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Measurement of E and H plane far field patterns for an open-ended 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 E-plane than for the H-plane, which is observed, measurements exhibit a sharper decay in the E-plane 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 E-plane. 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 Plane Wave Synthesis Approach for mitigating errors in antenna measurements caused by stray signals and imperfections in the measurement range illuminating fields has been demonstrated previously for a 2D scalar source [1]. This paper presents algorithms developed for the Range Transfer Function (RTF) method for a 2D vector source. Vector basis functions for both the field representation and the AUT representation are implemented to provide a robust numerical solution. The new algorithms are more stable because the plane wave angles and the antenna measurement angles may be completely general, provided that Nyquist rules of sampling are observed during both the field probing (to obtain the plane wave coefficients) and the antenna measurement (to obtain the raw pattern data).
Far-field uncertainty due to probe errors in planar near-field 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 near-field measurements on two partial planes is presented to reduce the multiple reflections between the probe and the AUT.
This work concerns the experimental validation of a probe compensated near-field – far-field 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 two-dimensional optimal sampling interpolation formula to recover the near-field data needed to perform the classical spherical near-field – far-field transformation. The good agreement between the so reconstructed far-field patterns and those obtained via the classical spherical near-field – far-field transformation assesses the effectiveness of the approach.
Method of moments (MoM) codes have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In an earlier AMTA paper [1], we presented a network model (NM) algorithm that uses a Gauss-Newton iterative nonlinear estimation method in conjunction with a MoM model to estimate the “as-built” materials parameters of a target from a set of backscatter measurements. In this paper, we demonstrate how the NM algorithm, combined with the basis pursuits (BP) l1 minimization technique, can be used to locate unknown defects (dents, cracks, etc.) on a target from a limited set of RCS pattern measurements. The advantage of l1 minimization techniques such as BP is that they are capable of finding sparse solutions to underdetermined problems. As such, they reduce the requirement for a priori information regarding the location of the defects and do not require Nyquist sampling of the input pattern measurements. We will also show how the BP solutions can be used to interpolate RCS pattern data that is undersampled or has gaps.
A balanced PIFA-inspired antenna design is presented for use with the 2:45 GHz ear-to-ear radio channel. The antenna is designed such that the radiated electric fields are primarily polarized normal to the surface of the head, in order to obtain a high on-body path gain (jS21 j). The antenna structure can be made conformal to the outer surface of a hearing instrument, such that the bandwidth of the antenna is optimized given the available volume. The radiation patterns, ear-to-ear path gain and available bandwidth is measured and compared to the simulated results. It is found that the antenna obtains a relatively high ear-to-ear on-body path gain, as well as a bandwidth that is large enough to cover the entire 2:45 GHz ISMband.
Over-the-air (OTA) testing is an established technique used to measure the wireless system performance of mobile devices in addition to characterizing the antenna subsystem. 3D radiation patterns of transmit power and receive sensitivity are used to determine a figure of merit (FOM) for the transmitter and the receiver performance, i.e., Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS), respectively. For LTE, most attention is focused on the MIMO receiver chain evaluation. Discussions have been ongoing for quite some time on how OTA testing can be updated to support LTE MIMO. The MIMO OTA decomposition approach [1], [2] determines separate FOMs for the key MIMO receiver chain subsystems. The conducted MIMO test is utilizing a fading simulator to introduce dynamic fading and is used to determine a FOM for the MIMO receiver performance. The radiated test is performed without the introduction of fading profiles inside the anechoic chamber and is used to determine a FOM for the MIMO receive antenna pair. The FOM for the overall MIMO performance of the DUT is a combination of the FOMs from each test (conducted and OTA). Splitting up the MIMO wireless system performance testing into two straightforward and cost effective tests provides more information about the DUT performance than performing a complex single test. The presented approach differentiates itself from competitive MIMO OTA approaches due to its simplicity, reduced complexity, and low cost.
The focused beam measurement technique has proven to be a solid technique for free space measurement of electromagnetic material properties. This paper presents the use of the focused beam method to measure swept frequency antenna gain as well as antenna patterns. A calibration and signal processing procedure has been developed to properly handle the range of incident waves inherent in the Gaussian beam illumination. One disadvantage of this technique is that the size of the antenna under test is limited by the spot size of the focused beam. However, the GTRI focused beam system uses lenses that are easily reconfigured to realize various spot sizes. The advantage of the focused beam illumination is that the number of measurements and thus measurement time is reduced by roughly an order of magnitude when compared to spherical near-field scanning techniques. More importantly, focused beam systems can be used in a lab environment and do not require large dedicated chambers. We present both model/theory predictions and measured data of how a too-small spot size of the focused beam leads to systematically lower peak gain measurements and wider beam widths.
The development of a wideband, high-power capable 18-45 GHz quad-ridge horn antenna for a small towed decoy platform is discussed. Similarity between the system-driven antenna specifications and typical requirements for gain and probe standards in antenna measurements (that is, mechanical rigidity, null-free forward-hemisphere patterns, wide bandwidth, impedance match, polarization purity) is used to assess the quad-ridge horn as an alternative probe antenna to the typical open-ended rectangular waveguide probe for measurements of broadband, broad-beam antennas. Suitability for the spherical near-field measurements is evaluated through the finite element-based full-wave simulations and measurements using the in-house NSI 700S-30 system. Comparison with the near-field measurements using standard rectangular waveguide probes operating in 18-26.5 GHz, 26.5-40 GHz, and 33-50 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 18-45 GHz measurements.
Two of the three algorithms require an estimate of the element pattern, which they assume to be common to all the elements. We describe our measurement of our array’s element pattern, as well as the use of the IsoFilter™ to center the element pattern and limit the edge effects.
TESTING LARGE WIRELESS DEVICES IN SMALL ANECHOIC CHAMBERS 100_0967100_0973 James D. Huff -20.00-18.00-16.00-14.00-12.00-10.00-8.00-6.00-4.00-2.000.00050100150200Relative Power (dB) Theta Angle (deg) Uncorrected Dipole Patterns 0,0,00,0,120,0,18-20.00-18.00-16.00-14.00-12.00
A spherical array designed for hemispherical coverage of satellite communications at S-band that is approximated by hexagonal and pentagonal planar panels. Ball Aerospace built a segment of a 10m diameter spherical array that has one center pentagonal panel and five surrounding hexagonal panels. This paper describes our efforts at testing one large hexagonal panel in a compact range.
This paper describes the behavior of the antenna radiation pattern for different planar near-field measurement errors superimposed on the near-field data. The disturbed radiating near field is processed using multilevel plane wave based near-field far-field transformation to determine the far-field. Errors like scan area truncation, transverse and longitudinal position inaccuracy of measurement points, and irregular sample spacing are analyzed for an electrically large parabolic reflector at 40 GHz. The error behavior is then compared with the standard transformation technique employing 2D Fast Fourier Transform (FFT) using the same near-field data. In order to exclude the effect of any other measurement or environmental error, electric dipoles with appropriate magnitude profile and geometrical arrangement are used to model the test antenna.
The radiation pattern of an antenna can be determined accurately by near-field measurement and transformation techniques. Low numerical complexity, full probe correction capabilities, and high accuracy of the transformed far-field pattern are important features of near-field transformation algorithms. The multilevel plane wave based near-field transformation algorithm achieves an efficient full probe correction by plane wave representations of antenna and field probe and realizes the low numerical complexity by hierarchical grouping of measurement points. Field translations are carried out to the boxes on the coarsest level and are further processed to the measurement points by disaggregation and anterpolation. Disaggregation is a simple phase shift of the plane waves and anterpolation reduces the sampling rate corresponding to the spectral content of the plane wave spectra on the various levels. The accuracy of the transformation is influenced by several variables where the number of buffer boxes between antenna and measurement point groups is crucial. A higher accuracy due to more buffer boxes can be achieved at the cost of increased computation time. Adaptive field translations structure the measurement setup such that individual groups are transformed with the required accuracy at lowest costs. A detailed investigation for a planar near-field measurement will be shown.
This paper compares 3 sets of far-field patterns of an S-Band Open Ended Waveguide (OEWG). The sources for the data are measurements from NPL, an EM-Model and the commonly used NIST analytical model. Both co-polarized (co-pol) and cross-polarized (x-pol) patterns are compared. Results indicate that accuracy improvements are possible by utilizing an EM-Model in certain applications. These applications as well as the pros and cons of doing this are discussed. Understanding the differences between these 3 independent sets of data enables near-field range engineers to better understand the directional dependence of probe correction accuracies over the majority of the forward hemisphere. Information and insight gained from this comparison, along with specific AUT requirements, better equips the near-field range user to address probe correction concerns and ultimately to determine if a calibrated probe solution is required for their unique testing scenario.
The use of computational electromagnetics (CEM) prediction codes in the analysis and interpretation of RCS measurements has become increasingly prevalent. This is in large part due to rapid advances in computing capability over the last several years, particularly for rigorous techniques such as the method of moments (MoM). In many instances, however, these codes are still limited to plane waves and/or elementary dipoles as the sources of target illumination. Modeling of the illumination from an arbitrary antenna therefore requires meshing and solution of the combined antenna-target geometry for each frequency and aspect angle, with an associated increase in the computational complexity of the problem, even if the interactions between the antenna and the target are negligible. In this paper, we describe a method by which measurements or predictions of the antenna pattern are used to develop an equivalent representation of the antenna in terms of an array of non-interacting elementary dipole current sources in a MoM code that uses RWG basis functions. The representation can then be used to efficiently derive the antenna’s illumination on the target as a function of frequency and aspect angle with only a minor increase in the computational burden relative to plane wave illumination. Results are presented using antenna pattern predictions for an ETS-Lindgren 3164-01quad-ridged VHF antenna which illustrate the accuracy and efficiency of the technique.
Two novel antenna designs using inexpensive planar printing technology are presented. The goal is to obtain antennas that maintain their pattern and other antenna parameters over a large range of frequencies so they can be used efficiently in applications with either multiple frequencies or broadband frequency range. The advantage of these structures is the design simplicity, cheap cost, and capability of operating at high frequencies. The design technique, the measurements of pattern variation of electric fields E and H, bandwidth, and the reflection coefficient parameters between 2 and 6.5 GHz are presented. Although the field strength of the pattern is reduced as the frequency increases, the pattern is within a 10 dB difference between 2 and 4 GHz and is mostly omni directional in this range.
Kriging fitting, originally developed for geological exploitation, is here applied for fitting an expected pattern to noisy, irregular in-flight measurements of a satellite antenna. The noise level in in-flight measurements is often so high that only the central part of the main beam appears. By the Kriging method, first a characteristic function, the regression model, is fitted to the measurements. For the main beam this is chosen to be described by a general second order polynomial. To this is added a more detailed correlation model which represents realistic deviations from the regression model but filters out the fast variations of the noise. The method is applied on simulated measurements on the Planck RF telescope and the presented results show a considerable reduction of the noise floor of the Figure 1 – The Planck double reflector antenna system pattern; even beam details invisible in the original with two ellipsoidal mirrors (aplanatic configuration). measurements (a shoulder) are revealed by the pattern From the antenna pattern obtained by the in-flight testing fitting1 .
In the paper [1] the principles of operation of high performance absorbing materials were described and the criterion for absorber performance optimization at UHF/VHF frequency bands was proposed and confirmed experimentally on a number of absorber components optimized for operation at low frequencies such as the VHF/UHF bands. C:\Publishing 2011\AMTA 2011\Papers\Absorbing Material Performance\freq dom 18 24 36 60.jpg The experimentally verified optimization criterion is intended to determine the optimum carbon loading of the absorber components, thus delivering optimal reflectivity of the full absorbing assembly (absorber components on a metallic backing plate) at the lowest possible operating frequency. The optimization is based on equalization of reflections in the time-domain from the front face surface of the absorbing component and from the backing metallic plate. Validity of the criteria was confirmed by measurements of the reflectivity of pyramidal absorbing components of various heights, (3’, 5’, 6’ and 8’ [3]) in a 40’ long coaxial line terminated in a metallic back wall [2,4]. In this paper, more details are highlighted explaining how the criterion is delivering the best absorber reflectivity at low frequencies. This is accomplished by implementing time gating post-processing to isolate two primary concurrent peaks corresponding to the reflections from the front surface and metallic backing substrate. It is shown that the improved reflectivity is achieved by a self-cancellation of the two signals delivering the “null” in the frequency domain, which, in turn determines the lowest operating frequency attributed to an absorber of a given height. It is shown that the “null” property of the reflectivity pattern, as well as the properties of the peaks in between “nulls”, can be scaled and, therefore, predicted based on the height of the absorber almost everywhere in the UHF band. Thus, it is possible to optimally choose the grade of the absorber necessary to meet or exceed given reflectivity specifications, or to manufacture the appropriate absorber grade which can deliver the optimum reflectivity at the specified frequency.