Design and characterization of meander line slow wave structures for W-band space traveling wave tubes

Socuellamos, Juan and Paoloni, Claudio and Letizia, Rosa (2021) Design and characterization of meander line slow wave structures for W-band space traveling wave tubes. PhD thesis, Lancaster University.

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Abstract

Satellite communication systems are nowadays facing the growing demand of different services such as internet, streaming or teleconferencing that consume a lot of resources and exhaust the available links. Increasing the frequency band of operation is becoming ever-more necessary to cope with the challenges of the future communication services. The exploitation of W-band frequencies (71-86 GHz) may offer many benefits for satellite communications. The wide band channel and high data rate permits to transmit and receive more information at higher velocities. The reduced size of the components decreases the weight of payload and the mission cost. To overcome the huge atmospheric attenuation at W-band, traveling wave tubes (TWTs) are the only solution to provide enough power to feed the link between satellites and ground stations. The amplification in a TWT is based on the interaction between an electromagnetic wave and an electron beam in such a way that, under specific circumstances, the electron beam is able to transfer part of its energy to the wave and amplify the signal. The main component of a TWT is the slow wave structure (SWS), whose role consists of slowing down the radiofrequency signal to a phase velocity comparable to that of the electrons in the electron beam in order to get amplification. Microwave TWTs are mostly based on helix SWSs. Going up in frequency to millimetre waves implies a reduction of the wavelength and, consequently, of the dimensions of the SWS. The helix is particularly affected by this reduction, making its manufacture unfeasible at W-band. New SWSs have been then recently investigated as an alternative to the helix. One of them is the meander line SWS which, in its standard form, consists of a dielectric substrate where a serpentine-shaped metallization has been patterned. Compared to other SWSs at W-band, meander lines are potentially capable of offering very good performance while lowering the beam voltage and enhancing the interaction impedance, providing higher efficiency and reducing the size and weight of the TWT. This is particularly important for space applications, where saving power and reducing the mass on spacecraft has direct implications for the final system and launch costs per satellite. Given the simplicity and straightforward fabrication of the meander line, this kind of SWSs are also adequate for low-cost and high volume production. Thanks to its many benefits, the meander line is seen as a very promising SWS for a new generation of low-cost, lightweight and compact TWTs for the establishment of future cost-effective satellite communication networks. This thesis will dedicate the first chapter (Ch. 1) to a brief summary of the history and evolution of the TWT and its role as power amplifier for satellite communications and, in particular, at W-band. The working principle and main components of the TWT will be described as well as the requirements for space applications. Special emphasis will be put on the SWS and the different options available at W-band. The second chapter (Ch. 2) will focus on the description of the characteristics and properties of the meander line SWS and will go through an extensive literature review to become familiarized with this kind of SWS and the different coupling transitions between the meander line and a rectangular waveguide. The main microfabrication techniques needed for manufacturing meander line SWSs will be also briefly explained.