You need to place the antennas outside where they can 'see' each other. A pre-made length of coax and a small fixing will be required. Try to think of ways to keep the coax cable run as short as possible as this will be a big help with the low power levels we generally use.
If you can put the whole radio module outside in a weather proof box then that would be another option.
Thanks @skywatch I'll experiment some more with different radios and antennas outside the barn. It's a bit frustrating because the barn has sensors in it, and I plan to have several sensors in opposite directions outside the barn. There's the pump shed 15m outside in one direction and a water tank on a hill 200m in the other direction.
I think I'm going to need sensors, acting as repeaters, on all 4 corners attached to the outside of the barn!