[werk_ag]

So in practice an antenna on the TX side is optional. At least for me.

I know that we are using a very powerful TX module with more than 10 mW transmiting power, but I strongly recommend to everyone to install an antenna in the TX module.
Your conclusion may be somewhat precipitated. Do you already have an anemometer connected to the TX unit? The wind data packets are sent every 5 seconds, I think you will don't want to lose to many data packets.
If the place of your installation is almost free of outside RF interferences at least install a simple coil antenna like this one below.