AMTA Paper Archive

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.

Radar sensor working at 76-77GHz band, because of its long detection range, high resolution and excellent performance in different weather and illumination conditions, has been used to develop on-road pedestrian collision avoidance system. Therefore, studying the pedestrian radar scattering features is important to develop reliable on-road pedestrian detection algorithm. In this paper, we first discuss the measurement setup requirement at 76-77GHz to obtain reliable radar cross section (RCS) data of human subjects. Then the RCS pattern of human subjects with different postures and different body features are measured and studied. The observed radar features could be further developed into stable radar signatures to improve the pedestrian identification 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 far-field 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 ^k-space 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.

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 broad-beam 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 far-field 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 phase-center offsets. Eand H-plane 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 Theta-Phi raster. In addition, multiple frequencies can be processed to determine the average 3D phase-center offset. The technique can merely report the phase-center 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.

Compact range measurement facilities have been used successfully for many years to characterize antenna performance as well as radar signature. This paper investigates strategies for improving compact range measurement accuracy by mitigating errors associated with ground reflections inherent in most range designs. A methodology is developed for strategically modifying, or patterning, the surface between the range source antenna and the reflector to reduce error terms, thereby increasing measurement accuracy. Candidate patterns were evaluated using a full-wave computational finite-difference time-domain (FDTD) model at VHF/UHF frequencies to determine baseline performance and develop trade rules for more advanced designs. Physical optics (PO) models were used to analyze the final design at the frequencies of interest.

Multi Input Multi Output (MIMO) antenna systems are needed to meet the increasing demands of users in wireless systems. MIMO technology has been used to improve the capacity of wireless systems; however, designers have faced challenges to reduce antenna-size and increase the isolation between the antennas in MIMO systems. In this paper, a compact MIMO antenna array platform is proposed for LTE MIMO and Handset applications. The proposed array was designed to operate at the 2.6GHz Long-Term Evolution (LTE) band for wireless communication systems. The proposed array consists of four compact patch antennas on a dielectric substrate with total dimension of 12.5x6.25x1.27mm3. Modification of the ground plane along with the systematic placement and orientation of the antenna elements on top of the substrate play a key role to reduce mutual coupling, which normally degrades the performance of MIMO antenna arrays. The performance of this MIMO antenna array has been evaluated through simulations and measurements of the scattering parameters [S] and radiation patterns. The minimum gain of a single antenna with all the other three elements terminated in 50O loads is 1.49dBi, while the isolation is over 25dB between all the MIMO antennas located in the array structure. The measured results suggest that the antenna is well suited for LTE MIMO applications as well as handset antennas.

We recently introduced large, lightweight, broadband plano-convex RF lens for close-range measurement of far-field 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.

This paper presents V-band radiation pattern characterization of both low- and high-directivity antennas. A fourarm micro-machined spiral antenna with monolithically integrated mode-forming network designed for dual circularlypolarized radiation represents the low-directivity antenna, while a standard gain horn is used for the highly directive antenna. All measurements were performed using an in-house NSI-700S- 30 system capable of spherical near-field measurements from 1-50 GHz and direct far-field measurements from 50-110 GHz. Complete comparisons of simulated, near- and far-field patterns show the feasibility of near-field measurements in V-band. Based on pattern comparison and measurement statistics conclusions are drawn about V-band near-field measurements.

Reconfigurable radar antennas with rapid, real-time control of the radiation pattern beamwidth provide expanded performance for many instrumentation radar applications, including RCS signature measurement and dynamic Time Space Position Information (TSPI) radar tracking applications. Adaptive adjustment of antenna radiation patterns was traditionally accomplished by electro-mechanically selecting predefined aperture dimensions that corresponded to desired beamwidths (e.g., ? ?/D). For an array antenna consisting of as few as 200 elements, beam shaping can be accomplished by adjusting the relative phase of individual array elements, a technique defined as beam spoiling or decollimation. This paper analyzes an operational radar antenna array incorporating reconfigurable beamwidth and beam shape through independent phase control of each subaperture. By adjusting the relative phase of radiating elements, the system can illuminate a programmable field of regard with full transmit power. For this array, the phase distributions across the elements map to a smaller "virtual aperture" displaced behind the physical array. Theoretical and measured results are presented to validate the reconfigurable array pattern control technique.

Abstract—In this paper, a novel feeding method of microstrip line array is presented, with which every line array can be fed from the side part instead of from the center part .At the same time a novel compact slot patch antenna loaded with a pair of spirals is proposed, which is 66% smaller than the conventional half-wave patch antenna. The simulated and measured radiation patterns at center frequency are both presented. The -18dB side-lobe level for the sum pattern and -37dB null depth for the difference pattern have achieved in the experiment.

Abstract—Nowadays, radar retro-reflector has been widely applied as a decoy, to seduce an incoming assault away from the target, or towards a less vulnerable part of it to communication systems and remote identification as their characteristics of low-profile, low-cost and Radar Cross Section(RCS) enhancement. A passive retro-reflector is a device which can be used to be reflected most of the energy incident upon it in the direction of the in-going wave. The Luneberg lens and a sphere are widely used as their self characteristics. In this paper one of the retro-reflector, is paid more attention as time goes by, is introduced. The retroreflector is consist of patch antenna arrays and feeding system and can be defined as Retro-directive arrays (RDA). It has a very simple structure and can focus outgoing waves back at the direction of incident waves. The character of the re-radiation pattern affected by the size and type of patch and width and length of feeding network related are optimized by the HFSS. The final results are validated experimentally.

Abstract—When antennas are measured in echoic environ­ments, there is usually a need to process the measured data in order to remove multipath contributions. Traditional measure­ment set-ups involving a single probe antenna provide limited information for the purpose of this separation. A multi-probe measurement technique whereby the AUT is measured with two sets of probe antennas is presented. One set of probe antennas are oriented such that they radiate mainly toward the AUT and the other set radiate away from it. This measurement technique allows for the separation of the direct AUT contribution from the multipath contributions. The acquired data is processed using a well-suited near-.eld far-.eld transformation algorithm with the echo sources considered as they were independent sources. The performance of the measurement technique is also evaluated for traditional spherical mode near-.eld far-.eld transformation whereby both incoming and outgoing spherical waves are consid­ered. The results show a substantial improvement in the obtained far-.eld patterns when compared with non-compensated far-.eld results.

Abstract: In order to characterize the Boeing 9-77 compact range, the empty chamber background was measured as a function of frequency, polarization, and the azimuth angle of the upper turn-table (UTT). The results exhibited a near-field diffraction pattern with enlarged hot-spots on a 4-fold symmetry [1]. A 2-D FFT on the diffraction pattern yielded a mapping on the relative arrangement of the absorbers on the UTT [2]. In this paper, we take a closer look at the scattering geometry of the UTT as illuminated by the residual field above and beyond the quiet zone (QZ). The different responses in VV and HH are discussed. The enhanced diffraction due to a “blazed grating” condition is identified and analyzed.

Abstract— Measurements and strategies for the calculation of radar cross-sections in the shorter millimeter wave region, especially of objects that include rough surfaces, are discussed. Because of decreasing wavelength, roughness becomes more significant in this spectral region, but also more difficult to characterize. A tabletop radar cross-section measurement system was set up to measure scattering from canonical objects and rough objects with regular or random patterns using a swept frequency continuous wave system. Random, rough objects of different surface roughnesses were measured and fit to statistical distributions governed by optical speckle theory. In this paper we consider the inclusion of optical speckle theory in the electromagnetic codes to address both issues associated with the characterization of target surfaces and the time required for numerical calculations.