[hvalentim]

Your conclusion may be somewhat precipitated. Do you already have an anemometer connected to the TX unit?

Not yet. No anemometer used. Just temp/hum and solar radiation.

at least install a simple coil antenna like this one below.

Of course. I will install a proper antenna regardless. I was just commenting the fact that meteoestareja's joke may prove real. If it worked 15 meters away with a few walls in between there is a chance it may work at 5 meters with far less obstacles in the way. I will let you know the result when I test that way with the anemometer connected.