Effect of Manufacturing Tolerances of Helix Diameter on TWT Performances Abhishek Jain, Dheeraj Kumar, N Purshotaman, Rajendra Kumar Sharma, Vishnu Srivastava, Sanjay Kumar Ghosh CSIR-Central Electronics Engineering Research Institute(CEERI) Pilani-333031,Rajasthan,India Email: itsabhi_85@yahoo.co.in,ghoshsk@rediffmail.com,Fax:01596-242294,Phone:01596-252358(O) Abstract:-Saturated output power and gain are important characteristics for TWT performances. An extensive study aiming to improve the comprehension of the effect of helix diameter manufacturing tolerances on the TWT performances has been completed. Effect of variation of helix diameter by ± 5%, ± 10% about its nominal value on TWT performance have been studied and presented. It can be seen that change in performance can be compensated by changing helix operating voltage without disturbing focusing structure. Keywords: Helix diameter; TWT performance; Tolerances. I. Introduction Helix traveling-wave tubes (TWTs) still represent a key device in high data rate telecommunication systems and in satellite transmitters, where high output power (up to hundreds of watts) in a large instantaneous bandwidth is required. Further, the merging of TWTs with solid-state technologies in the microwave power module (MPM) permits the realization of low weight, low noise, high power, and high reliability amplifier devices. Flat gain response or gain ripple is mainly depends on the helix slow wave structure (SWS) dimensions. However, during formation of helix there is possibility of spring back action and wearing of helix material during chemical etching, thus helix diameter deviates for its actual value causing a large gain ripple. In this paper, effect of variation helix diameter has been studied and presented here[1]. Variation of helix dimensions can be compensated by varying helix voltage without changing the focusing structure. Variation of helix diameter affects the dimension of helix bundle which is fitted into a barrel envelope. Due to variation of helix diameter, helix rod bundle may be loosen or may required manual sizing loosing concentricity[2]. Here, the simulation has been carried out, in CST-MWS, for TWT under development, operating in X-band[3]. II. Result And Discussion Propagation constant (Figure.1) and interaction impedance (Figure.2) of the structure has been obtained using CST-MWS. It can be seen that for a given pitch profile, TWT performance (power and gain) varies with helix diameter (Figure. 3). It can also be seen from Figure.3, that for a given beam voltage and helix pitch profile, output power reaches to a maximum value and then decreases with the increase in helix diameter. Figure.4 shows the change in beam voltage that is required to keep the desired tube performance if there is any change in helix diameter. About 900V change in beam voltage to be accommodated for -10% tolerance while only about 400V for +10% tolerances. Figure.5 shows that the TWT performances becomes approximate flat in the region of -10% to +10% tolerances of helix diameter as beam voltage is varied to a certain limit (Figure.4). From Figure.6 it can observed that an increase of +5% tolerances, 80% of the tubes fall in the range of acceptability, while in the -5% tolerance case the number of acceptable tubes is reduced to 40%. Finally, it can be observed that how TWT performance is degraded for a change in 5% tolerance in helix diameter. Figure 1. Propagation constant (β) versus helix diameter at different helix pitches 283 978-1-4673-0369-9/12/$31.00 © 2012 IEEE