Accurate calculation of terahertz attenuation of a dielectric-coated metal wire with an intervening air gap using the engineering approach

A dielectric-coated metal wire with an intervening air gap between the conductor and inner surface of the dielectric is presented and demonstrated by theoretical calculation at terahertz frequencies. The characteristic equation of such a modified Goubau surface-wave transmission line is derived for the general case of a lossy dielectric and imperfect conductor. The terahertz attenuation of the modified Goubau line is investigated by using the accurate classical relaxation-effect frequency dispersion model. The influences of the different dimensions, different metal and dielectric materials on terahertz attenuation are also analyzed. In addition, the errors introduced by adopting the traditional and much simpler classical skin-effect model are also quantified. By using various conductivity models, the variation of the conductor loss is changed from 2.8% to 5.5%, and the variation of total loss is changed from 2.4% to 4.7%. It is shown that for certain combinations of the electrical dimensions of the structure the improvement in the attenuation constant over the Goubau line can be higher than 5 dB and realize stronger field confinement at terahertz frequency. The numerical results are very useful for the development of the surface plasmon polaritons (SPPs) devices in the fields of terahertz spectroscopy, sensors and detectors.