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This contribution details a procedure to collect and process necessary data to describe the antenna patterns of PAAs using a planar near-field (NF) range. It is proposed that a complete characterization methodology involves not only capturing beam-steered antenna patterns, but also measuring embedded element patterns, exhaustive testing of the excitation hardware of the antenna under test (AUT), and performing a phased array calibration technique. Moreover, to demonstrate the feasibility of the proposed approach, the methodology is applied onto a 2x8 microstrip patch PAA, proving its utility and effectiveness. Finally, by means of the collected data, any array pattern could be predicted by post-processing, as proven by the great agreement found between a measured pattern and its computed predicted version.
Flared notch (“Vivaldi”) arrays have been a subject of great interest since the mid 1990s for use in broadband phased-array systems. These arrays are popular in large part due to their ultra-wide bandwidths, which can span multiple octaves, exceeding the bandwidths of the individual flared notch elements themselves. This effect is achieved via strong inter-element coupling, a departure from the conventional wisdom of minimizing mutual coupling between elements in a phased array. The benefits of this design choice have been widely reported on in the literature - however, this dependence on element coupling also places serious constraints on array performance, especially with regards to scan angle, active impedance, and array efficiency, which often go unreported. In addition, reliance on inter-element coupling necessitates an array that can be safely approximated as “infinitely” planar. If an array does not strictly meet this condition, significant VSWR issues can result, especially for elements near the edges of the array. This paper discusses the common pitfalls inherent in practical flared-notch array design that are often overlooked in the literature. To aid in this analysis, a network-centric approach to array modeling is demonstrated that allows for an examination of both element- and array-level performance metrics in a way that minimizes computation time and resources. Special attention is paid to parameters such as active impedance as a function of scan angle, which, though vital to array performance, are often mischaracterized by “infinite array” approximations commonly used by engineers in the design phase. The effects of mutual coupling on different array performance metrics, both beneficial and detrimental, are examined in detail so that an informed decision can be made on the suitability of the flared-notch topology for a given application.
Microstrip patch antennas are well known in the field of communications and other areas where antennas are used. They consist of a metallic conducting surface deposited onto a grounded dielectric substrate and are widely used in situations where a conformal antenna is desired. They are also popular antennas for array applications. But most patch antennas are typically resonant structures owing to the standing wave of current that forms on them. This resonant behavior limits the impedance bandwidth of the antenna to a few percent. In this paper we shall present an approach for improving the bandwidth of a resonant patch antenna which employs an engineered anisotropic superstrate. By proper design of this superstrate and its tensor, and proper alignment of it with the axis of the patch, an antenna with improved impedance bandwidth results. Some of the challenges associated with the measurement of the anisotropic superstrate will be discussed, ranging from 3D simulations to physical models tested in the laboratory. A final working model of the antenna will be discussed; this model consists of a stacked patch arrangement and was designed to operate at the GPS L1 and L2 frequencies. Data collected from 3D simulations using CST Microwave Studio along with laboratory and anechoic chamber measurements will be presented, showing how the bandwidth at both of these frequencies can be increased while maintaining circular polarization in both passbands. Tolerance to errors in alignment and fabrication will also be presented. Additionally, some lessons learned on anechoic chamber measurements of the antenna’s gain and axial ratio will be discussed.
Gain is a principal property of antennas; it is essential in establishing the link budget for communication and sensing systems through its presence in Friis’ transmission formula and the radar range equation. The experimental determination of antenna gain is most often based on a gain-transfer technique involving a reference antenna for which the gain has been calibrated to high accuracy; this is typically a pyramidal horn antenna [1]. The required accuracy of antenna gain obviously depend on the application; in some cases it can very high, ±0.1 dB or less, and this implies an even higher accuracy, of the order of ±0.01dB, for the gain reference antenna. This work investigates the accuracy to which a gain reference antenna can be calibrated; the investigation is based on experimental spherical near-field antenna measurements [2] and computational integral equation / method of moments simulations [3]. While calibration of gain reference antennas has been studied in many previous works, even works from early 1950s [4]-[6], this work is novel in systematically supporting measurements with full-wave simulations. Such simulations facilitate the study of e.g. the effect of multiple reflections between antennas at short distances. We study two absolute calibration techniques for the gain of pyramidal horn antennas. The first technique determines gain as the product of directivity and radiation efficiency; this technique has been referred to as the pattern integration technique [7] (which is not an entirely adequate designation since gain cannot be determined from the radiation pattern). The second technique determines the gain from Friis’ transmission formula [8] for two identical antennas; this technique is generally referred to as the two-antenna technique [1]. These two calibration techniques involve very different steps and contain very different sources of error; for both techniques our investigation involves measurements as well as simulations. For the pattern integration technique we compare experimental and computational results for the directivity and demonstrate agreement within one-hundredth of a dB. The radiation efficiency is calculated by different techniques based on the surface impedance boundary condition for the metallic walls of the pyramidal horn. This technique is not influenced by proximity effects or by impedance mismatch between the measurement system and the gain reference antenna. For the two-antenna techniques we compare experimental and computational results for the gain and we compare the calculated distance-dependence with that of the extrapolation technique [9]. It is demonstrated how the use of the phase center distance in Friis’ transmission formula notably decreases the necessary separation between the antennas for a required accuracy, but that multiple reflections may then become a limiting factor. This technique is highly influenced by the impedance mismatch that must be accurately accounted for. We compare the gain values resulting from the pattern integration technique and the two-antenna technique, including their very different uncertainty estimates, for a C-band standard gain horn. The work is related to an on-going ESA project at the DTU-ESA Spherical Near-Field Antenna Test Facility for the on-ground calibration of the scatterometer antennas of the EUMETSAT MetOp Second Generation B-series satellites. IEEE Standard – Test Procedures for Antennas, Std 149-1979, IEEE & John Wiley & Sons, 1979. J.E. Hansen, “Spherical Near-Field Antenna Measurements”, Peter Perigrinus Ltd., London 1987. www.wipl-d.com W.C. Jakes, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 160-162, February 1951. E.H. Braun, “Gain of Electromagnetic Horns”, Proceedings of the IRE, pp. 109-115, January 1953. W.T. Slayton, “Design and Calibration of Microwave Antenna Gain Standards”, Naval Research Laboratory, Washington D.C., November 1954. A. Ludwig, J. Hardy, and R. Norman, “Gain Calibration of a Horn Antenna Using Pattern Integration”, Technical Report 32-1572, Jet Propulsion Laboratory, California Institute of Technology, October 1972. H.T. Friis, “A Note on a Simple Transmission Formula”, Proceedings of the I.R.E. and Waves and Electrons, pp. 254-256, May 1946. A.C. Newell, R.C. Baird, P.F. Wacker, “Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique”, IEEE Transactions on Antenna and Propagation, vol. 21, no. 4, pp. 418-431, July 1973.
Wireless connectivity is rapidly expanding in both popularity and potential. Incorporating antenna arrays on both ends of the wireless channel realizes this potential by facilitating beam steering and increased directivity. From an operations vantage point, these capabilities reduce transmit power, increase data rates, and extend communication range. Antenna arrays also facilitate forming nulls toward antagonistic regions to hide information and thwart easily accessible jamming devices. These performance characteristics of antenna arrays address several critically important challenges for Unmanned Aerial Vehicle (UAV) operation, which is becoming attractive in both military and commercial sectors. Maintaining wireless communication channels over extended ranges that can potentially cross into antagonistic regions helps accomplish precise, adaptable mission objectives. In addition, efficiently utilizing power, a scarce commodity typically drawn from solar panels, facilitates extended flight durations. Finally, the reduced transmit power also reduces the aircraft weight, which can further extend flight duration. Although antenna arrays offer extensive advantages, the final design must account for the presence of the aerial platform including other electronic systems. Strong mutual coupling can then result from operating multiple wireless systems within a physically confined space. In addition, the surrounding environment can change the electrical characteristics of the antenna (e.g. input impedance and radiation pattern). Analyzing these electrical characteristics on a physically-confined platform becomes an electrically-large problem when operating a communication channel over the 2.4 GHz ISM band. Simulating the installed performance potentially requires significant computational resources unless research is conducted to understand the trade-offs between numerical methods in existing commercial software. The installed performance of an antenna array on an electrically-large platform can then be optimized in the most efficient manner. In this paper, we demonstrate a process that efficiently navigates the typical trade-offs engineers encounter when conducting an antenna placement study. This process involves designing a conformal antenna array in an isolated environment, analyzing potential installation locations and further optimizing the antenna array for the chosen location.
Antennas utilized as probes, sources, and for gain comparison are typically specified to have excellent cross polarization levels, often on the order of 50 dB below the primary polarization component. In many cases, these antennas are fed with a waveguide-to-coaxial adapter, which can be sourced from a multitude of vendors. Depending on the design and construction of the adapter, and the distance from the excitation probe to antenna aperture, the adapter itself can contribute significantly to the degradation of the polarization purity of the antenna. These adapters typically use one of several methods to achieve a good impedance match across their bandwidths, including tuning screws, posts and stubs. These tuning elements may be arranged asymmetrically and can cause the waveguide to be overmoded locally. Additionally, there is wide variance in the separation of the adapter excitation probe and waveguide electrical flanges, which may not be long enough to suppress the higher order modal content. In this paper, we study the effects of adapter to antenna aperture coupling, including the coupling of fields local to the current probe as well as those that are induced by design asymmetries. The results of the analysis lead to a number of rules of thumb which can be used to ensure that the antenna polarization purity is optimized.
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.
Mars rover Direct-to-Earth (DTE) communication is an exciting new development that can maintain transfer of high volumes of scientific data from Mars to Earth. Currently, large orbiting assets are used as a relay to return scientific data, often containing higher data rates than current DTE systems. Therefore, the goal of this paper is to investigate antenna array topologies to augment DTE systems to support high data rates. The antenna design is complex, having to simultaneously support dual-band, high gain, high power handling, and circular polarization capabilities. An exhaustive study of patch elements in literature shows that current geometries are infeasible for a Mars rover DTE system. A CP Half E-shaped patch element is developed, containing important dual band S11/AR performance in the required RX and TX bands while featuring a single-feed single-layer design. Moreover, various subarray architectures are evaluated to determine if the gain requirements can be achieved. To meet this gain requirement, a 4x4 subarray topology is designed which allows a modular, scalable, and high gain design. To feed each of the 4x4 element subarrays, a stripline feed network is developed, consisting of a binomial impedance transformer and a four stage 1:2 power divider. This feed network supported a broadside radiation pattern for the subarray topology. These components are then integrated, first through a full wave simulation in HFSS. This rigorous study showed support for Mars rover DTE communications systems. The integrated subarray design is then fabricated and measured using a spherical near-field chamber in the UCLA Center for High Frequency Electronics (CHFE) facilities where measurements showed a very good comparison to the simulation results. Overall this integrated subarray design was successful, showing dual-band, high gain, high power handling, and CP performance.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.