I think the antenna becomes a generator. Passing photons induce AC voltage in the antenna. The amplitude is greatest when no load is attached. The antenna produces a waveform which is a jumble of frequencies. We can feed it to a bandpass filter (tuning it to one frequency), or to an amplifier. Depending on the type of device, it may load the antenna, drawing current, and reducing voltage.
How does current flow in the antenna? Do the photons induce true AC, or is it AC with a DC component? Etc. The above is merely my non-expert thought experiment, and I could be wrong.