XXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE Design of Slow Wave Structure for G-band TWT for High Data Rate Links Rupa Basu. Laxma R. Billa, Jeevan M. Rao, Rosa Letizia, Claudio Paoloni Engineering Department Lancaster University Lancaster, UK c.paoloni@lancaster.ac.uk Abstract— The need of high data rate can be satisfied only by wide frequency bands in the millimetre wave region. This paper presents the design of a G-band (215 – 250 GHz) Traveling Wave Tube with 40 dB gain for wireless communications, based on the double corrugated waveguide. The structure of the TWT is based on a single section, instead of the typical configuration of two sections with a sever used at microwave frequency. This is possible due to the high losses at those frequency that permit a stable behaviour. This paper reports both cold and hot simulations. Keywords— TWT, G-band, double corrugated waveguide, millimetre waves I. INTRODUCTION The G-band (about 205 – 310 GHz) offer about 100 GHz useful bandwidth for high data rate internet distribution. So far data rate up to 40 Gb/s has been demonstrated [1]. The high atmosphere attenuation and the lack of enough transmission power limit the range to a few tens of meters, even by using high gain antennas. G-band solid-state amplifiers are still in development phase, however, it is unlikely to achieve output power above 50 - 100 mW. A rough link budget calculation provides that at least one Watt is needed to achieve useful range above 500m. Recently, traveling wave tubes (TWT) have been considered to as enabling devices for long links at millimetre waves. TWTs have proved to provide multi-Watt output power that satisfy the link specifications [1 - 4]. In this paper, the design ofa G-band (215 – 250 GHz) TWT, to power a multigigabit per second transmitter, will be described. Simulations of the cold parameters and large signal performance will be described. The TWT is designed with the double corrugated waveguide (DCW) as slow wave structure [5]. Differently, from microwave helix TWTs, it has been designed with a single section without sever. This approach is possible because of the high ohmic losses in the slow wave structure at these frequencies. A single DCW section permits to achieve about 40 dB gain. In the following, it will be discussed also the stable operation of the TWT. In a previous work, a DCW with 160 periods [6] was considered for a moderate gain TWT, based on one section without the sever. A study on a longer DCW to achieve about 40 dB gain is proposed. II. TWT DESIGN A. Double Corrugated Waveguide Design The design of the DCW started with the dimensioning of the geometry of the unit cell to achieve the useful bandwidth with proper beam synchronism, given the beam voltage. The voltage of the electron beam is set at 12.3 kV. This value is a compromise between the best focusing, the power supply cost and the energy of the electrons for a relatively high efficiency. The dimensions were optimised to assure the proper synchronism of the phase velocity with the electron beam over the 215- 250 GHz frequency band. B. S-parameters Once the correct dispersion is achieved, a complete circuit to connect the DCW to the input and output flanges was designed. The output and input coupler consist of a number of pillars tapered in height to provide the TE10 mode at the flanges. The DCW interaction section has 220 periods and each coupler includes 15 periods. The S-parameters of the complete circuit were computed by CST- transient solver. The reflection coefficient (S11) better than -20 dB is obtained over the desired operating band, assuring a high-quality matching (Fig. 2). Fig 2. S- parameters for the DCW. Fig. 1. Double Corrugated Waveguide Unit Cell