Wireless backhaul systems have been considered as a promising candidate of beyond 3G wireless broadband system for mobile communications. The achievable transmission performance over radio relay channel depends on antenna directivity and radiation patterns of each antenna element. To improve the transmission performance and keep radio relay channel in an acceptable condition, it is effective to control the antenna directivity by employing multi-antenna signal transmission and reception. In this paper, we propose a simple method to extend the existing single antenna relay node based on IEEE802.11a to multi-antenna system, where a cross polarized multi-antenna is applied to the existing relay nodes as external equipment. The multi-antenna system is controlled by detecting the antenna control information assigned to the lengths of the packets transmitted from the existing node. The proposed method does not require special change of the existing system such as hardware design on multi-antenna configuration. We also developed a prototype base/relay node with optional multi-antenna equipment and confirmed the effectiveness of the proposed method by experiment. To clarify the effectiveness of the proposed multi-antenna system in wireless backhaul networks, we evaluate throughput performance by system level simulations. Simulation results ensure that the proposed multi-antenna system with highly efficient packet forwarding protocol, called Intermittent Periodic Transmit (IPT), improves throughput performance of the radio relay transmission in wireless backhaul as compared with conventional omni-directional antenna system.