lens, the field amplitude variation across the same plane increases with the distance of the plane from the lens. The amplitude variation reduces the useful size of the "quiet zone". To reduce the amplitude variation, we propose to incorporate "matching layers" around the lens. As we shall demonstrate in the paper, these matching layers help to reduce the aforementioned diffraction and internal reflections. As a result, the amplitude variation of the field across the transverse plane is reduced (to within ±1dB), thereby increasing the size of the "quiet zone". The matching layers are effective even for lenses as small as 6 in diameter.

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