Electromagnetic Characterization of Misaligned Serpentine Waveguide Structures in Traveling-Wave Tubes at Microwave Frequencies

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Modern-day millimeter and microwave source technology has advanced considerably over the past century, but to meet the defense industry’s demand for high power and large bandwidth, vacuum electronic devices (VEDs) are still the ideal candidate to fulfill such requirements as opposed to their solid-state semiconductor counterparts. Of the numerous VEDs available, the traveling-wave tube (TWT) amplifier provides novel solutions in areas where size, weight, and power (SWaP), and bandwidth are of great importance such as on satellites and in electronic warfare applications.

The advancement in computer-aided design (CAD) and simulation has allowed for increasingly complicated device configurations to be designed with ease. Instead, challenges arise in fabrication as extremely tight manufacturing tolerances on the order of micron to submicron levels are necessary due to the very short wavelengths in the mm-wave and sub-mm-wave regimes. Without this level of manufacturing precision, VEDs will not operate at optimal levels in power, bandwidth, and efficiency.

We present a serpentine waveguide (SWG) design to be used as the slow-wave structure (SWS) in a TWT amplifier. Manufacturing techniques for the design are discussed, and a detailed study into how one-dimensional and two dimensional misalignments in the circuit’s half-plane affect the radio frequency (RF) signal that propagates through the device. Figures of merit include the device’s reflected power, or S11, the transmitted power through the SWG, or S21, the device’s cutoff frequency, and the SWG’s dispersion curves.

Computer simulations using Ansys’s High Frequency Structure Simulator, or HFSS, and cold test laboratory measurements for aligned and misaligned Ka-band (26.5 GHz – 40 GHz) SWG circuits are presented. Upon completing a thorough RF characterization of the Ka-band device, efforts will shift focus to designing a SWG circuit for a W-band (75 GHz – 110 GHz) TWT amplifier prototype.