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Microwaves, Antennas and Propagation, IEE Proceedings H

Issue 4 • Date Aug. 1991

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Displaying Results 1 - 22 of 22
  • A study of tropospheric scintillations using SODAR

    Page(s): 313 - 318
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (549 KB)  

    Tropospheric scintillations at 7.6 GHz over the Tiruttani-Tirupati line of sight microwave link have been analysed for the period of February 1987 to March 1988. The scintillation index (%) is observed to be predominantly a night-time phenomenon. Its occurrence is minimum during monsoon and postmonsoon seasons compared to premonsoon and winter seasons. The scintillation index has been observed to be positively associated with the initial radio refractivity gradient. The diurnal variation of structure intensity parameter Cn/sup 2/ computed from the scintillation data shows the value of Cn/sup 2/ to be minimum around midday and maximum at night. An attempt has been made to correlate tropospheric scintillations with SODAR structures. The studies show that the daytime convective conditions do not affect the microwave signal, whereas ground based inversions, elevated layers, rising inversions and wave motions are responsible for scintillations.<> View full abstract»

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  • Theory for cylindrical, open ended, complex cavities supporting all TE0n modes: Gyrotron application

    Page(s): 297 - 306
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (692 KB)  

    A theory is developed to accurately determine the resonant frequencies and quality factors in good conducting, open-ended cylindrical structures with step changes and weakly irregular changes in wall radius, supporting all TE0n modes. Analytical expressions are obtained which avoid the need to solve the complicated boundary value problem. Inadequacies in experimental quality factors are discussed and new definitions are developed which consider mode coupling effects. The theory is applied to complex cavity gyrotron structures. Very good agreement is shown with existing experiments. Linear and quadratic phase shifts result from slight tapers. Small changes in the open-ended cavity geometry significantly changes the quality factor of the desired mode. The theory is applicable in overmoded structures with a dense frequency spectrum as a consequence of the high accuracy in determining the resonant frequency View full abstract»

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  • Dispersion curves and fields for a chiral slab waveguide

    Page(s): 327 - 334
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (376 KB)  

    The paper discusses wave propagation and field distributions in slab dielectric waveguides having homogeneous isotropic chiral material. The analysis is based on field expansions in the guide and in the upper and lower half spaces. Enforcing the boundary conditions at the waveguide interfaces results in eigenvalue equations for the hybrid propagating and guided modes. The theory is illustrated by calculating dispersion curves and field distributions for weakly guiding waveguides View full abstract»

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  • Study of varactor tuned microstrip rhombic resonators

    Page(s): 383 - 385
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (152 KB)  

    An electronically tunable microstrip rhombic resonator is developed employing GaAs packaged hyperabrupt junction varactor diodes. A tuning bandwidth of more than 30% is achieved experimentally at C-band. This was found to be in very close agreement with the theoretically predicted values obtained using an equivalent circuit model based on transmission line theory. The analysis takes into account the dispersion effect of the microstrip as well as the influence of the bends inside the resonator structure. The resonator is simple to design and fabricate, it could find applications in microwave tunable filters and frequency agile communication systems View full abstract»

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  • Ultra-wideband beamforming in sparse arrays

    Page(s): 342 - 346
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (368 KB)  

    Related studies applied to wide-band radar transmitting a single cycle of a sine wave indicate that it may be possible to construct a steered phased array radar with ultra high resolution using very sparse array apertures. In narrowband phased arrays, grating lobe consideration require that the array elements be placed close together. In contrast, very sparse, ultra high resolution wide band arrays may be constructed without incurring grating lobe problems View full abstract»

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  • Analysis of a variable height metallic strip in a rectangular waveguide

    Page(s): 386 - 388
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (136 KB)  

    The paper presents numerical data on the complex reflection coefficient due to an offset vertical strip of variable height in a rectangular waveguide. The evaluation of this parameter is based on the moment method, using the pulse function and the point matching technique. A comparison between numerical and experimental results is presented. Perturbation in wall current is also evaluated View full abstract»

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  • Analysis of a junction between rectangular and circular waveguides with collinear axes

    Page(s): 265 - 268
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (224 KB)  

    The paper presents a variational formulation for a junction between collinear rectangular and circular waveguides coupled through a rectangular slot in the common transverse wall. The equivalent circuit, taking the effect of wall thickness into account, is determined. Numerical results on admittance loading, the reflection coefficient and the input VSWR are evaluated, and a comparison between the theoretical and experimental results on the variation of the VSWR with frequency is presented View full abstract»

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  • Discrete complex images of a three-dimensional dipole above and within a lossy ground

    Page(s): 319 - 326
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (392 KB)  

    The paper introduces a new method of discrete complex images, to evaluate the Sommerfeld integrals arising in the problem of a three-dimensional dipole above and within a lossy ground. The spectral domain quantity, which is independent of spatial co-ordinates, is accurately approximated by a short series of exponential functions. By use of the Prony method and non linear optimisation technique, and through the Sommerfeld equality, the Sommerfeld integrals can be analytically evaluated into simple closed forms of spatial complex images. Various dipoles (HED VED, HMD and VMD) above and within the lossy ground are studied. Numerical results show that for any parameters of the lossy ground, the closed-form Green function approaches the precision of the 'nearly exact' results of Sommerfeld integrals View full abstract»

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  • Symmetric-pair antennas for beam steering direction finding or isotropic-reception gain

    Page(s): 368 - 374
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (476 KB)  

    A novel type of phased array is described which seems particularly attractive in direction finding and adaptive beam steering applications, and allows considerable simplification in the necessary signal processing. A variant of the technique also permits gain with isotropic coverage for reception. The principles of the techniques are outlined, along with a discussion of several possible array configurations for use with direction finding, communications and spectrum monitoring. Experimental results are presented to confirm the predicted behaviour of the array View full abstract»

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  • Marching-on-in-frequency method for solving integral equations in transient electromagnetic scattering

    Page(s): 347 - 355
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (604 KB)  

    A new approach was recently proposed for the numerical solution of integral equations describing transient scattering problems. The idea is to first discretise in space, and then solve the resulting system of linear time-domain equations by carrying out a temporal Laplace transformation. This approach combines the efficiency of the marching-on-time method with the stability and the accuracy of frequency-domain techniques. The repeated solution of the Laplace-transformed system of linear equations thus obtained with the aid of iterative techniques is considered. Initial estimates are generated by marching-on-in-frequency and using a new extrapolation scheme. The feasibility of this idea is demonstrated by solving electric-field integral equations for the total current along a straight thin-wire segment and the current density along a perfectly conducting cylinder View full abstract»

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  • Some calculation methods and universal diagrams for measurement of dielectric constants using open-ended coaxial probes

    Page(s): 356 - 360
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (232 KB)  

    Several formulas for the calculation of the admittance of open-ended coaxial probes from the complex permittivity, ε, of the measured medium and the dimensions of the probe are presented. The inverse problem, calculation of ε from the measured admittance of the probe, is also studied in detail. These formulas are discussed and compared in terms of numerical calculations. New universal diagrams of solutions of these problems are presented which are both convenient and useful for practical measurements. Some distinguishing features of the relationship between ε of the measured medium and the admittance of the probe are discussed View full abstract»

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  • Versatile strategy for CAD of multistage low-noise amplifiers

    Page(s): 380 - 382
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (168 KB)  

    The introduction of a novel performance chart which allows the selection of simultaneous noise, gain and VSWR specifications for a multistage low-noise amplifier equipped with given active devices is described. In addition, the individual goal function to be assigned to each equaliser network to fulfil the chosen specifications is provided. A design example is also included View full abstract»

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  • Electromagnetic scattering from bodies symmetric with respect to mutually orthogonal planes

    Page(s): 361 - 367
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (428 KB)  

    The scattering from bodies symmetric with respect to h mutually orthogonal planes (1⩽h⩽3) is presented. For any incident field, the linear equation describing the phenomenon can be decomposed into 2h uncoupled equations, each of which involves an unknown function symmetric or antisymmetric with respect to each symmetry plane. In terms of the method of moments, this corresponds to transforming the equation into a matrix equation of the form Ax=d, where the matrix A is partitioned into 2h diagonal blocks, provided that suitable basis and weighting functions are chosen View full abstract»

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  • Effects of adding a small disk to a spiral antenna backed by a conducting plane reflector

    Page(s): 375 - 377
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (232 KB)  

    A small conducting disk is added to a spiral backed a plane reflector to extend the frequency range over which the spiral radiates a circularly polarised wave (CPW). The numerical analysis based on the moment method reveals that the frequency range for CPW radiation is extended to 1:1.42 (34.7%) with the small disk, from 1:1.18 (16.5%) without the small disk View full abstract»

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  • Analysis of rectangular corrugated structures

    Page(s): 335 - 341
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (372 KB)  

    The determination of the scattering matrix of rather general corrugated structures is considered. Three types of elementary sections are defined that can describe these structures. Two sections have already been treated in the literature. For the third, a section of corner-filled corrugated waveguide, expressions for the scattering matrix are developed. These expressions are based on the representation of the fields inside the corrugations by a small number of modes propagating in a direction perpendicular to the axis of the waveguide. The method is applied to the determination of the modal fields and dispersion diagrams for two usual types of rectangular corrugated waveguides. Some basic properties of the modal fields are discussed View full abstract»

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  • Hybrid quasistatic analysis for multilayer coplanar lines

    Page(s): 307 - 312
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (356 KB)  

    A new quasistatic analysis of a coplanar waveguiding structure with multidielectric layers is presented, using a hybrid approach that combines the Galerkin finite-element method and the conformal-mapping technique. In the study, Wen's (1969) mapping function is adopted, which not only transforms the infinity problem domain into a finite region, but also overcomes the field singularity difficulty around the strip edges. Numerical results for effective dielectric constants, characteristic impedances, charge distributions and field distributions for various multilayer coplanar line structures are presented and the magnetic wall assumption over the slot is also investigated View full abstract»

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  • Analysis of TEM cells using the finite-difference technique and the conjugate gradient method

    Page(s): 261 - 264
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (224 KB)  

    The electric scalar potential distributions within TEM cells are obtained by solving Laplace's equation using the finite-difference technique and the conjugate gradient method. Once the potential distributions are solved, other properties of TEM cells, such as the field distributions and the characteristics impedance, can be determined directly View full abstract»

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  • TEM wave current distribution on two coaxial arc strips

    Page(s): 277 - 282
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (308 KB)  

    The longitudinal current density in two coaxial arc-strip conductors surrounded by homogeneous dielectric is derived. The current density is found by converting a set of two coupled integral equations into a set of nonhomogeneous linear equations whose solution can be approximated by a single closed-form function. In the limiting case, corresponding to a two-conductor parallel-plate transmission line, a good agreement with previously published results has been verified. The method presented can also be used for treating other configurations of cylindrical conductors in the form of arc-sectors forming two- and multi-conductor transmission lines View full abstract»

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  • Frequency-scanned reflection gratings consisting of ring patches

    Page(s): 273 - 276
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (268 KB)  

    A periodic array of thin conducting ring patches etched on a dielectric substrate and placed over a ground-plane is investigated for the use as a frequency-scanned reflection grating (blazed grating). The theoretical analysis used is based on Floquet's theorem and the method of moments. Numerical examples are presented showing that frequency-scanned gratings can be designed to operate for circular polarisation with high blazing efficiency, i.e. with the property of efficiently scattering the power of an incident electromagnetic wave into the first order diffracted wave View full abstract»

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  • Elimination of false singularities in GTD equivalent edge currents

    Page(s): 289 - 296
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (460 KB)  

    The method of equivalent edge currents (MEC) has extended the applicability of GTD. The equivalent edge currents, however, generally suffer from false singularities outside the Keller cone. These singularities depend upon the coordinate for inner integration in the surface to edge integral reduction. The objective of this paper is to eliminate false singularities of the currents. A new co-ordinate for inner integration that maximises the phase variation of the integrand is proposed. Consistent use of this co-ordinate leads us to novel expressions for not only fringe but also PO and GTD equivalent edge current without any false singularities. These currents are verified for diffraction from a disk and a square plate. It is numerically confirmed that GTD-MEC using new currents predicts uniform patterns and is almost comparable to PTD View full abstract»

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  • Comparison between different rectangular horn elements for array antenna applications

    Page(s): 283 - 288
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (380 KB)  

    The coupling and directivity characteristics of an array with fundamental smooth-walled, soft and hard open-ended waveguide elements with rectangular cross-section are considered. Exact and approximate methods to analyse coupling in arrays are discussed. It is known that the coupling, which can be expressed by a quadruple integral across the rectangular apertures, can be reduced to double integrals by introducing a co-ordinate transformation. Calculations show that the soft waveguide elements are suited for applications where low return loss and mutual coupling are required. Arrays with hard waveguide elements yield high directivity for limited scan applications. Both elements can, in contrast to conventional smooth-walled horns, operate in dual polarisation with high isolation between the orthogonal modes View full abstract»

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  • Use of frequency-derivative information in two-dimensional electromagnetic scattering problems

    Page(s): 269 - 272
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (220 KB)  

    The use of frequency-derivative information incorporated with model-based parameters estimation to predict the scattering from conducting and dielectric two-dimensional targets over a frequency band is summarised. Instead of computing a method of moment solution using a point-by-point approach, a rational function model is used to approximate the current as a function of frequency. The model coefficients are computed using frequency-derivative information at one frequency within the band to improve efficiency of conventional moment method approaches. Results show that the model based approach gives excellent results over a limited frequency band, and is much more efficient than the conventional point-by-point approach View full abstract»

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