How to Model antenna as voltage source?
Antennas have something known as their characteristic impedance.  You can think of a antenna as a Thevenin or Norton source at this impedance.

A common dipole has around 75 &Omega; impedance at its intended operating frequency.  A folded dipole has about 300 &Omega; impedance.

For example, if a 75 &Omega; antenna picks up a 100 &micro;V signal, then you can think of it in two equivalent ways from the point of view of the circuit.  It could be a 100 &micro;V source with 75 &Omega; in series, or a 1.33 &micro;A source with 75 &Omega; across it.

Note that the above only applies at the antennas intended operating frequency.  The impedance can be very different at other frequencies.  For example, a dipole as infinite impedance at DC, and a folded dipole 0.  The impedance also become reactive (no longer purely resistive) at off frequencies.