Browsing by Subject "Periodic structures"

Now showing 1 - 3 of 3
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    Development of design Approaches for passive RF and microwave circuits using periodical structures.
    (2009-09) Kim, Ho Saeng
    This thesis deals with several design and analysis techniques for RF/microwave passive circuits such as interconnects, filters, and antennas that offer circuir size reduction and lower fabrication cost associated with future integrated communication systems. Presented are new design approaches to enhance performance of passive circuits and offer the ability to reduce size, minimize mismatch, reduce group delay variation, and alleviate unwanted odd-mode excitation. This work is developed by exporing and using periodic structure behavior in coplanar waveguide (CPW) designs. Periodic structures are known to have bandgaps such as those seen in electromagnetic bandgap (EBG) designs. Circuit based models were developed to predict the stop-band frequency range and to reduce computational time associated with full wave models. To reduce circuit size slow-wave designs can be employed. Thus, periodic structures in the form of interdigitated designs can be used to achieve this function when operating well below the bandgap frequency range. Herein, interdigitated coplanar waveguide designs are developed by increasing both inductance and capacitance per unit length to reduce circuit size, control signal phase, and match signal velocity. Geometrical characteristics of the proposed structures are analyzed with the S-paramters, effective dielectric constant, and attenuation constant. Design guidelines, which relate the effective dielectric constant to geometrical parameters, are also developed. To suppress slot-line mode excitation in circuits with structures, namely right-angle bends, slow wave structures can be used. In this work, novel designs for wire-bond free circular interdigitated bends are presented and compared to circuits with right-angle bends. Also shown is a novel alternative approach which presents a fast-wave design method and exploits it for suppression of slot-line mode excitation. In filter design, the interdigitatated approach is used to (1) reduce the size of a standard EBG based stopband filter by approximately 45% and (2) reduce the size of an ultra-wide band (UWB) bandpass filter by 40% when combined coupled line with meandered slot. Lately, annular ring slot antennas are miniaturized by leveraging the periodic nature of meander geometry to reduce the surface area of single- and dual-band annular ring slot antennas by 40% and 35%, respectively. The performance of all structures is evaluated by comparing modeled designs to the measured one in this thesis.
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    Investigation of Metamaterial Transmission line resonators for Ultra-High Field Magnetic Resonance Imaging RF Coils
    (2018-10) Panda, Vijayaraghavan
    The objective is to develop a highly efficient RF head coil on a thin substrate for the Ultra-high magnetic field (7 T and above) MRI systems. The artificial Metamaterial resonator is investigated for this purpose. Simulation and experimental results are provided for an 8-channel Metamaterial based RF coil in comparison with a standard high performance 8-channel dipole based RF coil for the 10.5 T MRI system. Each element is 180 mm (approximately a quarter of a wavelength λ0) long, identical, evenly spaced along the circumference of the cylindrical phantom, loaded with dielectric material, and referred to as inverted Metamaterial Zeroth Order Resonator. The resonator elements are open circuited, matched, and tuned to 447.06 MHz with the phantom. An unloaded to loaded quality factor ratio of 2.97 is obtained from the scattering matrix of the proposed design. The length independent nature of the proposed design and the flexibility of the lumped elements have provided an optimized element with a substrate thickness of roughly 3 mm (λ0/200). With the proposed design, a RF magnetic field strength (B1+) to √SAR ratio of 1.38 (compared to 1.46 for dipole) is obtained. Optimization of the physical design parameters, especially the distance between the element and the phantom, is performed to improve the transmission efficiency of the metamaterial based RF coil element. The amount of radiated power reaching 45 mm inside the phantom is used for the comparison. The optimal design for a 16 cm long, 7 T metamaterial resonator shows an increase of 1.1 dB and 3.2 dB in transmit power when compared to a dipole and microstrip element of same length. Similarly, the optimal design for an 18 cm long, 10.5 T metamaterial resonator shows an increase of 1.6 dB in power when compared to a 12 cm long microstrip and a 0.2 dB decrease in power when compared to an 18 cm long dipole. An electron band-gap (EBG) periodic structure is designed as the ground plane for the proposed metamaterial resonator. The simulation results show increased B1+ field magnitude when compared with the metamaterial resonator with a solid ground plane. Similarly, a technique of improving the surface current density by creating slots along the line is implemented in the 7 T microstrip resonator. The experimental results show an improved coil efficiency after including of the slots. The extendibility of the coil conductor length with the Metamaterial resonators is also shown by designing a 48 cm long, 10.5 T metamaterial loop body element (longer than a wavelength). Simulations and experimental results confirm the functionality of the loop element and show the comparison with a traditional loop element.
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    Nonlinear mechanisms of wave propagation in periodic structures: harmonic generation, dispersion correction, and their interplay
    (2020-12) Jiao, Weijian
    Periodic structures have been extensively investigated due to their unique dynamical properties, which have enabled a broad range of practical applications, especially in the context of wave control. Considering nonlinearity in periodic structures not only leads to a more complete description, but also opens new doors for the design of functional and tunable metamaterials. In this thesis work, we are interested in the dynamical behavior of periodic structures in the weakly nonlinear regime. In the case of quadratic nonlinearity, a well-known effect on wave propagation is second harmonic generation (SHG), which gives rise to a secondary harmonic in addition to the fundamental harmonic that is nearly identical to the linear response. This effect provides an opportunity to nonlinearly activate a second harmonic that exhibits complementary modal characteristics to those of the fundamental harmonic, thereby enriching the modal characteristics involved in the total response. Then, this modal enrichment functionality is explored in 1D periodic structures featuring internal resonators via numerical and experimental analysis, in which we use SHG as a mechanism to achieve energy trapping and localization in the resonators. Moreover, we extend our focus to experimentally demonstrate all the key components induced by SHG in 2D lattices of repulsive magnets supported by pillars. As for cubic nonlinearity, the effect on wave propagation is an amplitude-dependent correction of the dispersion relation, which can manifest either as a frequency shift or as a wavenumber shift depending on how the excitation is prescribed. Compared to the vast study on frequency shift, the scenario of wavenumber shift has only been marginally explored. To fill this gap, we first present a multiple scales framework to analytically capture the wavenumber shift on the dispersion relation of monatomic chains, showing that wavenumber shift is associated with harmonic boundary excitation. We then extend the framework to periodic structures with internal resonators to achieve tunability of locally resonant bandgaps. Last, we investigate the effects of the interplay between quadratic and cubic nonlinearities in periodic waveguides. Through two conceptual applications, we demonstrate that these effects can be leveraged to unveil an array of wave control strategies for the design of tunable metamaterials with self-switching functionalities.