Described is a simultaneous transmit and receive antenna system having a ring array of transmit antenna elements and a receive antenna element disposed on an axis that is perpendicular to and passing through the center of the ring array. Alternatively, the ring array includes receive elements and a transmit antenna element is disposed on the axis perpendicular to the ring array. Opposite antenna elements in the ring array differ in phase by 180° so that a radiation pattern null occurs at the antenna element at the center of the ring array. Also included are at least one ground plane and an electrically-conductive cylinder disposed on the perpendicular axis inside the ring array to provide a high degree of isolation between the transmit and receive antenna elements. The system may be configured for wireless communications, for example, according to WIFI IEEE standard 802.11 or WIMAX IEEE standard 802.16.