Dielectric Loading Research Articles

Transient response of a wide‐angle cone with dielectric loading

Introducing a dielectric material near a conical antenna affects the transmitted and received signals. In this paper, wide‐angle cones with dielectric loading are analyzed in both the transmitting and the receiving modes. A Gaussian pulse is used as the exciting signal.

Theory of helix traveling wave tubes with dielectric and vane loading

A time-dependent nonlinear analysis of a helix traveling wave tube (TWT) is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading are included in the formulation as is efficiency enhancement by tapering the helix pitch. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. The vane-loading model describes the insertion of an arbitrary number of vanes running the length of the helix, and the polarization of the field between the vanes is assumed to be an azimuthally symmetric transverse-electric mode. The field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. An overall explicit sinusoidal variation of the form exp(ikz−iωt) is assumed (where ω denotes the angular frequency corresponding to the wave number k in the vacuum sheath helix), and the polarization and radial variation of each wave is determined by the boundary conditions in a vacuum sheath helix. The propagation of each wave in vacuo as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. A dynamical equation for the field amplitudes is derived analogously to Poynting’s equation, and solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Electron beams with a both a continuous and emission-gated pulse format are analyzed, and the model is compared with linear theory of the interaction as well as with the performance of a TWTs operated at the Naval Research Laboratory and at Northrop–Grumman Corporation.

Characteristics of a shielded dielectric-loaded edge-coupled conductor-backed coplanar waveguide

Dispersion and impedance characteristics have been obtained for a completely shielded edge-coupled conductor-backed coplanar waveguide. It is demonstrated that dielectric loading can considerably improve these characteristics with respect to mode coupling. On the other hand, the unloaded structure can yield directivity couplers. © 1996 John Wiley & Sons, Inc.

Dielectric slab-loaded resonant cavity for applications requiring enhanced field uniformity

This paper introduces and analyzes a rectangular resonant structure that provides an alternative to the multimode resonant cavity in applications requiring enhanced field uniformity. The resonant cavity contains four dielectric loading slabs placed along the cavity walls. Its first resonant mode is related to the uniform field distribution supported in a rectangular TEM waveguide. The electromagnetic fields within the cavity are described using a closed-form approach, with approximations taken to account for the presence of two of the loading slabs. Application of the boundary conditions leads to an eigenvalue formulation, which is used to determine resonant frequencies and electromagnetic field distributions within the cavity. Measurements of both resonant frequencies and electric field magnitudes confirm the analysis. This work provides the basis for future analyses and implementation of slab-loaded cavities in both scientific and industrial settings.

Broadbanding of a gyro-TWT by dielectric-loading through dispersion shaping

A general cylindrical waveguiding interaction structure with a dielectric rod at its axis and a dielectric lining at its wall is field-analyzed to obtain a cold dispersion relation. The results of the cold analysis are combined with the hot dispersion relation of a dielectric-free gyro-traveling wave tube (TWT) to obtain the hot dispersion relation of the dielectric-loaded gyro-TWT. The dispersion relation is interpreted to obtain the conventional TWT-type, Pierce-form gain equation. The dielectric parameters are chosen for a wideband coalescence between the beam end the waveguide modes. For a TE/sub 0n/-mode gyro-TWT, the fundamental mode of operation (n=s=1, where s is the beam-harmonic mode number), gives a higher gain which reduces as one goes for higher values of n=s. The second harmonic operation (n=s=2), however, gives a larger bandwidth. The present analysis also suggests a method to select the values of the hollow beam radius relative to the waveguide wall radius for enhancing both the device gain and bandwidth. This study shows the feasibility of a wideband amplification in a gyro-TWT using dielectric loading, at higher gain values than those obtainable from other broadbanding schemes such as simultaneous profiling of the waveguide cross section and the background dc magnetic flux density.

Methods of suppression or avoidance of parallel-plate power leakage from conductor-backed transmission lines

Four useful methods are presented to suppress and/or avoid parallel-plate leakage from conductor-backed printed transmission lines. These include: 1) the use of shorting-pins; 2) the use of a dielectric-guide-coupled configuration; 3) using a two-layered conductor-backing configuration; and 4) dielectric loading on top. New analyses to model the leakage suppression due to the shorting-strips and the dielectric-guide-coupled geometries are presented, with selected demonstrative results and critical discussions. It is concluded that the unwanted leakage in many printed transmission lines, that are otherwise attractive for integrated circuits and phased array applications, can be successfully avoided and/or significantly suppressed using the proposed techniques.

Novel two‐mode cutoff‐waveguide filters and their new design method

AbstractThis paper describes a new method of configuration of a cutoff waveguide filter with attenuation poles. When a filter with attenuation poles is constructed, often the degenerate mode is used or the filter configuration uses two paths. In this paper, a new configuration is presented in which the negative coupling between the TE101 and TE201 modes in a waveguide cavity is used. The test fabrication and experiment are carried out for both the cavity resonator loading and the dielectric resonator loading. The effectiveness of the present filter configuration and the design method is confirmed.

Analysis of electromagnetic fields using the finite‐difference time‐domain method in a microwave oven loaded with high‐loss dielectric

AbstractThere are few examples for three‐dimensional electromagnetic analysis for a high‐loss dielectric water load which has one of the highest relative permittivities of 80.4 among food items placed in a microwave oven. In this paper, the three‐dimensional finite‐difference time‐domain (FD‐TD) method is used for the analysis of the electromagnetic field inside the water load. In the fundamental investigation using the high‐loss dielectric loaded rectangular waveguide, it is confirmed that the FD‐TD numerical results with Yee's cell pitch which is set at about one‐tenth the wavelength reduced within the dielectric, agree almost with the exact solution based on the transmission theory. After the confirmation of the validity, the electromagnetic field within a water load placed on a metal turntable is analyzed. The water load has a cylindrical shape with a diameter of 64 mm and a height of 80 mm. As a result, the reduction of heating unevenness by the turntable can be predicted visually and quantitatively. In this analysis, the steady‐state solution can be obtained after 300 iterations while the heating pattern is stabilized after 100 to 200 iterations.

Electromagnetic scattering from a coaxial dielectric circular cylinder loading a semicircular gap in a ground plane

An exact dual series solution of a plane wave incident on a coaxial dielectric circular cylinder imbedded in a semicircular gap of a ground plane is presented. Both TM and TE cases are considered here. The scattered field is represented in terms of an infinite series of cylindrical waves with unknown coefficients. By applying the boundary conditions and employing the partial orthogonality of the trigonometric functions the scattering coefficients are obtained. The resulting infinite series is then truncated to a finite number of terms to produce numerical results. For the sake of comparison with the published data some special cases are introduced first. The comparisons showed excellent agreement in all cases. >

Applications of high-T/sub c/ superconductors in sub-Gigahertz r.f. cavity resonators

The size of cavity resonators below 1 GHz may be reduced by using dielectric or helical loading. With normal conductors, such electrically small resonators suffer problems of losses and inadequate Q-factor. However, by using superconductor in the portions with the highest current density, very good performance may be obtained with very electrically-small cavities. A cylindrical dielectric resonator was used to load a corrugated cavity, having YBaCuO thick-film end plates, and operated at VHF. The results showed a significant improvement over corrugated cavities with copper end plates. Investigations have also been undertaken with high-T/sub c/ superconductor helical loading. The size of the cavities was electrically very small (about 0.02 of free space wavelength), and results with both bulk and thick-film helical loading elements were obtained. The results showed a significant improvement over a similar copper helically-loaded cavity resonator.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Guided plate waves in piezoelectrics immersed in a dielectric fluid. II. Experiments

Detailed ultrasonic reflection measurements in piezoelectric LiNbO3 are reported which are compared to the results of the theoretical analysis of the previous paper, part I. Differences in mode coupling and behavior are illustrated in several calculated examples. The calculated numerical results are demonstrated in a full image format. It is shown how dielectric fluid loading influences the reflection behavior and how strong piezoelectric coupling can lead to unexpected dispersion in the fundamental symmetric mode. The data confirms in each case the predictions of an exact partial-wave analysis of the problem, including the dielectric influence of the fluid. These phenomena play a role in the design and performance of piezoelectric sensing devices.

HEAT TRANSFER MODELLING OF A CYLINDRICAL ER CATCH

In electro-rheological (ER) devices the control of temperature is often of paramount importance. For a device through which the ER fluid is not able to flow continuously to and from a reservoir, where it may be cooled, this can be a problem. Such a situation occurs in the ER catch. The heat generated there will predominantly be dissipated from the outer surface of the drive/input. The rate of heat transfer is thus principally a function of the speed and area of that surface and the temperatures of the ER fluid and atmosphere. Since these factors reflect on the levels of electro-stress, current requirement and viscosity of the ER fluid, an opportunity exists for optimisation of catch performance. The paper shows the results of an investigation into the effects on the cooling/heating problem of varying the radii of the clutch rotors, their relative rotational speed and inter-electrode spacing. Equilibrium fluid temperatures are confirmed by experimental evidence. The effects of heat generation in run up to speed and clutch locked periods, (through electro viscous drag and dielectric loading respectively) are quantified and compared with the case of a contemporary ER fluid in a cylindrical catch on zero volts idling. At any given operating condition, uniform temperature, viscosity and constant electro stress are assumed throughout the fluid.

Mixed modes dielectric resonator filters

A new configuration of dielectric loaded cavity filters is presented. The filter combines the superior spurious free performance of TE/sub 01/ mode ring dielectric resonators with the advantage of HE/sub 11/ dual-mode dielectric loading. Resonant frequency, field distributions and unloaded Q of each cavity are computed by rigorous mode-matching technique. The fields are used to compute the coupling between two similar cavities by small aperture approximation based on formulae modified by Levy, which are extended to the cases of dissimilar cavities. Design aspects of the filter are discussed. This filter is used to construct an L-band triplexer. Excellent experimental results verify the theory and show the advantage of this type of filter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Tri-band frequency selective surface with circular ring elements

Theoretical analysis, physical reasonings, and experimental verifications are presented for a frequency selective surface with circular ring elements. Both double-screen and single-screen designs are generated for a tri-band system that reflects the X-band signal while transmitting through the S- and Ku-band signals. In these designs, the dielectric loading effect is used to reduce ring size and element spacing and thus avoid the grating lobe problem. The circular ring element is very insensitive to a large variation of the RF incident angle. >

Acoustic waves in a piezoelectric plate loaded by a dielectric fluid

Measurements and calculations of reflection spectra of an X-cut LiNbO3 plate immersed in a dielectric fluid have been performed, with the numerical results presented in an innovative image data format. The influence of dielectric fluid loading on all branches of the reflection coefficient function has been investigated. Strong piezoelectric coupling leads to unexpected plate wave dispersion in the fundamental symmetric mode, where the group velocity exceeds the phase velocity at low frequency. The experimental data are in excellent agreement with the results of an exact partial wave analysis. These phenomena and their effect on plate wave behavior can play an important role in the performance of chemical or physical liquid sensing devices.

Full-wave analysis of a transversely magnetized ferrite nonradiative dielectric waveguide

A full-wave analysis is applied to a nonradiative dielectric waveguide where the isotropic dielectric slab is replaced by a transversely magnetized ferrite. The characteristic equation is obtained and the corresponding effects are discussed. The above structure exhibits reciprocal propagation characteristics. Nonreciprocal effects are also possible with a proper dielectric loading. Several numerical results are presented in the form of dispersion curve and an operational diagram, as a function of several ferrite and guide parameters. Electronically tuned and nonreciprocal devices can be implemented using this simple structure. >

TE-Plane Wave Scattering from a Dielectric-Loaded Semi-Circular Trough in a Conducting Plane

The theoretical behavior of TE-plane wave scattering from a dielectric-loaded semicircular trough in a conducting half-space is investigated. The dielectric loading is made of a circular cylinder which is partially buried in a semi-circular trough in a perfectly conducting plane. The scattered field is numerically evaluated to investigate the angular scattering pattern under the various dielectric loading conditions. The presence of the dielectric loading in the trough can substantially affect the angular scattering behavior. The difference between the TE and TM-scattering angular patterns is also observed. The field magnitude inside the trough is found to be much greater than the outside, thus indicating that the trough focuses the incident beam inside the trough.

Propagation and radiation behavior of dielectric-loaded plural hybrid mode multiflare conical horn antennas

A dielectric-loaded multiflare conical horn exhibiting the desirable features of low sidelobes and cross-polarization with the additional advantage of increased on-axis gain is reported. This new horn is developed by combining the techniques of multimode operation for low cross-polarization and sidelobe levels, and dielectric loading for aperture efficiency enhancement. The condition for the support of balanced hybrid modes for low cross polar radiation in the dielectric-loaded multimode horn is derived. Also, the equation for the dielectric thickness for maximum aperture efficiency at a given frequency is obtained. This new type of multimode horn is potentially simple to design, analyze, and manufacture, yet it exhibits the desirable features of a corrugated conical horn and hence can be an attractive alternative to corrugated conical horns for communication applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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>

Reduction of RF penetration effects in high field imaging.

A method of correcting for the RF inhomogeneity in the body by dielectric loading of the coil-to-shield space in an RF resonator (coil and shield assembly) is described. The presence of the RF coil and RF shield have significant effects on RF homogeneity. Based on theoretical calculations, a method for adjusting the RF homogeneity by manipulating the axial propagation constant, kappa z, is proposed. This is accomplished by loading the coil-to-shield space with dielectric material of suitable relative permittivity so as to increase kappa z and decrease the radial propagation constant, kappa rho. In this manner, the radial wavelength (lambda rho = 2 pi/kappa rho) can be increased relative to the body dimensions, and the field amplitude variations in the axial plane minimized. Theoretical calculations indicate that a value of between 30 and 40 for the relative permittivity of the dielectric material in the coil-to-shield space would reduce the RF field inhomogeneity from +/- 15% to about +/- 3% over a central 30-cm-diameter region of a homogeneous 40-cm-diameter body at both 64 and 170 MHz. The theoretical model was verified in laboratory measurements of the [formula; see text] field generated in a test coil at 170 MHz which was scaled to correspond to a body at 64 MHz. However, the improved RF field homogeneity would be accompanied by increased RF power requirements and reduced coil sensitivity.