Yun's design does not support external antenna. The cable connector is for ART (Atheros Radio Test) only.The build in antenna's gain is only ~-0.5 dbi, plus AR9331 only has 17 dbm output power.
@sonnyYu,The external antenna connector is not good?Why have connector?Jesse
It is for calibrate radio (Atheros Radio Test) of AR9331 during manufacture stage, equalize all the channel (1-13) output power to make FCC happy. That answer why it only last ~100 times connect, for external antenna second connector is needed.