Xbee pro S3B antenna

I would like to know which type of antenna is best suitable for Zigbee pro XSC S3B module. I am using the module to send and receive data from end device which is passed through 2 routers. Total Xbee pro XSC S3B module used is 4 including the coordinator, router and end device. The total range from coordinator to end device is 45 km.