Islam A. Eshrah

Rectangular waveguide with dielectric-filled corrugations supporting backward waves

A new application for corrugated waveguides as left-handed (LH) meta-material guided-wave structures is investigated. The waveguide is operated below the cutoff of the dominant mode, where the waveguide has an inherent shunt inductance. The dielectric-filled corrugations are used to provide a series capacitance, which, along with the shunt inductance, create the necessary environment to support backward waves. A simple equivalent-circuit model is constructed, and proves quite accurate in determining the dispersion, as well as the scattering characteristics of the structure. Experimental verification of the occurrence of backward waves in the corrugated waveguide is presented. Very good agreement between the results obtained using the equivalent-circuit model and the full-wave finite-difference time-domain solution is achieved. The effect of the various design parameters on the LH propagation bandwidth is investigated. The advantages and possible applications of the structure are discussed.

Theory and implementation of dielectric resonator antenna excited by a waveguide slot

Excitation of dielectric resonator antennas (DRAs) by waveguide slots is proposed as an alternative to traditionally used excitation mechanisms in order to enhance the frequency bandwidth of slotted waveguide radiators and to control the power coupled to the DRA. The analysis is based on the numerical solution of coupled integral equations discretized using the method of moments (MoM). The dielectric resonator (DR) is modeled as a body-of-revolution based on the integral equation formulation for the equivalent electric and magnetic surface current densities. The analysis of an infinite or a semi-infinite waveguide containing longitudinal or transverse narrow slots uses the appropriate dyadic Greens function resulting in closed-form analytical expressions of the MoM matrix. The scattering parameters for a slotted waveguide loaded with a dielectric resonator antenna disk are calculated and compared with finite-difference time-domain results. Bandwidth enhancement is achieved by the proper selection for the antenna parameters.

Spectral analysis of left-handed rectangular waveguides with dielectric-filled corrugations

The spectral analysis of an infinite rectangular waveguide with dielectric-filled transverse corrugations is performed to investigate supporting backward waves in this structure. The corrugations act as a capacitive immittance surface which provides a suitable environment for left-hand propagation to occur in an undersized rectangular waveguide having an inherent shunt inductance of the evanescent transverse electric dominant mode. The analysis of the corrugated waveguide is based on Floquets theorem, where the problem is solved in terms of a unit cell; then Galerkins projection technique is applied to reduce the integral equation to the matrix form. The dispersion relation and the modal field expressions are derived and, for the special case of wall-to-wall corrugations with single current harmonic, rather simpler expressions are obtained. The effect of the different design parameters on the dispersion characteristics is studied with emphasis on the left-hand propagation band. Some representative field plots of the dominant mode of the corrugated structure are depicted. Sample results are compared with Ansoft High Frequency Structure Simulator (HFSS) and with an equivalent circuit model developed for the same structure. The results exhibit very good agreement.

Excitation of dielectric resonator antennas by a waveguide probe: modeling technique and wide-band design

Analysis of a dielectric resonator antenna (DRA) fed by a waveguide probe is presented. The probe is excited by the dominant mode of a waveguide and extends into the DRA through an aperture in the waveguide wall. The DRA has, in general, an arbitrary shape and resides on an infinite ground plane, which coincides with the exterior of the waveguide broad wall. A simple and efficient analysis procedure is implemented where the problem is divided into two parts. In the upper part, the input impedance of the DRA excited by a coaxial probe is obtained with respect to the feeding position on the ground plane independent of the waveguide part. Then the input impedance is transformed to the waveguide part as a concentrated load at the end of the probe connected to the waveguide wall. The effect of the wall thickness is taken into account by modeling the section of the probe passing through the waveguide wall as a coaxial cable transmission line supporting the transverse electromagnetic mode. Thus the DRA input impedance is transferred from the ground plane reference to the waveguide inner wall reference. Results obtained using the method of moments are compared with those obtained using the finite-difference time-domain method and exhibit very good agreement. The procedure is used to achieve a bandwidth of 50% for a stacked DRA excited by a waveguide probe.

Analysis of waveguide slot-based structures using wide-band equivalent-circuit model

Analysis of geometrically complicated waveguide-based slotted arrays and filters is performed using a simple equivalent-circuit model. First, the circuit parameters (inductance and capacitance) of a simple waveguide slot-coupler problem are obtained through moment-method (MoM) analysis. The values of the lumped LC elements are virtually constant over the frequency range of interest (the X-band) for specific waveguide and slot dimensions. Based on the equivalent-circuit model of a single slot of two coupled waveguides, more complicated structures are then analyzed, such as slot coupler arrays and slot-based waveguide filters. The scattering parameters of these structures are obtained through circuit analysis, and are verified using the MoM and finite-difference time-domain method. Excellent agreement is observed over a wide band of frequencies and is confirmed by experimental results.

GENERALIZED EQUIVALENT CIRCUIT MODEL FOR TRANSVERSE WAVEGUIDE SLOTS AND APPLICATIONS

A generalized equivalent circuit model for waveguide transverse slots is proposed for the efficient analysis of waveguide transitions and waveguide slot antennas and slot-excited antennas. The transverse slots on the broad and end waveguide walls analyzed in this paper are decomposed into three structures: two apertures and an auxiliary waveguide section. The slot aperture in the host waveguide is modeled as an inductance-capacitance-transformer (LCT) combination, for which the equivalent inductance and capacitance are determined by first considering the case of zero-thickness slot, then the transformer turns ratio is calculated for the slot on a finite thickness wall. The effect of the wall thickness is accounted for by a waveguide section having cross-sectional dimensions equal to those of the slot, and length equal to the wall thickness. The loaded slot aperture is modeled along with the external load as a lumped load impedance. Expressions for the inductance, capacitance and transformer turns ratio are obtained in terms of the slot length and width. The obtained expressions facilitate the design of a variety of structures, including waveguide couplers, feeding networks and radiators. The equivalent circuit model proved to be accurate compared to the method of moments solution over a wide frequency band. Comparison of the scattering parameters obtained from the circuit analysis, the method of moments, and the finite-difference time-domain solutions exhibit very good agreement.

Equivalent circuit model for a waveguide probe with application to DRA excitation

A simple equivalent circuit is used to model an arbitrarily loaded waveguide probe with emphasis on probe-excited dielectric resonator antennas. The values of the lumped inductance and capacitance are obtained via the method of moments analysis for the problem of a short-circuited probe acting as a waveguide scatterer. For problems involving the excitation of an external load, the aperture through which the probe extends to the load is modeled by a shunt capacitance, and an additional transformer is introduced to model the coupling to the load. Expressions for these circuit parameters are obtained using curve-fitting techniques and are employed to determine the lengths and positions of an array of waveguide probes used to feed external loads with feeding currents of desired relative values. The procedure is applied to synthesize waveguide-fed antenna arrays to achieve a specified radiation pattern. The scanning capabilities and the effect of the mutual coupling are studied by comparing radiation patterns obtained from the analysis of the whole array and from the pattern multiplication principle

Dyadic Green's function of an ideal hard surface circular waveguide with application to excitation and scattering problems

[1]xa0Greens function analysis of ideal hard surface circular waveguides is proposed with application to excitation and scattering problems. A decomposition of the hard surface waveguide into perfect electric conductor and perfect magnetic conductor waveguides allows the representation of dyadic Greens function in terms of transverse electric (TE) and transverse magnetic (TM) waveguide modes, respectively. In addition, a term corresponding to a transverse electromagnetic (TEM) mode is included in the representation of the Greens dyadic. The TEM term is extracted in closed form from the eigenmode expansion of TM and TE modes in the zero-cutoff limit. The electric field distribution due to an arbitrarily oriented electric dipole source is illustrated for representative TM, TE, and TEM modes propagating in the ideal hard surface circular waveguide. The derived Greens function is used in the method of moments analysis of an ideal hard surface waveguide excited by a half-wavelength strip dipole antenna. In addition, the scattering of the TEM mode by a thin strip is studied in the ideal hard surface circular waveguide.

Analysis of linear arrays using the adaptive basis functions/diagonal moment matrix technique

An efficient algorithm is proposed for the analysis of large finite arrays using the adaptive basis functions/diagonal moment matrix technique. Adaptive basis functions constructed using clusters spanning over an array element are used to generate a highly diagonally dominant moment matrix. The physical interpretation of the constructed set of adaptive basis functions is discussed. The new matrix equation is solved iteratively in a way that only the significant mutual impedances are considered. The proposed algorithm is applied to a linear array of bow-tie antennas, and results are compared to those obtained using the direct moment method solution and exhibit very good agreement. The relative computational time improves as the array size increases compared to the conventional moment method solution. A speedup factor of more than 100 is achieved for an array of 32 elements.

Modal analysis of corrugated rectangular waveguides supporting left-hand propagation

The purpose of this work is to investigate the possibility of supporting LH propagation in rectangular waveguides with dielectric-filled broad wall corrugations. In this paper, spectral analysis of a corrugated waveguide is performed based on the dyadic Greens function approach in conjunction with Floquets theorem analysis of periodic structures to determine the dispersion characteristics and modal field distribution.