When the metal film is thicker than the skin depth in the working frequency band, the transmission characteristics of outer coated type are superior to the transmission properties of inner coated type under the same size. Further more, the transmission properties of the single, double, three and four groove both for inner coating and outer coating terahertz (THz) polystyrene (PS) tubes are studied in this paper. In result, the transmission properties of single and double slot are good, but the three and four slots' transmission characteristics deteriorate. In addition, slots width affects the transmission characteristics of PS tubes evidently, and the attenuation coefficient of outer coated PS tube with single slot is proportional to the slot width, so as to optimize the transmission properties of PS tube. It is a compromise for the slot width (it is better to choose appropriate slot width).
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