We developed a wireless interface design of tan ultra-thin RFID reader module for wireless mobile devices. The module consists of a novel self-ground antenna, PCB mounted chip devices, and a covering case. The antenna we developed has a thin planar shape that can overlap the PCB without substantive volume increase. We also developed a computational design method to ensure the antenna exhibits good radiation characteristics. Tests on a sample device showed that the antenna has adequate electromagnetic characteristics with radiation efficiency no higher than -5dB and broad radiation patterns suitable for RFID reader modules. These results demonstrate the suitability of the device for application in commercial wireless mobile devices.