Sergei P. Skobelev

Analysis of conical quasi-TEM horn with a hard corrugated section

There is an interest in using electromagnetic bandgap (EBG) surfaces to realize quasi-TEM waveguides and horns. TEM horns will have a uniform aperture distribution and a corresponding high aperture efficiency and low cross polarization. In the present paper a conical quasi-TEM horn with a hard-walled section excited in the TE/sub 11/ mode from a regular waveguide is analyzed. The hard wall is formed by longitudinal dielectric-filled corrugations. The analysis is based on a stepwise cylindrical approximation of the conical surfaces with subsequent application of the mode-matching method in combination with the method of factorization. The modal field expansion of the longitudinally corrugated cylindrical sections is found by using the asymptotic boundary condition of the corrugated surface when the corrugation period approaches zero. The indicated approach refines the data for the limiting aperture efficiency corresponding to the open-ended waveguide, and reveals strong influence of the hard section length as well as the interaction between the aperture and throat on the horn performance. It is shown that in the preferable case when the corrugations occupy the whole inner horn surface, it is possible to provide the crosspolar level below -30 dB and aperture efficiency from 85% to 95% in some frequency band up to 20% depending on the horn parameters.

Single- and dual-band multimode hard horn antennas with partly corrugated walls

We introduce a partly corrugated hard horn, consisting of a smooth-walled horn with an attached longitudinally corrugated outer section. This alleviates the problems with the manufacturing when the longitudinal corrugations extend into the throat of the horn. The transition between the inner smooth walled part and the outer corrugated part is abrupt. This is used and controlled to design better and shorter single-band horns than otherwise possible. Furthermore, it enables the design of dual-band horns with low cross-polarization and high gain, for multifunction use at Ka-band with transmit and receive frequencies in the same antenna. Measurements on a model of such a dual-band horn are presented and discussed.

Performance of an array of circular waveguides with strip-loaded dielectric hard walls

An infinite planar periodic antenna array of radiating open-ended circular waveguides is considered. The conducting waveguide walls are covered with dielectric layers loaded with longitudinal conducting strips for providing the hard wall boundary condition. Analysis of the array is carried out by the mode-matching method. The waveguide modes involved in the method are calculated by using the asymptotic strip boundary condition. It is shown that they are split into an independent subsystem of TE modes for the whole cross section and two independent subsystems of TM modes: one is for the central region and another is for the layer region. The calculations show that the operation of the hard waveguides in an array with small element spacing is similar to that of the multimode smooth wall waveguides completely filled with dielectric. For large diameters and element spacing, the hard waveguides have significant advantages over the smooth ones. It is shown that unlike an individual hard waveguide, the aperture efficiency of such a waveguide in the array has a nonmonotonic dependence on the waveguide radius. The results characterizing the behavior of the aperture efficiency and cross-polarization level in a frequency band as well as the contribution of certain waveguide modes in the reflected power are presented and discussed. The examples of the element patterns corresponding to minimal cross polarization are also given.

Blindness removal in arrays of rectangular waveguides using dielectrically loaded hard walls

We analyze an infinite planar array of open-ended rectangular waveguides in which the E-plane walls are loaded with dielectric slabs in order to create a hard boundary condition. The analysis is done by using the mode-matching method with Floquet modes in the exterior region. The results show that the hard walls remove the blind spots otherwise present in the element patterns of wide-angle scanning arrays. This occurs both for rectangular and for triangular lattices. In addition, the array becomes better matched to free-space in a rectangular lattice. In limited-scan arrays of large square waveguides, the use of the hard walls increases the array gain by 0.9 dB, it reduces the grating lobe level in the H plane by 11 dB and considerably decreases the difference between the array characteristics in the E and H planes.

A Modified Holographic Technique for Planar Near-Field Antenna Measurements

A modification of the synthetic-plane-wave-based holographic technique for determining radiation patterns and aperture fields of medium-gain horn antennas is proposed. The modification is based on using both the sum and difference of the near field and reference signals to form the hologram via simple low-cost amplitude-only near field measurements. This approach when compared to the previous version, where only the sum signal is used, allows increased sample spacing, and also provides an improvement in quality of the final results. The effectiveness of the modified technique is confirmed by both calculated and measured results obtained for pyramidal and conical horn antennas at 10 GHz frequency.

Analysis of a hard strip-loaded conical horn by the method of generalized scattering matrices

A hard-walled conical horn fed in the TE/sub 11/-mode from a regular circular waveguide is analyzed. The hard wall is formed by a dielectric layer with cylindrical inner surface loaded with longitudinal conducting strips. The analysis is carried out by using a stepwise representation of the horn with subsequent application of the method of generalized scattering matrices in combination with the method of factorization. The approach used refines values of the limiting aperture efficiency by 5 to 15% for diameters up to 3.2 wavelength in comparison with the underrated data based on the Kirchhoff method, and reveals the effect of the sharp worsening of the hard horn performance at some values of the horn length caused by resonances of the strip-line mode excited in the loaded section. Beyond the resonance points, the aperture efficiency remains relatively high (from 80 to 85%), and the crosspolar level is a few decibels lower than that of a standard smooth-walled horn.

Mode-matching modeling of a hard conical quasi-TEM horn realized by an EBG structure with strips and vias

A conical horn with hard wall used for supporting the quasi-TEM-mode having a uniform field distribution over the cross section is modeled. The hard wall is realized as an electromagnetic bandgap (EBG) structure using a dielectric layer loaded with narrow longitudinal strips connected to the basic metallic wall by radial vias to prevent excitation of the strip-line mode. The modeling is based on a stepwise approximation of the horn surfaces by a set of cylindrical sections. Each section consists of a strip-loaded part and a longitudinally corrugated part representing the vias. The stepped horn characteristics are calculated by using the mode-matching method combined with the Winer-Hopf-Weinstein method for accurate account of the wave scattering at the aperture. The strip-line mode resonance effects are studied and demonstrated. It is shown that if the length of the strip part of each section sufficiently differs from the resonance value, the performance of the hard strip-loaded horn with vias is similar to high performance of the appropriate hard corrugated horn. Recommendations are given on appropriate choice of the maximum spacing between the vias providing the absence of resonance effects in the specified frequency band.

A Modification of the Kummer's Method for Efficient Computation of the 2-D and 3-D Green's Functions for 1-D Periodic Structures

A new modification of the Kummers method of Mth order for 2 ≤ M ≤ 6 is proposed for efficient summation of the spectral and spatial series representing the 2-D and 3-D Greens functions, respectively, for 1-D periodic structures in homogeneous media. The modification is based on transformation of the auxiliary series consisting of asymptotic terms of the original series and subsequently subtracted from the latter into a new series which, unlike the previous one, allows its summation in closed form. As a result, there are obtained new representations of the Greens functions in question consisting of rapidly converging difference series whose terms decay with rate n-(M + 1) as n → ∞, as well as new rigorous analytic expressions for the sums of the transformed auxiliary series. Some numerical examples and comparisons characterizing the effectiveness of the proposed method are also presented and discussed.

Shaping of flat-topped element patterns in an array of slow-wave strip structures excited by parallel-plate waveguides

A two-dimensional (2-D) model of a corrugated-rod antenna array in the form of an infinite periodic array of finite slow-wave strip structures arranged above the apertures of two-or three-mode parallel-plate waveguides excited by TEM modes is analyzed. The formulated problem is solved by the method of matching the waveguide and Floquet modes on the waveguide apertures in combination with the Galerkin method for calculation of the currents on the strips. The geometrical parameters of the strip structures are optimized for shaping the flat-topped element patterns. It is shown that high quality of the flat-topped patterns in the element spacing range from 0.65 to 1.3 of wavelength is achieved if the excitation of the optimized structures is performed using the second and third reflected waveguide modes returned back to the apertures with the corresponding optimum phases. The presented examples also show that the shape of the obtained patterns keeps well at least in a 10% frequency band.

Analysis of Doubly Periodic Inhomogeneous Dielectric Structures by a Hybrid Projective Method

The problem of plane electromagnetic wave scattering by doubly periodic structures with both homogeneous and inhomogeneous dielectric elements arranged on a homogeneous dielectric substrate is considered. A hybrid projective method involving the method of coupled waves, method of projective matching of fields on the boundaries of partial regions, and one-dimensional finite elements method, is proposed. The method avoids using excessive number of unknown coefficients of expansion taking place in the previous versions. The effectiveness of the method is validated by a few tests including convergence of the results with increasing the number the expansion coefficients and comparison with appropriate data available in literature. The capabilities of homogeneous lossless matching dielectric elements of various shape and some properties of ultra-wideband absorbing structures with pyramidal elements are studied. The potential advantage of using inhomogeneous absorbing elements is also demonstrated.