I searched the datasheet of the Semtech SX1280, but I can't find the output impedance of the SX1280 (RFIO's pin). Where can I find it?
1\$\begingroup\$ Why don't you ask Semtech Technical Support directly as they would know exactly the answer to your question ? \$\endgroup\$– citizenMar 8 at 11:00
I reccomend you make this question on ham.stackexchange. A similar question was asked there by @Phil.
So does this answer your question?
Inside the output of a transmitter is often a PA, consisting of a transistor and a matching network
there's no simple answer. The transistor is typically operating nonlinearly, which can be modeled as a current source with a shunt RC that changes with voltage. This leads to the output impedance depending on the exact state of the PA and the exact state of the PA depending on the load.
Typically, you would design (aka model and simulate before ordering) your antenna have the best power efficiency (less losses) on the frequency you will operate the IC/antenna for your transmission or receive operations, without taking into account the impedance of the IC, if it is not mentioned at the datasheet. After all, and correct me if I am wrong, but an antenna (typically and usually) does not have a GND plane under it, it is just a wire on the air.