20 dBm

2.4GHz

RF220SU/SU-EU TX Up to 3 miles (4.8km) at 250 Kbps using a 5.5dBi antenna

* Since it is only 20 dBm, you can increase gain on antenna to at least 9 dBi?

Datasheet: http://www.electronics-chips.com/c/part/synapse-wireless/RF220SU-EU#Pdp-datasheet

Datasheets and firmware for the RF220 series: https://developer.synapse-wireless.com/modules/rf220/downloads.html

* LR-WPANs https://en.wikipedia.org/wiki/IEEE_802.15.4