Input impedance of a multilayer insulated monopole antenna

An efficient modal expansion method was developed for the analysis of the sleeve monopole antenna fed by a coaxial line [Shen and McPhie, 1995]. This method is employed to calculate the current distribution and input impedance of a multilayer insulated monopole antenna fed by a coaxial transmission line. The modal expansion analysis is facilitated by the introduction of a perfectly matched boundary (PMB), which is the combination of an electrical wall and a magnetic wall. The resulting guided-wave structure is then divided into several sub-regions; the electromagnetic fields in these subregions are expressed by the summation of their modal functions weighted by some unknown expansion coefficients. These coefficients, which lead to the current distribution and input impedance of the antenna, are found by enforcing the boundary and continuity conditions at conducting surfaces and regional interfaces. An efficient recursive algorithm is presented to implement the analysis of an arbitrary multilayer insulated monopole antenna. The modal expansion method presented not only takes the effect of the coaxial feed line into account, but is also valid for the cases of a thick monopole antenna and a monopole immersed in a multilayer dielectric cylinder of arbitrary permittivity. Numerical results for the input impedance of a dielectric-coated monopole antenna and an air-insulated monopole are compared with experimental ones available in the literature.