30 degrees & RFID interference

This is a "I need to investigate something" note to myself. I've read lots of interesting stuff on RFID theft (schneier on security covers skimming today), it's something that's fascinating- I wonder if the person who invented the "tin foil hats" meme years ago could possibly have predicted tin foil wrapped passports. However, I have 3 proxcards on a chain that I carry around every day. When I need to scan one, I have to take the particular card that I want to read and either flip it out almost perpendicular to the other cards, or fan it out (as if I was holding some playing cards in my hand) so that there's about a 30 degree angle between the card I want and the others. Holding the cards stacked up on top of each other- as they usually are on the chain- means that none of them will scan. I think this must be some kind of radio wave interference, and I wish I knew the physics behind it. One more thing to look up some weekend. I really wonder why 30ish degrees is the magic angle.