F. Alessandri

Computer-aided design of beam forming networks for modern satellite antennas

Multicontoured and reconfigurable beam spacecraft antennas rely on sophisticated beam forming networks (BFNs). Accurate and efficient CAD tools are required to meet the stringent requirements on the power division, while avoiding any tuning or trimming of the BFN. Advanced field theoretical techniques for the analysis and optimization of microwave BFNs realized in both waveguide and square coaxial cable technologies are presented. Such advances consist in new segmentation techniques of the microwave components associated with efficient mode-matching formulations for the modeling of isolated as well as coupled discontinuities. The achieved numerical efficiency allows sophisticated synthesis procedures, based on repeated full-wave analysis in wide frequency bands, to be performed on small machines such as a 386 PC. The design tools developed have been widely validated through a comprehensive test campaign. >

A technique for the fullwave automatic synthesis of waveguide components: application to fixed phase shifters

An efficient computer synthesis technique for waveguide components, based on rigorous field-theoretical models, has been developed. A computer code has been specifically set up for the automatic design of fixed phase shifters in rectangular waveguide technology. Only the electrical specifications are required to generate, normally in 15 to 20 min on a 386/16-MHz IBM PC, the geometrical structure of the components. The agreement with the experiments is shown to be so accurate as to avoid any tuning of the circuits realized. The efficiency and accuracy of the code is based on (i) a suitable segmentation technique of the microwave structure to obtain a very simple but rigorous network model; (ii) the efficient representation of the modal series for the electromagnetic fields; and (iii) a synthesis procedure based on a simplified model to obtain a good initial guess for the final full-wave optimization routine. >

A fullwave CAD tool for waveguide components using a high speed direct optimizer

An extremely efficient optimization tool, where the fullwave mode matching simulator is driven by a quasi-Newton optimizer using the adjoint network method, has been developed for the CAD of a class of rectangular waveguide components. This includes filters, phase shifters, branch guide couplers, etc., with step in either the E- or H-plane. With respect to the conventional finite difference computation of the derivatives, a speedup factor of more than 10 times is easily achieved. >

A fullwave CAD tool of waveguide components using a high speed direct optimizer

Based on the adjoint network method (ANM) and on mode matching technique, an extremely efficient optimization tool has been developed for the CAD of a class of rectangular waveguide components. This includes filters, phase shifters, branch guide couplers, etc., with step in either the E- or H-plane. With respect to the conventional finite difference computation of the derivatives, a speedup factor of more than 10 times is easily achieved.<<ETX>>

The electric-field Integral-equation method for the analysis and design of a class of rectangular cavity filters loaded by dielectric and metallic cylindrical pucks

Rectangular cavity filters loaded by metallic and dielectric cylindrical pucks represent a wide class of structures used for the realization of compact microwave filters for space and terrestrial communications. The electric-field integral-equation method has been applied here to develop a software tool for the design of such a class of filters. Resonators simultaneously containing a number of dielectric and metallic cylindrical pucks can be analyzed using the method presented here. Dielectric loaded rectangular waveguide filters and comb-line filters are two typical examples where this approach can be applied. The input and inter-cavity couplings realized by slots have been modeled using the proposed integral-equation method. The full-wave analysis and optimization of an entire filter can be performed using the software presented here. Dielectric and metallic losses have been taken into account and the tuning screws have been also included into the analysis. The prototype of a single resonator and complete passband dielectric loaded rectangular waveguide filter have been realized and measured to test the software, and a commercial software has also been used to validate the code. A number of examples are presented here. A very short computation time of the order of 1-s/frequency point for the analysis of an entire filter has been obtained, making the full-wave optimization of an entire filter an affordable task.

A new multiple-tuned six-port Riblet-type directional coupler in rectangular waveguide

Directional couplers are fundamental components for the realization of power splitting and combining networks. A basic component is the four-port coupler realized in a wide range of different electrical and geometrical configurations. Using multiport directional couplers, with six or more ports, compact power splitting and combining networks can be designed. Most of the multiport directional couplers in a rectangular waveguide found in literature use an E-plane branch-line layout. A special configuration of a six-port narrow-wall short-slot directional coupler (Riblet type) is presented here. The coupler has only one central coupling region, following the Riblet concept based on the differences among the propagation constants of the modes through the coupling region. A number of different solutions have been investigated and some examples are presented here. Moreover, a multituning realized by using input resonators permits one to significantly enlarge the bandwidth. A six-port coupler equally splitting on the output ports the power injected at each input port has been designed for a 8.4% working band. The theoretical and measured responses presented here prove the effectiveness of the multituning concept.

Theoretical and experimental characterization of nonsymmetrically shielded coplanar waveguides for millimeter-wave circuits

A quasi-planar structure, the nonsymmetrically shielded coplanar waveguide (NSCPW), is proposed as a quasi-TEM transmission line with advantageous characteristics for millimeter-wave circuit applications. Advantages in terms of broadband behavior and ease of machining, as well as device mounting and substrate mounting, are pointed out. An experimental method is developed which allows the evaluation of the transmission line spectrum in a very wide frequency band (15:1) with a single transmission measurement. The experimentally evaluated propagation characteristics of the dominant and higher order modes are shown to be in excellent agreement with the theoretical predictions based on the generalized transverse resonance technique. This method is also used for an extensive characterization of the structure in terms of characteristic impedance and useful frequency band. >

Planar Integrated Waveguide Diplexer for Low-Loss Millimeter-Wave Applications

Low-loss H-plane waveguide filters for planar integrated assemblies are described showing high attenuation near the pass-band due to a special modification of the outer inverter circuits. The outer irises are replaced by short-circuited T-junctions resulting in at least one pole above or below the pass-band. Such filters favorably can be combined to low-loss diplexer circuits for mm-wave applications. Examples of a single filter as well as of diplexers in the 28 GHz and 38 GHz frequency range are given.

Rigorous and efficient analysis of hybrid T-junctions

A very simple, yet rigorous and efficient model for the full-wave characterization of hybrid T-junctions is proposed. The model is based on a suitable segmentation of the structure in conjunction with the admittance matrix description. When suitable modal expansions of the Greens functions are considered, a very high numerical efficiency is obtained. Numerical simulations show excellent agreement with measured data.<<ETX>>

Field theory design of a nove circular waveguide dual mode filter

A novel and simple configuration is proposed for the realization of dual-mode filters. The geometry consists of a circular cavity coupled with rotated rectangular waveguides through thick irises, with no need of tuning or coupling screws. The geometry is such that it can easily be analized and optimized using mode-matching technique without requiring any special numerical treatment. Filter performances are shown to be equivalent to those obtained with more complicated geometries.