Stefano D'Agostino

Accurate analysis of helix slow-wave structures

In this paper, the helix slow wave structure (SWS) of traveling wave tubes (TWTs) has been analyzed. Dielectric supporting rods of arbitrary cross section have been considered in this analysis. The inhomogeneous dielectric loading factor has been accounted for by modeling the discrete support with a number of continuous dielectric tubes of appropriate effective dielectric permittivity. The helix tape model has been used for the field analysis. Furthermore the thickness of the helix tape has been considered. A rigorous solution of the field equations, including the contribution of the space harmonics, was performed to evaluate the phase velocity and the interaction impedance up to millimeter-wave frequencies. The nonuniformity of radial propagation constant over the structure cross section has been also included. With respect to other approaches, a closed-form expression of the field constants has been obtained. A study to choose the optimum number of space harmonics and dielectric tubes to be used in the analysis, has shown how the results are more sensitive to the number of space harmonics than to the number of dielectric tubes, beyond a certain number of the latter. The validity of this theory has been proved by comparison between measurements and simulations for helix SWS with different dimensions, rod shapes and operating frequency band.

Sensitivity analysis of TWT's small signal gain based on the effect of rod shape and dimensions

An extensive study aiming at analyzing the effect of rod shapes and dimensions on the gain of helix traveling wave tubes (TWTs) is performed. The evaluation of tube small-signal gain is obtained by making use of a rigorous field analysis which takes into account the helix tape model and the dielectric inhomogeneous loading conditions. Computing time to perform the analysis is extremely low compared with the time required in the case of a full wave, three-dimensional (3-D) electromagnetic simulator. The accuracy of the simulation approach has been extensively verified in a previous paper. A novel expression for the attenuation constant has been introduced in the model to improve the quality of results. The proposed study allows a better understanding of tube behavior before fabrication highlighting the contribution of the shape, the mechanical tolerances and the /spl epsiv//sub r/ variation of the rods to the small-signal gain.

Effect of mechanical tolerance of T‐shaped rods in a TWT based on a helix slow‐wave structure

The effect of the mechanical tolerances of a dielectric rod on the normalized phase velocity and interaction impedance of the helix slow-wave structure used in traveling-wave tubes is analyzed. In this letter, one of the most common rod shapes, the T-shaped rod is considered, highlighting how the three geometrical dimensions, typical of this rod, affect the above-mentioned helix slow-wave characteristics

Analysis of high‐efficiency multistage depressed collectors based on the 3‐D finite‐element method

An important feature in the realization of modem TWTs is the design of high-efficiency multistage depressed collectors. The results presented in this letter show that the 3-D finite-element method provides an excellent and fast means for the simulation of complex structures. An axisymmetrical four-stage collector and an asymmetrical four-stage collector are presented and simulated, showing the 3-D mesh distribution and the electron trajectory plots

ON THE TWT BASED ON A HELIX SLOW-WAVE STRUCTURE SUSTAINED BY RECTANGULAR DIELECTRIC RODS

Inhomogeneous dielectric loading is a necessary approach to obtain an accurate simulation of a helix slow-wave structure in traveling-wave tubes. In this paper a new expression for the effective dielectric constant to characterize each dielectric coaxial tube in which the region between the helix and the envelope is divided for the ease of a rectangular rod is proposed. The use of this expression in a rigorous analysis method results in a more accurate simulation of the helix slow-wave structure

Effect of tape width and thickness on helix slow‐wave structure performance

A study on the effect of tape dimensions on the performance of a helix slow-wave structure in traveling-wave tubes is proposed. The influence of the rape dimensions (i.e,, width and thickness) on the normalized phase velocity and interaction impedance has been considered. A proper choice of tape dimensions is essential to achieve optimum performance for the slow-wave structure. A measured helix slow-wave structure operating in the 6-18 GHz frequency range was considered for the study

Experimental validation of a large-signal MESFET model for submicron-gate-length devices

The performance of a physics-based large-signal MESFET model, previously published and experimentally validated for a gate length equal to or greater than 1 mu m, has been evaluated for the submicron case. A satisfactory agreement for both the dc characteristics and the S parameter derived from the model compared with experimental data assures the validity of the model for a 0.5-mu m gate-length MESFET process

A finite-element 3-D method for the design of TWT collectors

This letter discusses the basic philosophy of a fully three-dimensional finite-element method for collector design together with ifs main features. In particular, attention is focused on the dedicated mesh generator and the strategy followed for the solution of the coupled electromagnetic and motional problem within the same finite-element discretization context. An example of simulation is carried out, and indications of future applications are also provided.

An optimization procedure for MMIC large-signal amplifiers

An optimization procedure for GaAs integrated circuits to achieve both small-signal and large-signal performance is proposed. Besides the geometrical dimensions of the the circuit passive elements, the process parameters are used as optimization variables to define two separate sets of optimization variables for small-signal and large-signal optimization. A large-signal MESFET model, based on simple expressions among circuit elements and process parameters, allows an effective implementation of the proposed optimization method in commercial CAD tools

Fast prediction and optimization of yield in gallium arsenide large-signal MMICs

A study on the effects of the geometrical and physical parameters of the GaAs MMIC process on the yield of large-signal circuits is presented, Large-signal yield analysis as well as large-signal yield optimization are performed using a large-signal lumped-element MIESFET model related to MMIC process parameters, and suitable for implementation in commercial microwave CAD tools, The characterization of all the statistical variables of a large-signal circuit provides a better understanding of the yield behavior, In particular, the sensitivity of large-signal yield to MMIC process parameters is computed and the statistical behaviour of each parameter is presented by means of yield sensitivity histograms