This page covers radio button tips & tricks.
Radiobuttons placed in a group will have one which is set to TRUE.
It is not possible to set them all to FALSE.
Use this trick : place a non-visual (hidden) radiobutton in the group and set it to TRUE.
Then all other visual radio buttons will show FALSE.
Don't do the above. Either have a visible radio button that is the "None" Option or use a set of tick boxes.