Inhomogeneous Dielectric Loading Research Articles

The effect of inhomogeneous dielectric loading on transmission through a slot in an infinite thick metallic shield

ABSTRACTTransmission through a slot in a an infinite metallic shield of finite thickness, inhomogeneously loaded with dielectric material, is modeled using an analytical approximation based on the slot being small compared to a free space wavelength. Resonances of relatively broad bandwidth (Fabry-Perot) as well as of very narrow bandwidth (Fano) are found. We show that by introducing a gap of different dielectric constant within the slot, we can control the separation between the broad bandwidth resonances, and the locations and magnitudes of the very narrow bandwidth resonances. The influence of grooves on both kinds of resonances is considered using the same analytical model. The effect of losses in the dielectric slot is also considered, using data for commercially available dielectrics.

Cutoff Frequencies of a Dielectric-Loaded Rectangular Waveguide With Arbitrary Anisotropic Surface Impedance

The coupled transcendental equations describing cutoff frequencies of TE and TM modes are deduced for a rectangular waveguide with arbitrary wall impedance. In the case of TE modes, their generalization is made to a rectangular guiding structure, which possesses both distributed wall impedance and inhomogeneous dielectric loading. Effective impedance is obtained for imperfectly conducting surface incorporating periodic rectangular corrugations along the direction of mode propagation. Circular waveguide bounded by such corrugated surface and rectangular impedance waveguide are considered as numerical examples. For these waveguides, the computations performed to determine the cutoff frequencies of TE guiding modes are validated against the results of the perturbation theory and the finite-element method.

Analysis of dielectric rods with arbitrary shape for low-dispersion slow-wave structures in helix TWTs

The introduction of rods with optimized cross-sectional shape in slow-wave structures (SWSs) for wideband helix traveling-wave tubes can sensibly improve the dispersion maintaining an adequate interaction-impedance level. In this brief, a procedure for analyzing helix SWSs loaded with rods of arbitrary shape, by using an analysis method adopting the inhomogeneous dielectric loading approach, is proposed. The advantages, in terms of SWS-dispersion improvement, of adopting optimized rod shapes in comparison to the typical rod shapes (i.e., rectangular, circular, and T-shaped) utilized up to now, are also demonstrated.

Nonrounded dielectric rectangular rods in helix traveling-wave tubes

Longitudinal dielectric rods in helix traveling-wave tubes (TWTs) contribute to the determination of the performance. A study on nonrounded rectangular rods that exhibit improved dielectric loading with respect to typical rectangular rods is proposed. The expressions for computing the effective dielectric constants for the inhomogeneous dielectric loading model are proposed. Simulated dispersion characteristics are compared with measured data from literature. An analysis method based on rigorous solutions of the field equations is adopted. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 47: 101–103, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21093

Peculiarities of dominant mode transformation in rectangular T-septum waveguide with inhomogeneous dielectric loading

A combined 2D numerical-analytical algorithm for electromagnetic (EM) fields computation in one-counter waveguides with arbitrary cross section is proposed. The developed algorithm is employed to investigate the influence of the normalized propagation constant and dielectric loading on the field distribution in a rectangular T-septum waveguide with an E-plane dielectric insert. Dominant-mode varieties of a given transmission line are represented. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14, 201–205, 2004.

On the TWT based on a helix slow-wave structure sustained by rectangular dielectric rods

Inhomogeneous dielectric loading is a necessary approach to obtain an accurate simulation of a helix slow-wave structure in traveling-wave tubes. In this paper a new expression for the effective dielectric constant to characterize each dielectric coaxial tube in which the region between the helix and the envelope is divided for the ease of a rectangular rod is proposed. The use of this expression in a rigorous analysis method results in a more accurate simulation of the helix slow-wave structure. (C) 1999 John Wiley & Sons, Inc.

Mode-matching analysis of top-hat monopole antennas loaded with radially layered dielectric

A top-hat monopole antenna with homogeneous or inhomogeneous dielectric loading over a ground plane is considered. Mode-matching analysis with proper enforcement of edge conditions is applied to the problem. The results of this technique are verified through independent calculations of admittance as well as by comparison of fields across matching regions. Measurements were also taken for comparison with the results from the model. It is shown that dielectric loading can reduce the electrical size necessary for self resonance, but only at the expense of a large increase in radiation Q.

Accurate analysis of helix slow-wave structures

In this paper, the helix slow wave structure (SWS) of traveling wave tubes (TWT's) has been analyzed. Dielectric supporting rods of arbitrary cross section have been considered in this analysis. The inhomogeneous dielectric loading factor has been accounted for by modeling the discrete support with a number of continuous dielectric tubes of appropriate effective dielectric permittivity. The helix tape model has been used for the field analysis. Furthermore the thickness of the helix tape has been considered. A rigorous solution of the field equations, including the contribution of the space harmonics, was performed to evaluate the phase velocity and the interaction impedance up to millimeter-wave frequencies. The nonuniformity of radial propagation constant over the structure cross section has been also included. With respect to other approaches, a closed-form expression of the field constants has been obtained. A study to choose the optimum number of space harmonics and dielectric tubes to be used in the analysis, has shown how the results are more sensitive to the number of space harmonics than to the number of dielectric tubes, beyond a certain number of the latter. The validity of this theory has been proved by comparison between measurements and simulations for helix SWS with different dimensions, rod shapes and operating frequency band.

Multiresolution analysis similar to the FDTD method-derivation and application

A three-dimensional (3-D) multiresolution analysis procedure similar to the finite-difference time-domain (FDTD) method is derived using a complete set of three-dimensional orthonormal bases of Haar scaling and wavelet functions. The expansion of the electric and the magnetic fields in these basis functions leads to the time iterative difference approximation of Maxwell's equations that is similar to the FDTD method. This technique effectively models realistic microwave passive components by virtue of its multiresolution property; the computational time is reduced approximately by half compared to the FDTD method. The proposed technique is validated by analyzing several 3-D rectangular resonators with inhomogeneous dielectric loading. It is also applied to the analyses of microwave passive devices with open boundaries such as microstrip low-pass filters and spiral inductors to extract their S-parameters and field distributions. The results of the proposed technique agree well with those of the traditional FDTD method.

The inhomogeneous dielectric loading effects of practical helix supports on the interaction impedance of the slow-wave structure of a TWT

The helical slow-wave structure of a traveling-wave tube is usually supported by wedge-shaped bars or rods or circular or rectangular cross section. Such supports present an inhomogeneous loading of the helix caused by the variation of the effective permittivity of the helix surrounding with the radial coordinate. In one of the approaches to the analysis of such structures for dispersion characteristics, the discrete supports are azimuthally smoothed out into continuous dielectric-tube regions, the number of such tubes being increased for converging results. This method is extended for the interaction impedance of the helix supported by a number of dielectric bars/rods of rectangular or circular cross section, the whole enclosed in a metal envelope. The method gives more accurate results than that given by a single tube, particularly for situations in which the relative permittivity and radial thickness of the dielectric supports and the frequency of operation are high. The analysis developed is rather general.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Finite Element Analysis of All Modes in Cavities with Circular Symmetry

A field analysis is presented of all modes in a hollow, conducting cavity with rotational symmetry about an axis. Cavities can be periodic along this axis, and the unit (or single) cell can be of arbitrary longitudinal section, with inhomogeneous dielectric loading. Modes of any angular dependence of arbitrary phase-shift per unit cell are analyzed. The finite element method is applied in the longitudinal plane, and uses a specially developed sparse matrix scheme.

Comments on "Ridged Waveguides with Inhomogeneous Dielectric Slab Loading" [Letters

In the above paper/sup1/, Magerl derives dispersion equations for TE/sub m0/ modes of ridged waveguide with inhomogeneous dielectric-slab loading. The existence of TE modes was evidently assumed a priori (a seemingly natural assumption); unfortunately, true TE modes do not exist in this waveguide (with a singuIar exception to be noted).

Application of a unimoment technique to a biconical antenna with inhomogeneous dielectric loading

Except for the most trivial cases, antenna radiation problems involving inhomogeneous media escape analytical solution. For these problems, it is necessary to seek computer solutions. To this end, we have investigated Mei's [1] unimoment technique as it applies to a biconical antenna immersed in an inhomogeneons medium. The technique is used in conjunction with finite difference methods to analyze some dielectric loading schemes with the goal of improving performance of the biconical antenna. The technique is shown to be sufficiently general to allow analysis of any rotationally symmetric antenna inside an inhomogeneous medium by using more flexible finite element methods. Results of experimental investigations bear out the validity of the technique.

Application of inhomogeneous dielectric loading to coaxial resonators

A technique for improving the Q factor of air-filled coaxial resonators by inhomogeneous dielectric loading is reported.

Waveguide structures for low-loss transmission around sharp bends in circular waveguide

Insertion-loss measurements over the frequency band from 28 to 39 Gc/s have been made on a number of curved waveguide structures of circular cross-section. These included a waveguide periodically loaded with both tapered and parallel-sided dielectric discs placed transversely, in addition to longitudinally and circumferentially corrugated waveguide. It was found that the inhomogeneous anisotropic dielectric loading was difficult to fabricate accurately. Dielectric losses limit the usefulness of this method. Longitudinal corrugations of the wall of the tube were used to provide an anisotropic surface reactance and so break the TE01-TM11 mode degeneracy. It was found, unfortunately, that the attenuation coefficient of extruded waveguide of this kind in straight lengths was excessive. Circumferentially corrugated waveguide was also used to remove the TE01-TM11 mode degeneracy with much more success. Provided that the waveguide is not too highly overmoded, such a structure gives a method of negotiating a sharp bend with low loss.