Meng Li

University of Wisconsin–Madison, Madison, Wisconsin, United States

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Publications (19)18.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We introduce a new technique for designing miniaturized-element frequency selective surfaces (MEFSSs) with bandpass responses and no spurious transmission windows over extremely large bandwidths. The proposed harmonic-suppressed MEFSSs consist of multiple metallic and dielectric layers. Each metallic layer is in the form of a two-dimensional arrangement of capacitive patches or an inductive wire grid with extremely sub-wavelength periods. Harmonic-free operation in these structures is achieved by using multiple, closely spaced capacitive layers with overlapping unit cells to synthesize a single, effective capacitive layer with a larger capacitance value. This allows for reducing the unit cell size of a conventional MEFSS considerably and moving the natural resonant frequencies of its constituting elements to considerably higher frequencies. Consequently, the spurious transmission windows of such MEFSSs, which are caused by these higher order harmonics, can be shifted to very high frequencies and an extremely broad frequency band free of any spurious transmission windows can be obtained. Using this technique, an MEFSS with a second-order bandpass response is designed to operate at 3.0 GHz with 20% fractional bandwidth and be free of spurious transmission bands up to 27.0 GHz. A prototype of this harmonic-free MEFSS is fabricated and experimentally characterized in the lab. Measurement results confirm harmonic-free operation of the proposed FSS for incidence angles in the $pm {hbox {60}}^{circ} $ range for both the ${hbox {TE}}$ and ${hbox {TM}}$ polarizations of incidence.
    No preview · Article · May 2014 · IEEE Transactions on Antennas and Propagation
  • Meng Li · Nader Behdad
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    ABSTRACT: We present a new technique for designing microwave lenses with broadband, true-time-delay response types. The lenses examined in this work are planar structures with circular apertures populated with numerous spatial time-delay units (TDUs). Each TDU is the unit cell of an appropriately designed lowpass frequency selective surface (FSS) that provides a desired time delay over a wide frequency range. The lowpass FSSs used in this paper are either metallo-dielectric or all-dielectric type multi-layer structures. A metallo-dielectric lowpass FSS is composed of a number of capacitive patch layers separated from each other by thin dielectric substrates. An all-dielectric lowpass FSS, on the other hand, is composed of high- εr and low-εr dielectric substrates cascaded sequentially. Two metallo-dielectric lowpass FSS-based true-time-delay (TTD) lens prototypes and one all-dielectric lowpass FSS-based TTD lens prototype with focal length to aperture diameter ratios (f/D) of 1, 1.5 and 1.3 are designed, fabricated, and experimentally characterized. They respectively operate over a bandwidth of 30%, 50% and 40% without any chromatic aberrations. This is demonstrated experimentally by characterizing the responses of these lenses both in frequency domain and in time domain. Moreover, all of these lenses demonstrates excellent scanning performances with fields of views of ±60°.
    No preview · Article · Aug 2013 · IEEE Antennas and Propagation Magazine
  • Meng Li · Nader Behdad
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    ABSTRACT: We present a new technique for designing planar microwave lenses with broadband, true-time-delay response types. The lenses examined in this work are planar structures with circular apertures populated with numerous spatial time-delay units (TDUs). Each TDU is the unit cell of an appropriately designed lowpass frequency selective surface (FSS) that provides a desired time delay over a wide frequency range. The lowpass FSSs used in this paper are all-dielectric type multi-layer structures that are composed of high-εr and low-εr dielectric substrates cascaded sequentially. An all-dielectric lens prototype with focal length to aperture diameter ratios (f=D) of 1.3 is designed, fabricated, and experimentally characterized. It is experimentally demonstrated that the fabricated lens can operate over a bandwidth of 40% without any chromatic aberrations. Moreover, the proposed lens demonstrates an excellent scanning performance with fields of views of ±60°.
    No preview · Conference Paper · Jul 2013
  • Nader Behdad · Meng Li
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    ABSTRACT: form only given. Ultra-wideband (UWB) antennas have been the subject of many studies due to their widespread applications in areas ranging from wireless communications and sensing to radar systems and microwave imaging. The size of a UWB antenna is primarily determined by its lowest frequency of operation. Therefore, in applications where relatively low RF/microwave frequencies are used, the physical size of a UWB antenna may become prohibitively large. Additionally, in certain low-frequency applications such as military communications at HF, VHF, and UHF bands, antennas with monopole-like radiation patterns (i.e., vertically-polarized with omnidirectional radiation patterns) are required. In such applications, besides having compact dimensions, reducing the overall height of the antenna and its overall visual signature becomes extremely important as well.
    No preview · Conference Paper · Jul 2013
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    ABSTRACT: This paper describes using thermoelectric generators to power wireless sensors to monitor spent nuclear fuel during dry-cask storage. OrigenArp was used to determine the decay heat of the spent fuel at different times during the service life of the dry-cask. The Engineering Equation Solver computer program modeled the temperatures inside the dry-cask during its service life. The temperature distribution of a thermoelectric generator and heat sink was calculated using the computer program Finite Element Heat Transfer. From these temperature distributions the power produced by the thermoelectric generator was determined as a function of the service life of the dry-cask. To investigate the ability to provide wireless signal transmission using this power source, CST Microwave Studio was used to determine the scattering parameter S2,1 for a horizontal dry-cask. Important parameters that can influence the transmission of the signal are antenna orientation, antenna placement, and transmission frequency. This analysis estimates that a thermoelectric generator can produce enough power for a sensor to function and transmit data from inside the dry-cask throughout its service life.
    No preview · Article · Apr 2013 · IEEE Transactions on Nuclear Science
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    ABSTRACT: We present a new technique for designing low-profile, ultrawideband, true-time-delay (TTD) equivalent microwave lenses. Such a lens is composed of numerous spatial time-delay units (TDUs) distributed over a planar surface. Each spatial TDU is the unit cell of an appropriately designed miniaturized-element frequency selective surface and provides a frequency-independent time delay within the frequency band of interest. Two TTD lens prototypes with focal length to aperture dimension (f/D) ratios of 1 and 1.6 are designed, fabricated, and experimentally characterized at X-band. The 3-dB gain bandwidths of these lenses are respectively 7.5-11.6 and 7.8-11.5 GHz. Each fabricated lens has an overall thickness of 4.76 mm, which corresponds to ~ 0.150λ0, where λ0 is the free-space wavelength at the center frequency of operation. Each lens uses spatial TDUs with physical dimensions of 6 × 6 mm2, or ~ 0.19λ0 × 0.19λ0. Both lenses have a system fidelity factor close to 1, when excited with a broadband pulse. Furthermore, due to their true-time-delay equivalent behavior, the fabricated lenses do not suffer from chromatic aberration within their operational bands. When used in a beam-scanning antenna system, each lens demonstrates an excellent scanning performance in a field of view of ± 60°.
    No preview · Article · Mar 2013 · IEEE Transactions on Antennas and Propagation
  • Meng Li · Nader Behdad
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    ABSTRACT: We examine the peak power handling capability of a class of miniaturized-element frequency selective surfaces composed entirely of non-resonant constituting elements. The effects of various design parameters on the peak power handling capability of these structures are investigated using electromagnetic simulations and methods for increasing their peak power handling capability are proposed. These methods are used to design a high-power microwave (HPM) frequency selective surface (FSS), which is expected to be capable of handling extremely high peak power levels. The power handling capabilities of these devices are also experimentally investigated using an HPM source with a frequency of 9.382 GHz, a peak power of 25 kW, and a pulse length of 1 μs. Unit cells of various FSSs under investigation are placed in a waveguide and excited with pulses with variable power levels. The time-domain reflection and transmission coefficients of each device is measured at various power levels and the power level at which the device breaks down is determined. The results of these experimental investigations follow the same trend observed in the simulations. Additionally, our experiments demonstrate that the HPM FSSs developed in this work are indeed capable of handling extremely high peak power levels.
    No preview · Article · Feb 2013 · IEEE Transactions on Antennas and Propagation
  • Meng Li · Yazid Yusuf · Nader Behdad
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    ABSTRACT: In this paper, we present a low-profile, compact, ultra-wideband antenna that uses a set of closely coupled radiators. The system of two coupled radiators has two different linearly independent modes of operation with complementary frequency bands of operation. These include the differential mode and the common mode of operation. When the antenna is excited in the common mode of operation, it acts as an ultra-wideband (UWB) antenna covering a broad frequency band. When excited in the differential mode, the antenna operates as a wideband dipole in a frequency range below that of the common mode. Thus, by appropriately combining the two modes using a suitably designed feed network, the bandwidth of the antenna can be extended and its lowest frequency of operation is reduced. Mode combining is achieved with a feed network that employs a frequency-dependent phase shifter. Using this feed network, the two modes of the antenna are combined and a single-port broadband device is achieved that has a bandwidth larger than that of either the common or the differential mode individually. A prototype of the antenna is fabricated and experimentally characterized.
    No preview · Article · Jan 2013 · Progress In Electromagnetics Research B
  • Nader Behdad · Meng Li · Yazid Yusuf
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    ABSTRACT: We present a compact, ultra-low-profile, ultrawideband (UWB) antenna with monopole-like radiation characteristics. The antenna is composed of two bent diamond-shaped loops that are fed in parallel and loaded with a common capacitive top hat to reduce their lowest frequencies of operation. A series capacitor is used at the feeding terminals of one of the loops to create an additional resonance below the lowest frequency of operation of one of the loops and, hence, further reduce the lowest frequency of operation of the antenna. At the lowest frequency of operation, the proposed antenna has electrical dimensions of L × W × H = 0.22λmin × 0.22λmin × 0.033λmin, where λmin is the free-space wavelength. The antenna demonstrates a VSWR lower than 3:1 and consistent monopole-like, omnidirectional radiation patterns over a 4:1 bandwidth. A prototype of the antenna with physical dimensions of 7 × 7 × 1 cm3, which operates in the 1.0-4.0-GHz frequency range, is fabricated and characterized using a near-field system.
    No preview · Article · Jan 2013 · IEEE Antennas and Wireless Propagation Letters
  • Meng Li · Nader Behdad
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    ABSTRACT: We examine fluidically tunable periodic structures acting as highly-selective frequency selective surfaces (FSSs) or spatial phase shifters (SPSs) capable of providing phase shifts in the range of 0°-360°. These devices are multi-layer periodic structures composed of non-rePsonant unit cells. The tuning mechanism is based on integrating small, movable liquid metal droplets with the unit cells of the periodic structure. By moving these liquid metal droplets by small distances within the unit cell, the structure's frequency response can be tuned continuously. Using this technique, a fluidically tunable FSS with a fifth-order bandpass response is designed and its tuning performance is examined for various incidence angles and polarizations of the incident EM wave. Additionally, electronically tunable counterparts of the same structure are also designed and their tuning performances are examined under short-duration high-power excitation conditions. It is demonstrated that such electronically tunable FSSs/PSSs demonstrate extremely nonlinear responses. Since the fluidically tunable structure examined in this work does not use any nonlinear devices, its response is expected to remain linear for such short-duration high-power excitation conditions. The tuning performances of these fluidically tunable periodic structures are also experimentally demonstrated by fabricating three prototypes and characterizing their responses in a waveguide environment.
    No preview · Article · Jun 2012 · IEEE Transactions on Antennas and Propagation
  • Meng Li · Nader Behdad
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    ABSTRACT: We present the design and analysis of a frequency selective surface (FSS) capable of providing a third-order bandpass response. The transfer function of the proposed FSS has a transmission null, the frequency of which can be tuned to suppress strong interference signals with frequencies close to or within the main transmission band. The principles of operation of the device are presented along with an approximate analytical procedure for synthesizing the desired FSS response. Two prototypes are fabricated and characterized using a free-space measurement setup and the FSS's different modes of operation are experimentally demonstrated. Finally, the tuning performance of the FSS's transmission null is experimentally demonstrated in a WR-90 waveguide environment.
    No preview · Article · Apr 2012 · IEEE Transactions on Antennas and Propagation
  • Meng Li · N. Behdad
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    ABSTRACT: In this paper, a new technique for designing low-profile, ultra-wideband, true-time-delay (TTD) microwave lenses is proposed. Such a lens is composed of numerous spatial TTD units distributed over a planar surface. Each spatial TTD unit is the unit cell of an appropriately-designed metamaterial structure, which is composed entirely of non-resonant constituting elements. The proposed TTD lens is a low-profile structure with an overall thickness of only 0.15λ0 and uses spatial TTD units with dimensions of 0.19λ0 × 0:19λ0, where λ0 is the free-space wavelength at the center frequency of operation. Unlike most metamaterial lenses, which tend to be narrow band and highly dispersive, the proposed lens has a wide operational bandwidth of 30% and a high system fidelity factor close to 1. Furthermore, due to its true-time-delay nature, the fabricated lens prototype does not suffer from chromatic aberration within its operational band. When used in a beam-scanning antenna system, the fabricated prototype demonstrates an excellent scanning performance in a field of view of ±60°.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: This paper presents a novel fully integrated X-band energy harvester, with on-chip antenna, matching network, and a multi-stage RF-DC rectifier implemented in IBM 0.18-μm RF CMOS technology. We investigated different matching schemes, antennas, and rectifiers with focus on the interaction between building blocks. Among different matching topologies, the series inductive scheme demonstrated the best performance for X-band to DC energy conversion. Our measurement results show that the proposed X-band RF-DC rectifier generates a 1.23 V DC output with 304 KΩ load at 9.2 GHz with -8.0 dBm input RF power, providing a power conversion efficiency of 3.1%.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: Many insects possess acute directional hearing capabilities and are able to localize sound sources of interest with an astonishing degree of precision. An analogy can be drawn between the auditory systems of such insects and electrically small antenna arrays that demonstrate enhanced sensitivity to the direction of arrival of an electromagnetic wave, compared to regular arrays occupying the same aperture. Inspired by this, we discuss the design of biologically inspired electrically small antenna arrays that mimic the hearing mechanism of such insects. A method for designing such antenna arrays is presented, and the tradeoffs involved in achieving this enhanced sensitivity are discussed. Simulation and measurement results of two fabricated prototypes are also presented and discussed in this letter.
    No preview · Article · Feb 2011 · IEEE Antennas and Wireless Propagation Letters
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    ABSTRACT: Despite their small size compared to the sound wavelength, many small animals are able to accurately localize a sound source of interest. Noteworthy among these small animals is the parasitoid fly Ormia Ochracea that can determine the direction of arrival of a sound wave of interest with an astonishing degree of precision. An analogy can be drawn between the hearing mechanisms of this insect and an electrically small antenna array, composed of two isotropic receiving elements, that is capable of resolving the direction of arrival of an electromagnetic wave with a fine angular resolution. Inspired by this, we introduce the concept of biomimetic, super-resolving electrically small antennas in this paper. A method for designing such electrically small antenna arrays as well as simulated and measured results of a fabricated prototype are also resented and discussed in this paper. These biomimetic antenna arrays could be used in numerous applications ranging from miniaturized RF sensors and direction finding systems to small aperture, high- resolution microwave imaging systems and radars.
    No preview · Article · Jan 2011
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    ABSTRACT: We present a class of metamaterials capable of operating under extremely high power microwave (HPM) fields. The proposed metamaterials are in the form of periodic arrangements of unit cells with sub-wavelength dimensions and periodicities. Each unit cell is composed of multiple thin impedance sheets providing non-resonant reactive surface impedances separated from each other by ultra-thin dielectric spacers. The proposed structures can be designed to operate as spatial filters with highly selective spectral responses capable of handling power density levels in the order of 1.0 MW/cm2. Such filters could ultimately be used as counter high-power-microwave devices in HPM systems. In addition to their filtering characteristics, over a certain frequency range, the unit cells of these structures could be treated as spatial phase shifters or true-time-delay units (TTDU). We will discuss how tunable versions of these TTDUs can be used to design wideband, tunable HPM lenses for phased-array applications. Finally, we will discuss the possibility of using non-linear versions of these HPM lenses for pulse shaping applications at high-power-microwave levels.
    No preview · Article · Nov 2010
  • Meng Li · Bin Yu · Nader Behdad
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    ABSTRACT: In this letter, a new technique for designing tunable frequency selective surfaces (FSSs) with the second-order band-pass response is presented and experimentally verified. The technique is applied to an FSS utilizing nonresonant, subwavelength, constituting unit cells. Frequency tuning is achieved by adjusting the location of a liquid metal slug within capillary tubes embedded between the FSS's metallic layers. A wideband flexible frequency tuning covering nearly the entire X band is tested in a waveguide environment and excellent agreement between the measured and simulated results is demonstrated.
    No preview · Article · Sep 2010 · IEEE Microwave and Wireless Components Letters
  • Meng Li · N. Behdad
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    ABSTRACT: In this paper, we apply this technique to a class of MEFSSs with second-order band-pass responses capable of providing a phase shift of 0180° in their pass-band. We show how dynamic phase shift tuning can be obtained. To prove the proposed tuning concept, a waveguide version of a second-order band-pass MEFSS is fabricated and tested at X-band.
    No preview · Conference Paper · Aug 2010
  • Meng Li · Nader Behdad
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    ABSTRACT: A new technique for design low profile FSS with high order band-pass performance is presented. By choosing the hybrid resonator properly, single band with third order performance or dual band with second order behavior at upper band and first order behavior at lower band can be realized.
    No preview · Conference Paper · Aug 2010