Analysis of the probe-fed annular microstrip antenna for hyperthermic applicators

The exact image theory is applied to obtain the exact field distribution of an annular microstrip radiator over a two layered biological medium. First, the image functions for the three layer structure (substrate-skin-fat) are derived by expanding the Fourier domain reflection coefficient into a sum of pole terms, known also as the Mittag-Leffler expansion. Secondly, the current distribution on the strip is solved by using the Galerkin’s method with continuous trigonometrical basis functions. In order to reduce the focus of the antenna, the dependence of the focus size on the radius of the antenna is calculated.