Ku-band 140W short-length space TWT has been designed for two different beam parameters. Design-I was carried out at beam voltage/current 5.9kV/100mA for gain (>25dB) and electronic efficiency (>25%). Design-II was done at beam voltage/current 4.4kV/188mA for high gain (>50dB) and efficiency (>17%). Higher-order harmonics and inter-modulation (I/M) components for both designs were analyzed using in-house developed SUNRAY- 1D and SUNRAY-2.5D codes. Accuracy of these codes was ascertained from comparison of the simulated results with the experimental results against an in-house designed and developed C-band 60W Space TWT. Both harmonic and inter-modulation components are less for design-II compared to design-I. Gain variation over the frequency band is also found less for Design-II. Major advantage of Design-I is its much higher electronic efficiency.