915 mhz telemetry

i need long range antenna atleast can 15 to 20 kilometers range… what antenna shouls i use? thank you

The RFD900 set should do it fine using the standard antennas they come with. Maybe use a directional antenna at the ground station.

witch one is better the plus version or the X version?

The RFD 900x is the newer model. RF Design also have a range of extra antennas too.
I’ve personally heard of these radios doing 70km unobstructed from a very reliable source.