Ok, in case you didn't read my replies in another thread...
Skylanders are RFID MiFare. 13.56Mghz, it's a standard. ALMOST ALL systems use a similar method.
The portal only powers the figures when the game is active and wants to read them, otherwise as mentioned, it would drain batteries.
While different shapes and lengths of antenna WILL work, the whole system is "tuned" to work best with certain shapes and lengths.
If you want to power a shelf of Skylanders your best bet would be a rectangle, just deep enough to fit the figures inside it but wide enough to fit say 8 Skylanders.
This simply has to be a loop of wire in multiples of I believe 30cm.
A crystal oscillator circuit at 13.56mghz will generate the right signal, then a power transistor on the output to power the Antenna and figures will do the job.
There are 2 ways to generate 14.56mghz. A ready made 4 pin Oscillator (most seem to be awkward surface mount these days) which simply needs 5v, gnd and has a 13.56mghz output.
OR, a 13.56 crystal and a small circuit with a couple of capacitors and an old logic chip.
This circuit is well documented and is a basic electronics design all over the web.
To "tune" the circuit, when it's powered and skylander is lit (or not) experiment with varying the length of the Antenna, this may be as much as mm adjustments or whole cm.
I have like 5 Portals, 2 of which are dedicated for hacking. They are in pieces right now with many wires soldered to them so you can trust me on this info
Ok, I filmed this quickly... As you can see, making a receiver for the power is incredibly easy.