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Online ISSN 1751-8733 Print ISSN 1751-8725

IET Microwaves, Antennas & Propagation

Volume 5, Issue 15, 9 December 2011

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Volume 5, Issue 15

9 December 2011

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    • Dyadic Green's functions for multi-layered uniaxially anisotropic media with arbitrarily oriented optic axes
      • Author(s): A. Eroglu ; Y.H. Lee ; J.K. Lee
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1779 –1788
      • DOI:  10.1049/iet-map.2010.0499
      • Type: Article
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      • p. 1779 –1788 (10)
        Dyadic Green's functions (DGFs) for multi-layered uniaxially anisotropic media are developed analytically when the optic axis of each layer is arbitrarily oriented. DGFs are expressed in terms of two characteristic waves – ordinary and extraordinary, in the k-domain with Fresnel reflection and transmission coefficients. The Fresnel coefficients are obtained by the application of boundary conditions at the anisotropic interfaces. The results have been checked in the limiting cases with existing results analytically. The results of this work can be used in electromagnetic radiation and scattering problems in the presence of multi-layered anisotropic media.
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      Cosecant-squared pattern synthesis using a weighted alternating reverse projection method
      Non-linearities modelling and post-compensation in continuous-time ΣΔ modulators
      • Author(s): C.A. Schmidt ; J.E. Cousseau ; J.L. Figueroa ; R. Wichman ; S. Werner
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1796 –1804
      • DOI:  10.1049/iet-map.2011.0204
      • Type: Article
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      • p. 1796 –1804 (9)
        The authors present a post-compensation technique for continuous-time (CT) ΣΔ modulators based on efficient finite-order Volterra models. At first, the authors develop a behavioural model for a CT sigma–delta modulator (SDM). This model includes non-linear and non-ideal behaviour and leads to a finite Volterra representation of the SDM. Then, they derive and discuss two novel compensation blocks that are special cases of Volterra models. These models, a memory polynomial (MP) and a modified generalised memory polynomial (MGMP), can be interpreted as generalisations of classical block-based non-linear models, like Hammerstein and Wiener systems, respectively. The authors show that the MGMP compensator offers a better distortion cancellation because of the inclusion of cross-terms at the output of the model, at the price of increasing complexity. Simulation results, based on a transistor level circuit model for the SDM, show a good agreement between the SDM and the developed models. In addition, the authors also verify good performance of the proposed compensators.
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      Ultra-wideband statistical propagation channel model for implant sensors in the human chest
      • Author(s): A. Khaleghi ; R. Chávez-Santiago ; I. Balasingham
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1805 –1812
      • DOI:  10.1049/iet-map.2010.0537
      • Type: Article
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      • p. 1805 –1812 (8)
        Implant medical wireless sensors for monitoring physiological parameters, automatic drug provision, and so on represent a new promising healthcare technology. Inherent characteristics of ultra-wideband (UWB) radio make this technology highly suitable for the wireless interface of implant sensors. A communication channel model is essential for developing these wireless systems. However, there are currently few models describing the radio propagation inside the human body. To address this problem, a statistical model is presented for UWB propagation channels inside the human chest in the 1–6 GHz frequency range. The proposed statistical model is developed from numerical simulations using a heterogeneous anatomical model that includes the frequency-dependent dielectric properties of different human tissues. Mathematical formulas for the computation of path loss, scattering and the statistical implementation of the channel impulse response at different depths inside the chest are described. Two typical depths for implanted sensors in the chest, namely 20 and 80 mm are analysed in detail. Average path loss of approximately 20 and 50 dB is observed in each case, respectively. Moreover, the channel exhibits little time dispersion with a root-mean-square delay spread below 1 ns in both cases. These results aim at facilitating the tasks associated with the design of in-body medical communication systems.
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      Parallel fast multiple method for electromagnetic scattering from one-dimensional large-scale two-layered rough surfaces for large angles of incidence
      • Author(s): L. Guo ; A. Wang ; C. Chai
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1813 –1821
      • DOI:  10.1049/iet-map.2010.0510
      • Type: Article
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      • p. 1813 –1821 (9)
        A fast multiple method (FMM) is proposed for the fast implementation of electromagnetic scattering from one-dimensional (1-D) two-layered rough surfaces. Both horizontal (HH) and vertical (VV) polarised incident waves are considered. In particular, the focus in this study is on large angles of incidence. The parallel FMM is presented based on the message passing interface (MPI) of personal computer clusters. The bi-conjugate gradient method is adopted to solve the unsymmetrical matrix equation and is parallelised according to the property of MPI. The simulation time and parallel speedup ratio with different processors are also provided for one surface realisation. The algorithm is validated by comparing the angular distribution of bi-static scattering with that of the finite-difference time-domain (FDTD) technique. Bi-static scattering coefficient (BSC) by two-layered rough surfaces with different incident angles is exhibited. Finally, the influences of the characteristic parameters of rough surfaces, the relative permittivity of the media, as well as the average height between the two rough surfaces on the BSC for large angles of incidence are discussed.
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      Improved electromagnetic inverse scattering procedure using non-radiating sources and scattering support reconstruction
      • Author(s): G.L. Gragnani  and  M. Diaz Mendez
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1822 –1829
      • DOI:  10.1049/iet-map.2011.0132
      • Type: Article
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      • p. 1822 –1829 (8)
        A new approach to solve the inverse electromagnetic scattering problem is presented. A closed-form expansion of radiating and non-radiating sources is used in an equivalent inverse source problem, together with a detection of the minimum convex scattering domain, in order to solve the original inverse scattering problem and to provide a method for electromagnetic imaging. Preliminary numerical investigations show the capabilities of this approach in achieving a good solution to an inverse scattering problem. Furthermore, a comparison between the results obtained by the present method and the simple reconstruction of the scattering object without the scattering support information is also presented, showing an improvement of the results obtained with the method here described.
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      Multiband slot-ring antenna with single- and dual-capacitive coupled patch for wireless local area network/worldwide interoperability for microwave access operation
      • Author(s): C.Y.D. Sim ; F.R. Cai ; Y.P. Hsieh
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1830 –1835
      • DOI:  10.1049/iet-map.2010.0462
      • Type: Article
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      • p. 1830 –1835 (6)
        A compact size slot-ring antenna design with a dual operating band that satisfies both wireless local area network and worldwide interoperability for microwave access operation is proposed. To achieve optimum impedance matching for both lower and upper operating bands, an inner circular radiating patch is capacitively coupled with the open end of a coplanar waveguide feed line, thereby demonstrating wideband characteristics from 1.97 to 4.25 GHz with a voltage standing wave ratio (VSWR) of 2:1 for the lower band, whereas the broad upper band which is excited owing to a pair of protruding stubs from the feed line can cover between 4.92 and 5.93 GHz. Further investigation on the method for a dual-capacitive coupled patch design is performed, whereby the inner circular patch is divided into two halves that are coupled with each other. The design process and initial characteristics of the two proposed antennas will be discussed in detail via simulated, and experimental results with satisfactory performance are observed within all bands of interest.
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      Suppression of microstrip filter spurious responses using frequency-selective resistive elements
      • Author(s): Frederick Huang
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1836 –1843
      • DOI:  10.1049/iet-map.2010.0532
      • Type: Article
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      • p. 1836 –1843 (8)
        The spurious responses of microstrip filters were suppressed to extend the stop band. Even-order spurious resonances are suppressed using resistive loads at the voltage nodes of the fundamental resonance, which are the antinodes at the spurious frequency. Odd orders are removed using additional resonators which are deliberately lossy, and whose fundamental resonant frequency is equal to the spurious resonance of the main resonators. Equations for initial estimates for losses and required coupling coefficients are given. Of three filters designed and measured, two show 30 dB stop band attenuation, at the cost of only a small increase in pass-band loss and a small increase in substrate area.
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      Design of a circularly polarised radio-frequency identification reader antenna with high return loss
      • Author(s): H.-W. Son ; H. Park ; H.-G. Jeon
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1844 –1848
      • DOI:  10.1049/iet-map.2010.0386
      • Type: Article
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      • p. 1844 –1848 (5)
        A circularly polarised (CP), radio-frequency identification (RFID) reader antenna with high return loss (RL) is presented. The antenna consists of a dual-polarised microstrip patch with high isolation and a Wilkinson power divider with a 90° phase shift between its two outputs. A novel analytical approach is developed for improving the RL of the dual-feed CP antenna and verified experimentally by fabricating and measuring a prototype antenna. The experimental results show that the RL of the prototype antenna is more than 30 dB over the frequency range of 866–936 MHz, which fully covers the bandwidth of North America (902–928 MHz) as well as South Korea (917–923.5 MHz) for a passive ultra-high-frequency RFID system. The peak antenna gain is above 8.1 dBic and the axial ratio is below 0.7 dB over the frequency range of 902–928 MHz.
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      Effective interpolation scheme for multi-resonant antenna responses: generalised Stoer–Bulirsch algorithm with adaptive sampling
      • Author(s): Y. El-Kahlout  and  G. Kiziltas
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1849 –1856
      • DOI:  10.1049/iet-map.2011.0021
      • Type: Article
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      • p. 1849 –1856 (8)
        Multiple analysis calls and high computational demand for each analysis present themselves as important bottlenecks in conducting practical electromagnetic design optimisation studies, particularly for complex material designs in metamaterial studies. To enable efficient re-analysis in such studies, the authors develop an algorithm for rational interpolation by generalising the Stoer–Bulirsch algorithm to allow for non-diagonal Neville paths rather than the known standard diagonal path to enhance its interpolation capability. The algorithm is then integrated to an adaptive sampling strategy that exploits the non-diagonality and finds an optimum Neville path that provides more efficient and reliable fittings for multi-resonance antenna response functions. The resulting technique is applied to return loss responses of antenna models with textured material substrates and complex conductor topologies. Interpolation results are compared with the performance of a standard Stoer–Bulirsch algorithm. Results show that the proposed generalised scheme outperforms the existing Stoer–Bulirsch technique in terms of computational accuracy by detecting resonances while still maintaining minimum number of support points. To demonstrate the capability of the proposed generalised algorithm, it is adapted to a large-scale antenna design optimisation example, achieving significant bandwidth performance enhancements for a novel antenna structure within practical timespans.
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      Analysis and design of longitudinally corrugated cylindrical tips for reduced diffraction in the shadow region of a conducting wedge
      • Author(s): M.A. Christou  and  A.C. Polycarpou
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1857 –1862
      • DOI:  10.1049/iet-map.2011.0096
      • Type: Article
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      • p. 1857 –1862 (6)
        The mode matching technique is used to formulate the problem of full-wave scattering from longitudinally corrugated cylinders centred at the tip of a conducting wedge. A line source with an impressed electric or a magnetic current is used to excite a cylindrical wave for soft (TMz) or hard (TEz) polarisation, respectively. The cylindrical tip has multiple corrugations with or without a dielectric filling. The underlined mode matching formulation is validated for the particular geometries by comparing against a two-dimensional nodal finite-element method. The rigorous mode matching formulation was used in order to develop a code in pursuit of numerous parametric studies for the design of optimum corrugated cylindrical tips that mitigate diffraction in the shadow region. From the parametric analysis performed, it is evident that a carefully designed corrugated cylindrical tip effectively reduces strong diffractions in the shadow region of a conducting wedge.
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      Method of moments with an equivalent Green's function technique for modelling broadband cavity-coupled microstrip vertical transitions
      • Author(s): J.-C. Cheng  and  E.S. Li
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1863 –1871
      • DOI:  10.1049/iet-map.2011.0115
      • Type: Article
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      • p. 1863 –1871 (9)
        A method of moment technique is presented to model the frequency response of broadband cavity-coupled microstrip vertical transitions. The technique divides the original problem into two kinds of regions, and then calculates the electromagnetic field distributions in each region by Green's functions. However, the closed forms of Green's functions are not always available. For regions without the closed-form Green's functions, equivalent ones are derived by the finite-element methods. This gives maximum flexibility in the physical structures. Two designs proposed earlier for the broadband cavity-coupled microstrip vertical transitions are selected for the practical applications of this technique. The simulation results are compared with those obtained from other numerical techniques, and the accuracy and validity of this technique is assessed by measured data. Numerical convergence of the equivalent Green's function technique is examined and computational efficiency and required memory usage are also studied.
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      Bandwidth enhancement techniques for planar inverted-F antenna
      • Author(s): H.T. Chattha ; Y. Huang ; M.K. Ishfaq ; S.J. Boyes
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1872 –1879
      • DOI:  10.1049/iet-map.2010.0505
      • Type: Article
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      • p. 1872 –1879 (8)
        The planar inverted-F antenna (PIFA) is widely used in mobile radio systems because of its excellent performance. However, it is not yet employed as an ultra wide band (UWB) antenna because of its perceived narrowband characteristics. This study introduces three techniques for enhancing the impedance bandwidth of PIFA, which are (i) changes in the widths of feed plate and shorting plate, (ii) addition of an inverted-L-shaped parasitic element and (iii) addition of a rectangular parasitic element. It is shown that PIFAs with a very wide fractional bandwidths (up to 110%) can be achieved. Using these techniques, an UWB PIFA antenna is designed, manufactured and tested. Simulated and measured results are provided to verify the conclusion.
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      Non-linear radio frequency model identification using a hybrid genetic optimiser for minimal user intervention
      • Author(s): J. Hu ; J.Q. Lowry ; K.G. Gard ; M.B. Steer
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1880 –1890
      • DOI:  10.1049/iet-map.2010.0435
      • Type: Article
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      • p. 1880 –1890 (11)
        An adaptive reduced-order procedure for developing grey-box models of radio frequency systems is presented. The stochastic model extraction combines a modified genetic algorithm and the Nelder–Mead simplex algorithm to present a user with a range of possible good models from which the user can use intuition to select the most physically realistic candidate. The procedure is ideal for identifying models given incomplete observations, noisy data and inexact model structure. This procedure replaces the commonly used human-in-the-loop ad hoc deterministic approach in which a skilled operator must guide model fitting. Being able to use an expanded range of model architectures beyond the Wiener and Hammerstein family, the stochastic approach, utilisation of incomplete observations and extraction of multiple distinct model candidates, enables the insight and intuition of a skilled operator to be used to advantage. The extraction of a multistage microwave amplifier exhibiting long-term memory effects is used as an example.
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      Dual-band unequal power divider with simplified structure
      • Author(s): M.-J. Park
      • Source: IET Microwaves, Antennas & Propagation, Volume 5, Issue 15, p. 1891 –1896
      • DOI:  10.1049/iet-map.2011.0119
      • Type: Article
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      • p. 1891 –1896 (6)
        A new dual-band unequal power divider scheme is proposed with particular advantages in structural simplicity. The proposed divider employs only two stub lines without any additional lumped reactive elements and is only two cascaded sections long from the input to the output ports. The required number of transmission line sections and the stub lines is minimised by a novel combination of a stubbed dual-band power divider and single-stub, dual-band impedance matching networks through the application of the stub cancellation technique. A systematic design procedure is provided using a set of simple design formulas obtained from rigorous circuit analysis of the proposed divider.
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