Design and simualtion of a Ku-band input coupling structure TE10□ → TE11○ for gyro-TWT

In this paper, the design and simulation of a Ku-band input coupling structure TE
10
^□
→ TE
11
^○
for the gyrotron-traveling-wave-tube (gyro-TWT) amplifier is presented in detail. The input mode TE
10
^□
from the rectangular R180 is converted into the circular polarized mode TE
11
^○
by the input coupling structure, which consists of the mode converter part and the mode polarized part. It is a quite complicated process to optimize the input coupling structure since the parameters such as the disk thickness of the windows, the space between the disk and the axis of the waveguide, the radius of the waveguide, the length of the mode polarizer and so on, are all adjustable reciprocally. In order to reduce the reflection loss and expand the bandwidth of the input port the matched method of the phase and the amplitude is adopted in the process of designing the input coupling structure. With the optimization of the parameters the bandwidth of the input coupling structure is equal to about 1.0 GHz and 2.0GHz respectively when the VSWR (voltage standing-wave ratio) of the input port is equal to 1.2 and 1.4, which also corresponds to the 6.1% and 12.1% of the centre frequency respectively.