What is the major function of an antenna tuner (antenna coupler)?
It matches the antenna system impedance to the transceiver's output impedance✓Bonne réponse
It helps a receiver automatically tune in weak stations
It allows an antenna to be used on both transmit and receive
It automatically selects the proper antenna for the frequency band being used
Explication
An antenna tuner is a matching network placed between the transceiver and the feed line, transforming whatever impedance the antenna system presents into the 50 ohms the transmitter expects. It does not retune the antenna itself, and it does nothing to help receive weak stations or switch antennas.