This document describes a multimode high-isolation antenna system and associated methods and systems. The described antenna system is implemented on a generally-circular printed circuit board and can be used for wideband and ultra-wideband applications. The multimode high-isolation antenna system includes two orthogonal antennas separated by a decoupling structure. This arrangement provides high isolation between the antennas and enables five unique resonant modes of operation for the multimode high-isolation antenna system.