Silly suggestion I know, I am just throwing out there. What if a copper coil was not a circle, but a exact size and shape (or pattern)?

Some antennas like wifi catch certain frequencies, like the biquad antenna. It seems to catch a frequency at a certain size and shape.  


http://www.lecad.fs.uni-lj.si/~leon/other/wlan/biquad/

Also you see some electric wires have Ferrite Beads which helps to reduce radio wave interference in electronics.  


So some wave frequencies can interfere with wires and certain sizes and shapes capture certain frequencies. Now is you shape the copper coils into a shape to capture a specific frequency would the voltage needle jump a bit if you put a capacitor at the end of the coidl and tested it out after a while of radio wave bombardment? Obviously a lot of loops can not fit in a small specific shape, so a bigger design would be needed.

So instead of something small like this with many coil turns...
or this
Maybe something bigger that this would be the way.

Or even more elaborate to gradually increase in diameter to capture a lager range of frequencies?

This can all be wrong and I am just throwing it out there.

Yes, an antenna captures energy, That's what they do.

I guess what you mean is "useable energy that can be redirected to power something else." In that case, you have a fairly limited device that relies on the presence of other man-made sources of energy. Not really a profitable direction to pursue, IMHO.