All the other Y-8GX planes don't have them either. Its a possibility that the aerial may be used to communicate with older planes and ships. Perhaps something they may have figured out that they lacked on the KJ-2000s or the other Y-8GXs and decided to add it here as a remedy. Jets like the KJ-2000 or the SAAB AEW fly at speeds that using wire antenna may not be wise, not to mention clearance with the disk.