I think this should work - although it didn't work for my capture file
LINK: http://www.question-defense.com/2010/03/29/tshark-strip-wpa-wireless-captures-by-essid-with-tshark
LINK: http://www.question-defense.com/2010/03/29/tshark-strip-wpa-wireless-captures-by-essid-with-tshark