The transmission characteristics of cylinder and rectangular metal overmoded waveguides frequently used in 0.14 THz frequency band are numerically studied in this paper. At first, skin depths and losses of overmoded cylinder waveguides for different waveguide materials and terahertz wave modes are theoretically calculated and validated by electromagnetic finite difference time domain (FDTD) method. Loss as high as 5 dB/m is obtained for stainless steel and TM01 mode, and it increases with mode orders and decreases with waveguide radius. Also the mode conversion is observed in the straight overmoded cylinder waveguide with finite conductivity. Then transmission characteristics of overmoded rectangular metal waveguide including bend waveguides, straight waveguides, and transition waveguides are researched. The significant mode conversions in the overmoded E and H bend waveguides are found even ignoring the effect of finite conductivity. Only a few of the TE10 mode within frequency band of 0.13-0.16 THz is successfully transported through the standard bend waveguides in Ka band. The transmission characteristics of overmoded straight rectangle waveguide are almost the same as the cylinder one’s. Non-cutoff modes for the two waveguides connected by transition waveguide can transfer through it without losses, while the rest modes can’t.
This paper has successfully setup a calibration system for terahertz devices at frequencies from 0.11 THz to 0.17 THz in waveguide WR6.5. The system consists of a terahertz wave generator, a power meter, a digital oscilloscope and a computer, and is able to accomplish calibrations for some devices automatically. Calibrations of a 10 dB attenuator, a 20 dB directional coupler, and a Schottky diode detector are carried out by the system and a detailed discussion is given to the results. The results obtained have shown that the characteristics of the devices under calibrated remained essentially the same throughout the experiment and met rather good agreement to the factory data. Due to the use of GPIB, it is proved that the system can also improve the efficiency of work greatly. The calibration approach described in this paper can be a necessary supplement to the other calibration techniques and play an important role in terahertz measurement especially at lower frequencies.