An output circuit of a helix traveling wave tube was modeled in High Frequency Structure Simulation (HFSS) 3D software. A design change was attempted to make the circuit have a better VSWR than 1.5:1 in the frequency range of 12-18GHz.
Traveling Wave Tubes are high frequency wide band amplifiers that are used in nearly all communication systems. They operating frequency can range from 1-100GHz and can put out power from Watts-Megawatts. Microwave tubes are the answer to the failure of operation of solid state devices at high frequency. In solid state devices as the wavelength approaches the element size, the lumped elements behave in an unpredictable manner. As a result of this fact Traveling Wave tubes dominate the high frequency spectrum and find use in most military applications.
The electron gun is responsible for generating the beam needed for amplification. The slow wave structure makes the interaction between RF and electron beam possible. The collector is on the opposite end of the gun and consumes the remainder of the beam from the interaction. The RF wave amplification process occurs by extracting energy from electron beam that generated by the cathode. An electron beam is generated in the same manner as most conventional televisions. That is why all the TWT have to maintain a vacuum level. A cathode is heated up to its proper temperature so it can start emitting electrons. The beam then enters the slow wave structure and it held together by the magnetic field of magnets. The traveling speed of electron is a lot slower than the RF wave and that is why a slow wave structure is needed. A slow wave structure can be defined as any geometry that can slow down the RF wave to match the speed of the beam. In our case the slow wave structure is the helix.
http://www.engr.sjsu.edu/rkwok/projects/Report.OutputWindowModel.gurvinder.ankush.pdf
