Working into a mismatched load with the output set that high, the RFM69 module is easily damaged.
Quite the opposite actually.
An end-fed full wave antenna exhibits a high impedance, as does an end-fed half-wave antenna.
The RFM69 is designed to work into a 50 Ohm load, thus both would cause an increase in reflected power.
The solution for a monopole is an end-fed quarter wave antenna with a ground plane.
More info, and how-to, here: Improving RF signal strength emonTH emonTX - #16 by Paul