trying to receive RF signals sent bij an outdoor weather station sensor

Hi Reinoud, even the first few bits of signal might be enough - at least you could figure out what the bit rate is, and from that get some idea of the protocol.

Personally, I'd crack open the transmitter and look inside. There's often a transmitter module with a clearly marked 'data' pin, and if you hook onto that then you can get a nice clear signal to analyse (if you go from the radio receiver, you'll probably get all kinds of stuff and it won't be obvious which one is from the weather station).