It's the same thing as the Eton loop I've seen at Fry's for $30 or so. It probably works much the same as any other similarly sized loop (S-A-T, Terk loop, etc.) -- which Radio Jay Allen also says.
I have a Select-A-Tenna, Radio Shack loop (no longer available -- same size as the S-A-T) and a box crate loop I made with an old 365 pf cap and 110 ft of wire. The box crate loop has more gain, understandably, because it's bigger, but the two smaller loops work the same as each other, are very handy to use, and I use them more often.
They each add about a DB to my PR-D5, and maybe 1-2 DB or more (just going by ear) to my Superadios, depending on conditions. A necessity for DXing with a headset radio like a Sony SRF-59, and very helpful for DXing with smaller portables, unless you're in a high signals area.
