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Far Field
Acquisition, Reconstruction, and Transformation of a Spiral Near-Field Scan
Brett Walkenhorst, Scott McBride, October 2017
The topic of non-redundant near-field sampling has received much attention in recent literature. However, a practical implementation has so far been elusive. This paper describes a first step toward such a practical implementation, where the practicality and generality are maximized at the expense of more acquired data points. Building on the theoretical work of faculty at the University of Salerno and University of Naples, the authors have acquired a set of near-field data using a spiral locus of sample points and, from those data, obtained the far-field patterns. In this paper, we discuss the acquisition system, the calculation and practical implementation of the spiral, the phase transformations, interpolations, and far-field transforms. We also present the resultant far-field patterns and compare them to patterns of the same antenna using conventional near-field scanning. Qualitative results involving aperture back-projection are also given. We summarize our findings with a discussion of error, uncertainty, acquisition time, and processing time in this simplified approach to non-redundant sampling in a practical system.
A Novel and Innovative Near Field System for Testing Radomes of Commercial Aircrafts
Marc Le Goff, Nicolas Adnet, Nicolas Gross, Luc Duchesne, Arnaud Gandois, Ludovic Durand, October 2017
The maintenance of aircraft radomes is of particular importance for the commercial aviation industry due to the necessity to ensure the correct functioning of the radar antenna, housed within such protective enclosures. Given that the radar component provides weather assessment, as well as guidance and navigation functions (turbulence avoidance, efficiency of route planning in case of storms, etc.), it is imperative that every repaired radome be tested with accuracy and reliability to ensure that the enclosed weather radar continues to operate in accordance with the after-repair test requirements of the RTCA/DO-213. Recently, this quality standard was updated and published under the name RTCA/DO-213A, establishing more stringent measurement requirements and incorporating the possibility of measuring radomes using Near-Field systems. Consequently, a compliant multi-probe Near- Field system concept – AeroLab – has been specifically designed to measure commercial aircraft nose-radomes, in order to meet the new standard requirements. AeroLab performs Near-Field measurements. Near-Field to Far-Field transformations are then applied to the results. Such a Near-Field system allows the test range to be more compact than traditional Far-field test ranges, and thus be independent from the updated Far-Field distance which has progressed from D²/2l to 2D²/l in the new standard RTCA/DO-213A. AeroLab enables the evaluation of the transmission efficiency and beamwidth. It also allows for accurate evaluations of the side-lobe levels by providing improved visualization of principal cut views selected from 3D patterns. Moreover, depending upon the weather radar system inside the radome under test, 2 distinct scan sequences must now be taken into account: “elevation over azimuth” and “azimuth over elevation”. AeroLab emulates both of these motion sequences through a monolithic gimbal. Furthermore, thanks to its multi-probe array, such measurements are performed in a fraction of the time spent in current mono-probe test facilities (less than 4 hours, i.e. 1/3 less time than single probe scanners). Keywords: RTCA/DO-213A, radome measurement system, after-repair tests, multi-probe measurement system, Near-Field system.
Multi-Band Compact MIMO Antenna System for LTE and WLAN Communications
Jiukun Che, Chi-Chih Chen, October 2017
In this paper, a novel compact 2-channel MIMO antenna design for all cellular and Wi-Fi communication needs from vehicular is discussed. The entire antenna system fits within the 13cm (diameter) by 9cm (height) volume. It consists of 2 vertical multi-band cellular antenna elements and two vertical multi-band Wi-Fi antenna elements. All four antennas share a 13cm diameter circular ground plane. Each antenna element design is a PCB based slot-loaded multi-band monopole. This particular element design as well as their mounting positions were chosen to minimize mutual coupling and blockage in order to maximize MIMO performance, i.e. diversity gain. In addition, the center region of the antenna volume also accommodates a raised L1-band GPS antenna. A prototype antenna was subsequently fabricated. The measured antenna performance compared well with simulated results before and after being mounted on a 4 feet diameter ground plane. The effect of the radome was also assessed and was found to be insignificant. The cellular antenna produced realized gain of over 2 dBi in lower cellular band (0.7 GHz to 1 GHz), and over 5dBi in the higher cellular band (1.7-2.1GHz and 2.3GHz-2.5GHz). The Wi-Fi antenna produced realized gain of over 5dBi in both 2.4 GHz and 5.8 GHz bands. The far-field pattern correlation coefficient was also calculated to evaluate the diversity gain performance of antenna system. For the cellular band, the correlation number is lower than 0.55 for 0.7 to 1 GHz, and lower than 0.35 for all the other band. For the entire Wi-Fi band, the correlation number is lower than 0.4.
Near-Field Far-Field Transformation for Circular Aperture Antennas using Circular Prolate Wave Functions
Amedeo Capozzoli, Claudio Curcio, Angelo Liseno, October 2017
In the last years different advances in Near-Field (NF) measurements have been proposed. Among the others, the ones of interest here are: the determination of the number and spatial distribution of sampling points, the introduction of scanning strategies aimed to reduce the measurement time, the adoption of a proper representation, for the unknowns of interest, able to improve the reliability of the characterization [1]. In particular, the use of Prolate Spheroidal Wave Functions (PSWFs) for the expansion of the aperture field has proven effective to take into account for the quasi-band-limitedness of both the aperture field and the Plane Wave Spectrum. Furthermore, using a proper expansion is an important step of the Singular Value Optimization (SVO) approach, wherein the number of the spatial distribution of the NF samples are determined as the ones reducing the ill-conditioning of the problem [1]. Up to now, rectangular PSWFs has been successfully exploited to perform optimized NF characterizations of rectangular aperture antennas. Recently, we tackled the extension to the case of circular apertures. The difficulties related to the stability and accuracy of the numerical evaluation of the Circular PSWFs have been assessed in [2], showing the benefits due to the use of a proper expansion, with respect to standard backpropagation. Furthermore, the circular PSWFs expansion correctly takes into account for the spectral radiating support, with respect suboptimal representation of the rectangular case. The aim of the paper is to show how the circular PSWFs expansion can be fruitfully exploited in the NF characterization of circular aperture antennas. Experimental results will be presented to support the performance of the method. [1] A. Capozzoli, C. Curcio, G. D’Elia, A. Liseno, “Singular value optimization in plane-polar near-field antenna characterization”, IEEE Antennas Prop. Mag., vol. 52, n. 2, 103-112, Apr. 2010. [2] A. Capozzoli, C. Curcio, G. D’Elia, A. Liseno, “Prolate Function Expansion of Circularly Supported Aperture Fields in Near-Field Antenna Characterization”, European Conference on Antennas and Propagation 2017, Paris 19-24 March 2017.
Broadband Additive Spiral Antenna
Tommy Lam, October 2017
As part of the Lockheed Martin (LM) Additive Manufacturing (AM) Initiative, the Rotary Mission System antenna group has been developing a new and improved Additive Spiral Antenna (ASA) for both transmit and receive applications. This is a collaboration effort between LM engineering and LM manufacturing for a low cost and high performance antenna for manyultra-wide band(UWB) applications in both military and commercial market sectors. Unlike other conventional spiral designs, thisrecently emerging Additive Manufacturing capabilities allow extra spiral antenna miniaturizations without additional gain bandwidth performance penalties. This is achieved by leveraging unique low cost AM abilities to form complex and thus much more efficient 3D shapes to increase spiral antenna radiation efficiency, approaching the Chu’s gain bandwidth limitation. An initial prototype ASA was designed and tested in 2016 and showed very encouraging results. The measured ASA performance indicated nearly the same antenna performance as our current conventional production spiral antenna having multi-decade frequency band performance. More importantly, the ASA aperture size was significantly reduced by more than 50% with excellent transmit and receive gain efficiency and power handling capabilities. This paper will describe this ASA prototype design approaches and antenna near field and far field compact range measurement results along with material characterizations to demonstrate Additive Manufacturing technology can enhance antenna performance that otherwise not realizable with conventional fabrications. In addition, an integrated optimum balun length electromagnetic band gap (EBG) cavity design further reduces the antenna depth by over 70% will be presented. This is realized by use of high power and high temperature honeycomb absorbers in conjunction to electromagnetic band gap (EBG) cavity design for achieving high efficiency and low cavity profile, with total antenna volume reduction by nearly 3x. Some discussions will be provided for solving high thermal issues associated with ASA’s transmit capabilities.


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