Renato Orta

A novel design tool for waveguide polarizers

This paper presents a novel method for the design of broad-band waveguide polarizers. It consists of an interactive procedure where the designer places the zeros of the reflection coefficient for one polarization, simultaneously also controlling the frequency response of the other one and vice versa. In this way, the best matching condition for both polarizations can be found. This design tool is based on an automated phase control procedure and on an algorithmic characterization of the relationship between the two frequency responses. The spurious effects of multimodal interactions, frequency dispersion, and losses are taken into account and compensated by characterizing them with suitable transfer functions. Designs of iris-loaded polarizers in square and circular waveguides are described, with operative bandwidths up to 30%. Measurements on a 10% bandwidth Ka-band prototype show reflection and cross-polarization levels of approximately -50 dB, with a phase error of approximately 0.4/spl deg/.

Reduced-order optimized mode-matching CAD of microwave waveguide components

The optimum designs of microwave devices require highly accurate and efficient computer-aided-design systems. This paper describes a novel reduced-order model for the mode-matching technique, which is derived from the application of the Krylov subspace concept and of the singular-value-decomposition algorithm to the coupled integral equations solved by the mode-matching technique. The design example of a high performance 12-cavity Ka-band ridged-filter confirms the accuracy and the numerical efficiency of the proposed model. In particular, the presented method also enables to accurately control the metal losses of the devices.

Orthomode transducer for millimeter-wave correlation receivers

This paper presents a novel orthomode-transducer (OMT) architecture,which is particularly suitable for correlation receivers at millimeter waves. By exploiting an on-axis reverse-coupling structure, a compact OMT configuration is obtained, which provides high levels of channel equalization. The Ka-band prototypes exhibit very good electric performances in terms of isolation, cross-polarization, return loss, and channel equalization.

A microwave measurement procedure for a full characterization of ortho-mode transducers

Ortho-mode transducers (OMTs) are key components in both dual-polarized antenna feed systems for telecommunication and radioastronomy applications. The evaluation of the overall system performance requires the measurement of the return loss, isolation, and cross-coupling levels of the OMTs. In this paper, a novel technique for the microwave measurements of the full 4/spl times/4 scattering matrix of such devices is reported, which is based on different measurements at the single-polarized ports.

Full-wave design of broad-band compact waveguide step-twists

Rectangular waveguide twists are components that provide an on-axis rotation of the transverse waveguide section. In this letter, a step twist configuration, obtained by the direct connection of rotated rectangular waveguide lengths, is considered. The advantages of this configuration are the easy manufacturing with tight control on the relative rotation angles and the reduction of the overall dimensions. By means of a full-wave synthesis approach various wide-band devices were designed and reported in the letter. In particular, a couple of 45/spl deg/ twists was manufactured and integrated in the Ka-band OMT realized for the SPOrt project. The experimental results match the excellent predicted data.

Combined-Phase-Shift Waveguide Polarizer

A waveguide polarizer exploiting two different phase shift phenomena is presented in this letter. Iris-type discontinuities are in fact introduced in a waveguide structure having different propagation constants for the two principal polarizations. In this way, the required 90 differential phase shift is obtained combining the iris phase shift with the waveguide one. Several operative conditions arise from the combination of the two contributions. This approach has been used to design a broadband waveguide polarizer for the C-band antenna feed system of the Sardinia radio telescope with -40 dB reflection coefficients and a -35 dB cross polarization level in a 30% bandwidth.

Optimum-Iris-Set Concept for Waveguide Polarizers

Novel profiled irises for the design of waveguide polarizers are presented. The two degrees of freedom of their piecewise-circular profile allow one to better control the frequency behavior of the differential phase shift. Thanks to this feature, an optimum iris set can be found by selecting the discontinuities for which the differential-phase-shift deviation from the average value is minimum in the operative bandwidth. This optimum iris set can be identified for several values of the ratio r/lambda0, where r is the waveguide radius and lambda0 is the free-space wavelength at the central frequency. Hence, even if both the waveguide size and central frequency are assigned in the specifications, polarizers with a very low deviation from 90deg of the total differential phase shift can be designed. In this way, the number of waveguide transitions in the antenna feed system can be reduced and the effect of spurious higher-order-mode coupling can be minimized. A Ka-band polarizer in circular waveguide with piecewise-circular-profile irises is described, showing a return loss better than 55 dB and a phase error of about 0.9deg in a 10% bandwidth

Piecewise-uniform bends in rectangular waveguide

A new configuration of improved rectangular-waveguide bend for advanced millimeter-wave applications is presented. It consists of few lengths of constant-curvature bends, properly designed in order to obtain enhanced matching conditions, without matching elements. Comparisons between the proposed bends and the uniform ones are reported, showing that the formers exhibit a better return loss. Measurements on a Ka-band prototype, in its operative condition, and a worst-case analysis on a W-band design, confirm the high electrical performances also in presence of significant manufacturing uncertainties.

Analysis of piezoelectric strip waveguides based on the effective index and pseudospectral element methods

Integrated acousto-optical circuits in LiNbO3 are attractive devices for applications especially in advanced WDM systems. In order to increase the scale of integration and to reduce the RF driving power of these devices, one promising approach is to use acoustical waveguides with smaller lateral dimensions. In this paper the combination of a pseudospectral elements method (PSEM) and an effective index method (EIM) for the analysis of film-loaded surface acoustic waveguides (SAWGs) is presented. Numerical results demonstrate the exponential rate of convergence of the PSEM and agree with the computations performed via the transverse resonance technique. Due to the exponential rate of convergence of the PSEM and to a mapping transformation in the substrate, the piezoelectric problem in the depth direction of the structures is evaluated by a low number of Legendre polynomials. The method is applied to the analysis of several layered SAWGs of practical interest for AO devices, such as metal/SiO2/X - YLiNbO3, in which the metal is either Au, Al, In, or Ti.

Film-loaded SAW waveguides for integrated acousto-optical polarization converters

We report on a detailed theoretical and experimental investigation of film-loaded surface acoustic wave (SAW) waveguides in lithium niobate (LiNbO/sub 3/) for integrated acousto-optical (AO) polarization converters. The numerical analysis is based on both a scalar and a full-vectorial model. Dispersion plots and figures of merit for several structures are given, which lead to design parameters for optimized polarization converters. It is pointed out that very attractive structures are metal/dielectric/LiNbO/sub 3/ strip waveguides and dielectric/LiNbO/sub 3/ slot waveguides, in which metal is either gold (Au) or aluminum (Al), and the dielectric film is an optical transparent material such as silicon oxide (SiO/sub 2/), magnesium oxide (MgO), or aluminium oxide (Al/sub 2/O/sub 3/). Polarization converters with the designed acoustical waveguides have been realized and characterized by optical conversion and laser probing measurements.