A novel feeding structure for stacked circular microstrip antenna operating at higher order modes

This paper introduces a feeding structure for stacked circular microstrip antennas, operating at the fundamental TM₁₁ and higher order TM₂₁ and TM₀₂ modes. The latter mode has its strong field at the patch center, and prevents the use of a central feeding. However, this mode also has an electric field null within its cavity, but at a finite radius from its centre. Since this TM ₀₂ mode has the largest patch, which must be placed on the lowest layer, placing the top layer feeds at this radius, does not affect its field