If one 1km EM-wave is 1 photon and very weak, how is it absorbed? In QM, antennas emit photons that are absorbed by the receiver, which excite electrons (raising the local Fermi level) which yields potential difference thus current. The antenna gets very slightly charged up on one side. This should build up a net increase of heat or energy, but is lost in ground or as heat eventually. Think of it like a solar panel, all that heat and energy has to be converted or dissipated.
Photon probability waves behaves just like classical EM-waves before the collapse, so you get a higher photon intensity on one side of your dipole antenna, back and forth.
But how can copper absorb such large photons?  First of all you need a quite large antenna to cover as much of the probability wave as possible. But also, aren't those energies too low to excite electrons in copper?
You'd need a really high intensity of those low energy photons. But at some point, you can't even absorb them?
 A: First, it's conduction electrons in a metal that move back and forth due to an EM wave. No problems, they are very good conductors because they have so many near free electrons.  
Second, the photons may be low energy, but unless there's enough of them to have the necessary power or electric field you can't get enough current, or electrons to move, to detect something. So you need enough of them, might as well analyze it using classical EM
Third, a 1 km wavelength is a frequency of 300,000 HZ, or 300 KHz. It's in the so called the LF, low frequency band, and often have used tall monopole antennas (like the ones you see on a highway from afar). Called mast antennas. They use vertical E polarization, and usually propagate mostly as ground waves, with little attenuation for hundreds and thousands of miles because their long wavelengths makes them diffract over almost all obstacles and hills and mountains. There can be other antenna forms, including just a long wire, and short ferrite loop antennas that are just a foot or so long; see some of the other forms in the second reference below.
The large antennas usually aided by some coils or loads at the bottom to get a reasonable impedance can work pretty efficiently. The largest such antenna finally collapsed in Poland in 1991, it was for a 1.3km wavelength, and was 646   meters high. It was half a wavelength, and most of the LF antennas are so-called electrically short because they are half a wavelength or less. When much less they are less efficient, but are used. See mast antennas at
https://en.m.wikipedia.org/wiki/Mast_radiator
LF has been used for radio navigation (the Loran C system, other ground, air and over water), emergency communications, long wave radio, submarine communications, clock radio, and other things. It works, the antennas are not and don't have to be that efficient, just long enough to have some varying E field. See more on LF at https://en.m.wikipedia.org/wiki/Low_frequency
