Peter wrote:
"What amplitude can it be?"

A receiving antenna has a radiation resistance. The load which extracts
most power is a conjugate match for the antenna. With radiation
resistance perfectly matched to receiver input resistance, the radiation
resistance becomes the Thevenin equivalent source resistance feeding an
identical receiver input resistance. 1/2 the voltage induced into the
receiving antenna is used to re-radiate energy received by the antenna.
The other 1/2 appears across the receiver`s input resistance.

For a professional explanation of this topic, see Kraus` 3rd edition of
"Antennas" page 32. "All about antennas at a glance".

Best regards, Richard Harrison, KB5WZI