Er, and what does it do with the absorbed radiation?
Er, previous comment was written for people who think one or two steps ahead. A parabola is great for directing EM energy. (Think searchlight.) The rest was left as an exercise for the reader.
and so you also need to make it magically able to absorb limitless energy without failing.
There is indeed a fail, but not the one you envisioned. (EDIT: Don't feel too bad. That discussion on spacebattles was presaged by similar discussions on USENET in rec.arts.sf.science from over 25 years ago. I have a bit of a head start.)
So its a magic surface where the inside is a perfect reflector and emitter, the outside is a perfectly absorbent and transmitter (to the inside)? That (though it requires multiple levels of magic) solves one problem, but of course then its also a drive system you can't control or turn off that turns all of your internal energy use and all incoming EM energy into thrust in the direction opposite the outlet of the cone, in addition to creating a detectable (by reflection from anything that passes through it) plume of EM emissions from the back end of your ship that will expand in such a way as to be visible even in the area from which your ship itself is shielded from direct observation.
So its a magic surface where the inside is a perfect reflector and emitter, the outside is a perfectly absorbent and transmitter (to the inside)?
Not a surface. It's a paraboloid shaped machine. (With a "stamen" in the center.) It seems magical to us now, but within the realm of physical possibility, so it's fair game for science fiction speculation.
its also a drive system you can't control or turn off that turns all of your internal energy use and all incoming EM energy into thrust in the direction opposite the outlet of the cone
Yes. It will be tricky to use. Swarms of these units will be implementing strategies devised by AIs in cooperation with interfaced human geniuses, heuristically solving the horrendous geometric and continuous thrust trajectory puzzles formed by the partially and probabilistically known configuration of enemy units. (EDIT: There will also be some mass driver propulsion available, but this will also be limited.)
plume of EM emissions from the back end of your ship that will expand in such a way as to be visible even in the area from which your ship itself is shielded from direct observation.
I must admit I haven't worked out the numbers for how hot these things can get or how concentrated their waste EM plumes can be before it's counterproductive because the near-vacuum gas/plasma starts emitting too much. As mentioned elsewhere in these threads, I'm not envisioning a Star Trek or Star Wars style ship. These are clouds of small autonomous drones. Small size, unmanned operation, and larger numbers are going to be a big win for such technology. In other words, I'm not trying to stealth something big and hot in the first place. (Unless it's the "mother ship" and that will only operate far outside of enemy space.)
Start with clouds of small, dim, but potent drones, then take the arms race in a particular direction.
Er, previous comment was written for people who think one or two steps ahead. A parabola is great for directing EM energy. (Think searchlight.) The rest was left as an exercise for the reader.
and so you also need to make it magically able to absorb limitless energy without failing.
There is indeed a fail, but not the one you envisioned. (EDIT: Don't feel too bad. That discussion on spacebattles was presaged by similar discussions on USENET in rec.arts.sf.science from over 25 years ago. I have a bit of a head start.)