QuickForm and Radios

Hi everyone. My name is Ricardo, I am developing a complex form with multiple radio buttons groups, my problem is around setting them as null value (unchecked by default) to all radio buttons. ¿How can I do this?. Actually I assign to smarty engine the radio array, and smarty sets it as checked the first radio element of the group. Thanks for your time and sorry for my bad english. Ricardo Bruzos