Christine T. Chevalier

Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to <0.5% and <10%, respectively. Saturated gain predictions, being approximately half as sensitive to these parameters, appear to require correct specification of phase velocity and interaction impedance to within /spl sim/1% and 20%, respectively. Although all models generate sufficiently accurate predictions of the interaction impedance, not all generate sufficiently accurate predictions of the effective axial phase velocity.

Traveling-wave tube cold-test circuit optimization using CST MICROWAVE STUDIO

The internal optimizer of CST MICROWAVE STUDIO (MWS) was used along with an application-specific Visual Basic for Applications (VBA) script to develop a method to optimize traveling-wave tube (TWT) cold-test circuit performance. The optimization procedure allows simultaneous optimization of circuit specifications including on-axis interaction impedance, bandwidth or geometric limitations. The application of MWS to TWT cold-test circuit optimization is described below.

Characteristics of Double Exponentially Tapered Slot Antenna (DETSA) Conformed in the Longitudinal Direction Around a Cylinder

The characteristics of a double exponentially tapered slot antenna (DETSA) as a function of the radius that the DETSA is conformed to in the longitudinal direction is presented. It is shown through measurements and simulations that the radiation pattern of the conformed antenna rotates in the direction through which the antenna is curved, and that diffraction affects the radiation pattern if the radius of curvature is too small or the frequency too high. The gain of the antenna degrades by only 1 dB if the radius of curvature is large and more than 2 dB for smaller radii. The main effect due to curving the antenna is an increased cross polarization in the E-plane

Robust Optimization of High-Frequency Traveling-Wave Tube Slow-Wave Circuits

An optimization algorithm has been developed to provide robust designs for slow-wave circuits of high-frequency traveling-wave tubes (TWTs). The algorithm utilizes the optimization method of simulated annealing in a TWT simulation code. By considering the effects of dimensional variations during the optimization, a phase-velocity profile of a slow-wave circuit is generated. A simulated statistical performance test of a robust design for a 94-GHz folded-waveguide circuit shows significantly less sensitivity to dimensional tolerance variations

A comparison of three-dimensional simulations of traveling-wave tube cold-test characteristics using CST MICROWAVE STUDIO and MAFIA

Previously, it was shown that MAFIA (solutions of Maxwells equations by the Finite Integration Algorithm), a three-dimensional simulation code, can be used to produce accurate cold-test characteristics including frequency-phase dispersion, interaction impedance, and attenuation for traveling-wave tube (TWT) slow-wave structures. In an effort to improve user-friendliness and simulation time, a model was developed to compute the cold-test parameters using the electromagnetic field simulation software package CST MICROWAVE STUDIO (MWS). Cold-test parameters were calculated for several slow-wave circuits including a ferruled coupled-cavity, a folded waveguide, and a novel finned-ladder circuit using both MWS and MAFIA. Comparisons indicate that MWS provides more accurate cold-test data with significantly reduced simulation times.

Ka-band waveguide hybrid combiner for MMIC amplifiers with unequal and arbitrary power output ratio

The design, simulation and characterization of a novel Ka-band (32.05±0.25 GHz) rectangular waveguide branch-line hybrid unequal power combiner is presented. The manufactured combiner was designed to combine input signals, which are in phase and with an amplitude ratio of two. The measured return loss and isolation of the branch-line hybrid are better than 22 and 27 dB, respectively. The application of the branch-line hybrid for combining two MMIC power amplifiers with output power ratio of two is demonstrated. The measured combining efficiency is 92.9% at the center frequency of 32.05 GHz.

Microwave Characterization Of Electro-Optic Polymers

Electro-optic polymer thin-films were characterized at microwave frequencies, for possible applications in GHz modulators. Single layers of the polymers were spin coated on low loss magnesium oxide substrates for characterizing the polymers at microwave frequencies. Coplanar waveguide test structures were designed for evaluation of the relative dielectric permittivity as well as the loss-tangent of the polymers at microwave frequencies. A conformal mapping technique was used for the extraction of relative dielectric constant of the polymer films based on the microwave scattering parameter measurements. As an example, a DR1-PMMA polymer had a relative dielectric permittivity of 3.01 and a loss-tangent of 0.075 at 8 GHz. The design of the test structures, the procedure for the accurate determination of the dielectric permittivity and the loss-tangent of the polymer films are discussed in this paper.

Characteristics of corrugated double exponentially tapered slot antenna

In this paper, a double exponentially tapered slot antenna (DETSA) is bent into a corrugated shape to physically shorten the DETSA while keeping the electrical length the same. Measured and simulated radiation patterns and measured return loss are presented

Robust Design Algorithm for High-Frequency Traveling-Wave Tube Slow-Wave Circuits

An optimization algorithm has been developed to provide robust designs for slow-wave circuits of high frequency traveling-wave tubes. A simulated statistical performance test of a robust design for a 94-GHz folded waveguide circuit shows significantly less sensitivity to dimensional tolerance variations

HIGH EFFICIENCY KA-BAND MMIC SSPA POWER COMBINER FOR NASA'S SPACE COMMUNICATIONS

In this paper, we will review the design, construction and performance of the two-way Ka-band waveguide branch-line and asymmetric magic-T based unequal power combiners. The manufactured combiners were designed to combine input signals that are equal in phase and with an amplitude ratio of two. Next, the design, construction and performance of a three-way branch-line unequal power combiner, achieved by serially interconnecting two 2-way branch-line hybrids and optimizing the dimensions using software tools, is presented. The application of the two-way and three-way combiners for combining the output from two or three MMIC PAs was demonstrated. The observed efficiencies for all three power combining configurations are 90 percent or greater at Ka-band (31.8 to 32.3 GHz).