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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.
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.
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.
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.
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.