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Impedance
Measurement of Complex Permittivity Using Artificial Neural Networks
Azhar Hasan,Andrew Peterson, November 2010
In this paper, a Neural Network based methodology is presented to measure the com­plex permittivity of materials using monopole probes. A multilayered Arti.cial Neural Net­work, using the Levenberg Marquardt back propagation algorithm is used to back solve the complex permittivity of the medium. The pro­posed network can be trained using an analyt­ical model, numerical model, or measurement data spread over the complete range of param­eters of interest. The input training data for the non linear inverse problem of reconstruct­ing the complex permittivity comprises the com­plex re.ection coef.cient of the monopole probe. For the results presented in this paper, the net­work is trained using the analytical model for impedances of monopole antennas in a half space by Gooch et al. [1]. In addition to computational ef.ciency, the proposed approach gives 99% ac­curate results in the frequency range of 2.5­5 GHz, with the values of permittivity varying across a range of 3-10 for the real part, and 0 -0.5 for the imaginary part. The accuracy and the effective range of real and imaginary components of the complex permittivity that can be reconstructed using this approach, depends upon the accuracy and robustness of the model / system used to generate the training data. The analytical model used in this paper has a limited range for the values of loss tangent that it can model accurately. However, the performance of the back solving algorithm remains independent from any speci.c model, and the scheme can be successfully applied using any reliable ana­lytical or numerical model, or re.ection coef.­cient training data generated through a series of measurements. The methodology is likely to be employed for experimental measurements of complex permittivity of dissipative media.
Wireless Measurement of UHF RFID Chip Impedance
Toni Björninen,Atef Elsherbeni, Lauri Sydänheimo, Leena Ukkonen, Mikko Lauri, Risto Ritala, November 2010
Accurate knowledge of an RFID IC’s input impedance enables the design of performance-optimized RFID tags with a given IC. For this purpose, the most valuable information is the IC’s input impedance at its wake-up power, but as the impedance itself is power-dependent, few simple methods exist to extract this information. This paper presents a method, based on the joint use of computational electromagnetics, wireless RFID tag measurements and Monte Carlo simulations, to determine the input impedance of an UHF RFID tag chip at the wake-up power of the IC and the measurement uncertainty related to the result.
A Cable-Free Technique for Measurement of Radiation and Scattering Characteristics of Electrically Small Antennas
Jiaying Zhang,Olav Breinbjerg, Sergey Pivnenko, November 2010
Impedance and gain measurements for electrically small antennas represent a great challenge due to influences of the feeding cable. The leaking current along the cable and scattering effects are two main issues caused by the feed line. In this paper, a novel cable-free antenna impedance and gain measurement technique for electrically small antennas is proposed. The antenna properties are extracted by measuring the signal scattered by the antenna under test (AUT), when it is loaded with three known loads. The tech-nique is based on a rigorous electromagnetic model where the probe and AUT are represented in terms of spherical wave expansions (SWEs), and the propaga-tion is accounted for by a transmission formula. In this paper the measurement results by the proposed technique will be presented for several AUTs, includ-ing a standard gain horn antenna, a monopole an-tenna, and an electrically small loop antenna. A com-parison of measurement results by using the proposed method and by using other methods will be presented.
Antenna Miniaturization Using Artificial Transmission Line Concept
Chi-Chih Chen, November 2010
Antenna miniaturization will continue to be a key issue in wireless communications, navigation, sensors, and RFIDs. For instance, each cellular tower is often populated with many antennas to cover different angular sectors and different frequency bands. Each modern notebook computer is likely embedded with multiple antennas to provide service in WWAN (824 MHz to 2170 MHz) and WLAN (2.4 GHz and 5.5 GHz), Bluetooth, etc. Also automobiles, vessels, and aircrafts will require more antennas to compete for very limited real estate. This dire situation is changing antenna designer worldwide with a goal to develop a new generation of physically small antennas that multi-bands or wideband. This paper presents several generic miniaturize antenna design examples that applies the concept of artificial transmission line concept for artificially control phase velocity and impedance. This miniaturization approach can be applied to reduce the size of both narrowband and wideband antennas using minimal amount of materials. Thus improves antenna’s efficiency, and reduces its cost and weight.
A Standard for Characterizing Antenna Performance in the Time Domain
E. Farr, November 2011
We derive here a simple function describing antenna performance in the time domain. This single function describes antenna performance in both transmission and reception, and in both the time and frequency domains. The resulting equations are as simple as one could hope for. The function isolates the performance of the antenna from the impedances of the source, load, and feed lines. From this function one can simply derive such conventional frequency domain quantities as gain, realized gain, and antenna factor. It is hoped that this function will be adopted as an IEEE standard for time domain antenna performance.
Wireless Measurement of the Wake-Up Power and Impedance of UHF RFID IC
T. Björninen,K. Koski, L. Ukkonen, L. Sydänheimo, M. Lauri, R. Ritala, A. Elsherbeni, November 2011
Accurate knowledge of the wake-up power and the corresponding input impedance of an RFID IC is valuable for RFID tag and IC designers, as it enables the design of performance-optimized tags using a specific IC and provides means for validation of both tag and circuit designs. However, the IC’s power-dependent front-end makes it difficult to measure this information. This article presents a method, based on the joint use of computational electromagnetics, wireless RFID tag measurements and Monte Carlo simulations, to determine the wake-up power and the corresponding input impedance of a mounted RFID IC.
Frequency and Impedance Agile Real-Time Tuning Network for 200-400 MHz Antennas
N. Smith,C. Chen, J. Volakis, November 2011
Mobile communication devices have many different requirements; namely they often have stringent size constraints, and must efficiently radiate over several frequencies in a myriad of different environments. Furthermore, the antenna is often electrically small or unintentionally loaded by environmental effects which cause unpredictable changes in antenna impedance. Therefore an agile matching network that is self-tuned to increase matching efficiency is desired. Most existing tuning approaches minimize reflections looking into the matching network. It will be demonstrated that this approach does not guarantee optimal performance due to circuit losses. A better approach is to also maximize power transmitted through the matching network. This paper presents a real-time frequency and impedance-agile tuning design that automatically matches a very wide load impedance range (0.5. < Re{ZL} < 1K. and -1K. < Im{ZL} < 1K.) from 200 to 400 MHz by the use of varactor diodes with impedance tuning stubs.
Wideband Performance for Planar Antenna-PMC Configuration
D. Voltmer,E. Wheeler, E. Wandel, November 2011
Planar low-profile antennas over high-impedance surfaces show improved performance compared to that over metal ground planes. Unfortunately, these high-impedance surfaces often operate over narrow bandwidths. This paper describes an approach to high-impedance surfaces which permits improved performance over a broader bandwidth. Current approaches to the design of high-impedance substrates typically employ identical unit cells with the same resonant frequency to produce high-impedance behavior over a relatively narrow frequency range. The wide bandwidth performance described in this paper derives from cells having a size and subsequent resonant frequencies that vary with position on the PMC substrate. This approach is explored through simulations using CST Microwave Studio which show the improved performance of these wideband structures.
A Conformal X-Band Cylindrical Patch Antenna Array System
U. Olgun,C. Chen, November 2011
Fig. 1: Cylindrically conformal antenna array system This paper discusses an example of a unique cylindrical conformal array design approach. This exemplary design is composed of six subarrays of series-fed microstrip patch antenna operating in X-band. The diameter of the conducting cylinder is 7.5 inches in diameter. Each subarray contains seven patch antennas connected in series configuration and is connected to a single coaxial probe located at the center element. Such feed arrangement greatly reduces the number of feeding cables, increases the feed-line isolations, and minimizes the feed-line radiation. The elevation pattern of each subarray is controlled by the number of patches and impedance matching tapering via varying the width of connecting microstrip lines. The azimuth pattern is controlled by the subarray height above cylinder surface, substrate width, cylinder radius, and surface treatments between subarrays. Measurement results exhibited good impedance matching and broad antenna coverage in X-band.
Wideband And High-Power Performance Of Printed Spiral Antennas
James Bargeron, Dejan Filipovic, November 2012
Spiral antennas have been well established as good radiators of circularly polarized radiation that are capable of achieving very large bandwidths. Though traditionally used for receiving applications, this paper will show that printed spiral antennas are also capable of performing as high-power radiators. Several printed 4-armed spiral antennas are presented, along with measured data that attest to their ability to handle hundreds of watts of continuous wave (CW) power at microwave frequencies. This high-power data includes temperature, electric field, and return loss readings recorded during the tests. Such high power performance is achieved through the use of a high-power capable substrate, lossless cavity, multi-arming, and applying a dual-layering technique which serves to reduce the current density and improve the spiral antenna’s match to 50O. Radiation and impedance measurements are taken to fully verify wideband performance. Analysis of the current densities from simulations is also presented. Data from the high-power tests indicate that the chief factors limiting the spirals’ power handling are their beamformers and resistive terminations.
On The Development Of 18-45 Ghz Antennas For Towed Decoys And Suitability Thereof For Far-Field And Near-Field Measurements
Matthew Radway, Nathan Sutton, Dejan Filipovic, Stuart Gregson, Kim Hassett, November 2012
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.
A Microstrip Fed Slot Antenna with a Dual Band Frequency Response for WiMAX Applications
Gökhan Murat Eryilmaz and Mustafa Turkmen, October 2013
In this study, a novel and compact microstrip-fed slot antenna, which has a dual-band resonance characteristic, is proposed for wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) applications. The proposed antenna has a simple geometry. It has a microstrip feed line on one side and a ground plane having simple slots on the other side of the substrate. The prototype is fabricated by using mechanical mill-etching technique on a 1.27 mm thick RT/duroid 6006 substrate with the relative permittivity of 6.15, and a loss tangent of 0.0027. The return loss (dB) characteristics of the proposed antennas are measured. The results show that the antenna can provide dual impedance bandwidths of 180 MHz centered at 2.44 GHz, 200 MHz centered at 5.56 GHz, which covers the 2.4 GHz (2400-2484 MHz) WLAN band, 2.3 GHz (2300-2500 MHz) and 5.5 GHz (5250-5850 MHz) WiMAX bands. A good agreement between the results of the numerical and experimental studies has been observed. Consequently, the proposed antenna with simple structure and dual-band frequency response can be suitable for WLAN/WiMAX applications.
Broadband Optically Modulating Scatterer probe for near field measurements
Ghattas Lama,Serge BORIES, Mervi HIRVONEN, Dominique PICARD, November 2013
In the literature, one can find a low scattering photodiode modulated-probe for microwave near field imaging. The frequency response of the probe is computed at 2.45 GHz. In this paper, however a new formulation for computing the scattered field for low frequencies (from 150 MHz) by a broadband near field probe based on the impedance matrix is developed. In addition, a method to increase the scattered power by controlling matching will be shown.
Test and Evaluation Challenges for Well-Matched Multi-layer UWB Antennas
Stephen Blalock,Paul Friederich, Charles Hunter, Rick Moore, Eric Kuster, November 2013
 surfaces  overagroundplane  hasbeenusedfor  anumber  ofultra-­-wideband(UWB)  applications.Thispaperdiscussespracticalchallenges  associatedwithaccuratecharacterization  andverificationofmultilayer  UWBantennasdesignedfor  highpowerapplications.Akeydesignconstraintforhighpowerantennasisanextremelylowimpedancemismatch  atthefeedterminalsneededtoefficientlycoupleenergyintothe  antenna.Carefuldesign  and  optimization  oftheantennalayersandradiatingsurfacecantheoreticallyachievetherequiredefficiencies.However,verificationofthedesignischallengingduetopracticallimitations  ofboththeantennafabricationprocessand  measurementmethodologies.Analysisoftheantennaarchitecturefromatestandevaluationperspective  isusedtoidentify  potentialmeasurementriskareasandguidethedevelopmentofanincremental  testmethodologydesignedtomitigateorbetter  understand  therisks.Laboratory  testcouponswere  fabricatedandmeasuredtodetermineconstitutivelayerandcompositestack-­-up  performance.Throughtheincrementaltestingmethodology,developerswereabletodeterminethat  prototypeperformancewas  limitedduetothevariabilityofetchedresistor  valuesonthe  drivenlayer.Thepaperconcludes  withashortdiscussionoffrequencyscalabilityandsummaryofplansforfuturetests.
Four-Arm Wideband Log-Periodic Antenna and its High Power Measurements
Rohit Sammeta,Dejan Filipovic, November 2013
Abstract—Four arm Log-Periodic (LP) antennas are frequency independent antennas that are capable of producing dual circular polarizations from the same aperture and over the same bandwidth making them more versatile than commonly used spiral antennas. In this paper we present a four arm LP that is capable of being a high power radiator. Each pair of arms of the LP is fed with a microstrip line that functions as both an impedance transformer and a 180° balun, thereby greatly simplifying the required beamformer. The antenna is tested successfully up to 500W of input CW power. Post high power characterizations of the antenna (far-field gain, radiation patterns, and VSWR) for linear polarization are presented and the stable high power performance of the antenna is demonstrated. With an appropriate beamformer, good quality circular polarization can be expected. Presented results should pave the way for use of the LP in relevant wideband high power applications.
Novel Bowtie Nanoantenna Design for High-Efficiency Thermophotovoltaics
Sangjo Choi,Kamal Sarabandi, November 2013
Abstract—High absorption rate due to the field enhancement at the terminals of a bowtie nanoantenna is utilized to develop a nano-meter thick and highly efficient thermophotovoltaic (TPV) system. A nano-meter size block of Indium Gallium Arsenide Antimonide (InGaAsSb), a low bandgap semiconductor (Eg = 0.52 eV) is used for infrared (IR) energy absorption and also used as an antenna load. At the desired frequency (180THz) where the maximum quantum efficiency of the dispersive InGaAsSb material is observed, InGaAsSb load presents a high resistance and capacitance. For conjugate impedance matching, a high impedance plasmonic bowtie nanoantenna operated at its anti-resonance mode is developed and an inductive nano transmission line stub is used to compensate for the high capacitance of InGaAsSb load. The bowtie nanoantenna loaded with 30 cubic nanometer InGaAsSb block shows 23.5 of the field enhancement which is the ratio between field intensity at the antenna’s terminals and the incident field intensity. The infinite array of the bowtie nanoantenna backed by a metallic reflector is shown to absorb ~ 95% of the incident power at the desired band. A nano-meter thin TPV system using the bowtie nanoantenna array can show 1.5 times higher efficient than the bulk InGaAsSb TPV cell.
Metal-backed Antenna Miniaturization Based on Reactive Impedance Surface
Jiangfeng Wu,Kamal Sarabandi, November 2013
Abstract— This paper presents a two-layer mushroom-like reactive impedance surface (RIS) and patch antenna miniaturization with potential application in matel-backed antennas. RIS, known as meta-substrate, has shown the ability to miniaturize printed antennas with omni-directional radiation pattern, when served as the substrate for the antenna [11]. However, the area of conventional RIS substrate usually has to much larger than that of miniaturized antenna, since the cell’s dimension is comparable with the antenna, even using a high dielectric constant. Here an RIS with very small unit cell dimensions (cell area reduction by 95% compared to traditional RIS) is proposed and utilized to design a miniaturized antenna over the RIS substrate with the same size as the antenna itself. A microstrip transmission line over the RIS substrate model is studied and shown to have a high propagation constant near the resonant frequencies of the RIS. This model is used to predict the much reduced resonant frequency of patch antennas over the RIS. Applying the two-layer RIS substrate and an optimized miniaturized patch antenna topology, several UHF band patch antennas working around 400MHz have been designed and fabricated. Using this approach a miniaturized antenna with dimensions .0/11.4× .0/11.4 × .0/74, including the RIS substrate is developed.
Surface Electromagnetic Wave Characterization Using Non-invasive Photonic Electric Field Sensors
James Toney,Vincent E. Stenger, Peter Pontius, Andrea Pollick, Sri Sriram, Chi-Chih Chen, November 2013
Abstract— Electromagnetic properties of aircraft and missile skins have a large effect on radar cross sections and determine the level of stealth that is achieved over the various RF bands currently in use. RF absorption, reflection, and propagation along the skin surface all serve as important measures of the electromagnetic performance of the coated surfaces. Non-invasive probing of the electromagnetic field just above the propagating wave at multiple spots along the propagation direction can be used to determine and measure wave propagation parameters, including effective RF index, loss per length, wave impedance, and frequency dependent material properties of the coatings. Wide-band photonic electric field sensors have been demonstrated for probing of dielectric layers by measuring the traveling waves along the coated aircraft surface. The photonic E-field sensors are extremely linear and produce an exact real time analog RF representation of the electric field, including phase information. These ultra-wideband (UWB) photonic RF sensors are very small and contain negligible metal content, allowing them to be placed at close proximity without perturbing the RF surface waves. This is very important in accurately characterizing highly damped surface waves on absorber layers. This paper discusses the linearity, bandwidth, polarization, and sensitivity of the unique UWB photonic E-field sensor design. Experimental results are presented on surface-wave characterization measurements using these sensors.
Signal to Noise Ratio of Electrically Small Antennas Impedance Matched using Non-Foster Circuits
Aseim Elfrgani,Roberto Rojas, November 2014
Electrically small antennas (ESAs) are neither efficient nor good radiators because their radiation quality factor is considerably high.  It is therefore critical to add appropriate matching networks (MNs) to the antenna to enhance its realized gain and therefore its performance over a large frequency range.  For receiver applications, the important factor for the electrically small antenna is the signal-to-noise ratio (SNR). In this paper, the design of stable non-Foster circuits (NFCs) to improve the performance of the ESA in terms of the realized gain and the SNR has been achieved. Measurements of the signal and noise of the electrically small antenna with and without non-Foster circuits has been performed in an outdoor environment.  Key steps of the measurements will be shown including the post-processing of the data, which is an important step to reduce the effect of undesired signals.  Based on the measured data, it is shown that the Non-Foster circuits improve both antenna gain and SNR by more than 15dB over a wide frequency range, namely, 100MHz to 700MHz, with respect to the case without a NFC. To the best knowledge of the authors, this improvement is maintained over the widest frequency band among all the published work.  Excellent agreement between simulation and measurement results in terms of gain and signal improvements is obtained and will be highlighted in our presentation.
Probe-corrected Phaseless Planar Near-Field Antenna Measurements at 60 GHz
Javier Fernández Álvarez,Sergey Pivnenko, Olav Breinbjerg, November 2015
Antenna measurements at increasing working frequencies carry the difficulty of reliably measuring the signal phase, due to effects of cable bending, thermal drift, etc, and the resulting impedance mismatch which introduces uncertainty in the measurement results. In this paper we investigate the problem of phaseless measurements and phase retrieval for planar near-field measurements, together with the application of probe correction of the retrieved results, to the best of our knowledge the first experimental case of probe correction in phaseless near-field antenna measurements. A phase retrieval method based on an iterative Fourier technique (IFT) is proposed and tested with measurements of a Standard Gain Horn at 60GHz acquired at the planar near-field (PNF) scanner facility at the Technical University of Denmark. The obtained results indicate good agreement with a measured reference pattern within the region of validity when the probe correction is applied after performing the phase retrieval from a pair of uncorrected probe signals. Application of the probe correction before the phase retrieval, on the other hand, shows not satisfactory results. Additional improvements are obtained by introducing spatial filtering at the AUT aperture, thus enhancing performance of the algorithm by reducing phase noise of retrieved fields. Also, a “double-iterated” approach is explored, with additional phase-retrieval iterations after probe correction, with the aim of introducing true electric fields into the IFT.


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