[werk_ag]

You can either hermetically seal them into a waterproof box, or put the pcbs into a vented enclosure with the vent being a breathing dessicator, expensive though!

Sharing my experience...
I'm using a big waterproof case, but this case is not sealed, it have a small hole on the bottom. I never saw any sign of condensation inside, and I live in a place where air moisture levels, specially during winter, are usually high.
A small case is more prone to condensation than a bigger one.

My transmitter unit is here since 3 years, working as new. As seen on the picture, my TX unit even is one of the older versions which don't have an on-board SMA connector for the antenna.

This transmitter unit is only sending data of my wind instruments, all the other sensors (temp / hum,  rain gauge, solar Rad / UV) are connected to another transmitter unit.